MORPHOLOGYANDEVOLUTION OFTHEINSECTTHORAX RYUICHIMATSUDA EntomologyResearchInstitute CanadaDepartment of Agriculture Ottawa,Canada MEMOIRSOF THEENTOMOLOGICALSOCIETYOFCANADA- No.76 Editor:D.P.Pielou TomyMother PublishedbyTHEENTOMOLOGICALSOCIETYOFCANADA,OTTAWA 1970 CONTENTS PAGE INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 I.GENERALDISCUSSION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 1.TheExternalStructures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 TheNeckRegion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 TheTergum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 TheSternum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17 TheIntersegmentalRelationship. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 TheOriginofthePleuron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24 HomologiesofthePleuralParts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33 TheWing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40 TheCoxa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46 Positionsof the Thoracic Spiracles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46 2.TheMusculature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48 ThoracicMusculatureinLepismasaccharina. . . . . . . . . . . . . . . . . . . . . . . . .48 ThoracicMusculatureinthePterygota. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56 MajorEvolutionaryFeaturesofThoracicMusculature. . . . . . . . . . . . . . . .76 II.SPECIALDISCUSSION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88 TheProtura. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88 TheCollembola. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .90 TheDiplura. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .94 TheThysanura. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99 TheEphemeroptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .111 ThePlecoptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122 TheEmbioptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .132 ThePhasmida. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .139 TheBlattaria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .143 TheIsoptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .154 TheMantodea. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .157 TheGrylloblattodea. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .167 TheOrthoptera 169 TheDermaptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .187 TheColeoptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .199 TheZoraptera. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .218 ThePsocoptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .222 TheMallophaga. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .232 TheAnoplura............................................ .........239 TheThysanoptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .241 TheHemiptera(Homoptera)...................................... :248 TheHemiptera(Heteroptera). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .267 3 4MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA PAGE TheNeuroptera.....................................................283 TheMecoptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 293 TheDiptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .304 TheTrichoptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .326 TheLepidoptera(Zeug!opteraandDacnonypha). . . . . . . . . . . . . . . . . . . . . .334 TheLepidoptera(MonotrysiaandDitrysia). . . . . . . . . . . . . . . . . . . . . . . . . .339 TheHymenoptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .354 TheStrepsiptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .373 TheSiphonaptera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .382 TheOdonata. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .386 LiteratureCited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .402 IndextoTerminologies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .427 IndextoAuthors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .429 MORPHOLOGYANDEVOLUTIONOFTHEINSECTTHORAX RYUICHIMATSUDA EntomologyResearchInstitute,CanadaDepartmentofAgriculture,Ottawa INTRODUCTION This isthesecondof aseriesof worksin whichthestructuralevolutionofinsectsis studied.This volumedealswith theevolutionof the thorax throughouttheclassInsecta. As in thefirstvolume,dealing with the head( 1965), the primary objectiveistoestablish homologiesoftheexternalstructuresandmuscles. Homologizationsofstructuresrequire,amongotherthings,aclearunderstanding of the phylogenetic relationshipsof groups,and viceversa.Whenever this work deemed tothrownewlight,therefore,thephylogeneticrelationshipsamongordersofinsects arealsodiscussed,andindoingsoterminologiesproposedbyHennig( 1950,1953, 1966)arefollowed. Realizingthatathoroughstudycoveringallmajor problemsoftheinsectthoraxis hopelesslytimeconsuming,Ihaveleftasidecertainimportantaspectssuchasthewing venation,thelegs,andsoon.FlightmechanismineachorderisdiscussedpTimarily fromtheanatomicalviewpoint,solongasthephysiologicalfactorsdonotoverridethe anatomicalconsiderationsinimportance.Thepatternofinnervationisalsoomitted, sinceithasbecomeincreasinglyclearinrecentyearsthatatthepresentstateofour knowledgeandtechniquestheinnetvationpatterncannotbeaprimarylandmarkin determininghomologiesof musclesandexternalstructures. Thematerialonwhichthepresentworkisbasedhasbeenderivedmainlyfrom publishedworks,althoughaconsiderablenumberofactualdissectionswerenecessary to fillthegapsin knowledge.The resultof this workwillshow wherewestandtoday in ourknowledgeoftheinsectthoraxandwhatistobestudiednext.Thelastcareful search of literature was made in thesummer of 1968. ACKNOWLEDGMENTS Thisstudywasstartedinthesummer1965attheMuseumofZoology,University ofMichigan,AnnArbor,underagrantfromtheNationalInstituteofHealth.The work wascontinuedat theEntomologyResearchInstitute,Ottawa,sinceFebruary1968, andwascompleted in April1969.Inthesummerof1966thisworkwascarriedoutat theZoologicalInstitute,UniversityofMainz. Assistancecamefromvarioussourcesduringthecourseofthis study.Besidesthe giftsof specimensfrommycolleaguesinAnnArborandOttawa,Iowethematerialfor dissectiontoProf.G.W.Byers(Lawrence),Dr.A.B.Gurney(Washington,D.C.), Dr.R.Kinzel bach(Mainz),Dr.G.Mickoleit(Ttibingen),Prof.S.Miyamoto (Fukuoka),Dr.E.S.Ross(SanFrancisco),Dr.L.M.Roth(Natick),andDr.P. Wygodzinsky(NewYork). Sectionsofthemanuscriptwerereadandvaluablecriticismsandsuggestionswere receivedfromDr.S.Asahina(Tokyo),Dr./ R.McAlpine(Ottawa),Dr.J.M.T Campbell(Ottawa),Dr.G.Mickoleit(Tiibingen),Dr.M.Parsons(Toronto),Dr. W.R.Richards(Ottawa),Dr.E.G.Munroe(Ottawa),Dr.J.Sharplin(Edmonton) andDr.J.R.Vockeroth(Ottawa).Mr.J.A.Downes(Ottawa)reviewedthewhole manuscriptandgavecriticismandadvicefromaneditorialviewpoint.Ishouldalso mentionusefulconversationswithDr.J.Barlet(Liege)andwithDr.G.Mickoleit (Ttibingen). s 6MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA I.THEGENERALDISCUSSION Inthissectionemphasisisplacedonfindingtheprimitiveorganizationofthoracic structures,toenablehomologizationsofstructuresinthehighergroups.Since,how-ever,thissectioncannotbeindependentofthefollowingspecialdiscussion(PartII), whichdealswitheachorderofinsect,itservesalsoasasummaryofthenextsection. GeneralprinciplesofstructuralevolutionthroughouttheclassInsectaarealsodis-cussed here. 1.THEEXTERNALSTRUCTURES THENECKREGION Theoriginoftheneckregion:TheOriginoftheneckregion,whetheritderives fromtheheador fromtheprothoraxorfromboth,hasbeenamatterofcontentionin thepast.DuringtheembryonicdevelopmentofSi/pha,accordingtoSmreczinski (1932),theventraland lateral partsof thelabialsegmentparticipate intheformationof thehead,andtherest(thedorsalportion)connectstheheadandthethorax.This observation suggests that in theadult the definitiveventrolateral part of thelabialsegment musthavebeencompletelyintegratedintothehead.It follows,then,thattheventral partoftheneckregionmustbeprothoracicinorigin,andthatonlydorsallycanthe neckregionbelabialinorigin.Holmgren(1909)alsosaidthatapartoftheneck membrane probably belongs to thelabialsegment. Themusclesoccurring intheneckareat leastpartlylabialinorigin.InCarausius Wiesmann( 1926)showedthattwoofthethreelabialembryoniccoelomicdiverticula (medialandlaterodorsal)giverisetotheventrallongitudinalmuscles(oftheneck region),thedorsallongitudinalmuscles,andtherotatorofthehead,andthatonlythe ventralcoelomicdiverticulumgivesriseexclusivelytolabialmuscles.InSilpha (Smreczinski1932)thelabialcoelomicsacgivesrisetothelabialmuscles,partofthe flexormuscleofthemandible,therotatorsofthehead,partofthemidgutmusculature, etc.It isevidentthatatleastsomeneckmuscles(rotatorsofthehead)areformed fromthelabialsegmentinSilphaandCarausius.Othermusclesoccurringintheneck regionare,therefore,perhapsprothoracicinorigin. Nervedistributionissometimesusedindelimitingsegments,onthepremisethata nerveinnervatesthe segmenttowhichtheganglionfromwhichthenervearisesbelongs.' Matsuda(1956),Schmitt(1962)andMark!(1966),amongmorerecentworkers,have pointedout,however,thatthiscriterioncannotalwaysbereliablyused,becausetwo neighboringnervesmayfuseinvaryingdegreesoramusclemayreceivemorethanone nerve;furthermore,individualvariationinthedistributionalpatternofanervemay sometimesbeconsiderable.Yetifthegeneraldistributionalpatternofthenervesis moreorlessconstantinvariousgroupsofinsects,andiftheabovedifficultiesare relativelyunimportant ininterpretingtheanatomicalareainquestion,innervationcan be usedasanadditionalcriterionindeterminingasegment. Thus,inDissosteira(Fig.1),asSchmitt(1959)showed,apurelysuboesophageal nerve(secondcervicalnerveofSchmitt)innervatesmusclesop-t1,2,3( 47-49of Snodgrass,1929a)and cv(d)-t1 (56).Thefusionof theanterior ganglionicconnective and the prothoracic anterior dorsalnerveinnervatest-p2(58),t-cv3 andt-s(cv) 1(52), t-cv2(53),andsI,2(55).p-t1and2arealsoinnervatedbyaprothoracicnerve. InPeriplaneta(PipaandCook1959)apurelystiboesophagealnerve(SGNGLofPipa andCook)innervates op-tIandop-t2(52,53of Carbonell,1947).InPeriplanetathe mostanteriorprothoracicnerve(nerve2ofPip aandCook)fuseswiththeanterior ganglionicconnectivearisingfromtheinterganglionicconnective(nerve1ofPipaand Cook), and thefusednervedivergesfurtherandisjoinedbyalateralnervearisingfrom themesothoracic ganglion.Determination of thesegmental originof musclesinnervated by thenervesof such complex origins cannot be made,unlessthefibersof each individual nervearedistinguishedhistologically.Nevertheless,itisclearthatinPeriplaneta,asin 1Theknowndevelopmentalfactssupportingthispremiseareveryfewandfragmentary.Therelationship betweennervesandmusclesmustbestudiedfromtheirearliestdevelopmentalstages,andinmanyforms beforeitcanbemorereliablyused.' 2NDCERVICAL _____,. NERVE MATSUDA:TilEINSECTTIIORAX ~ - - - - SUBOES.GANGLION r--- ANT.GANGL.CONNECTIVE t-cv 2 t-cv 3, t-s 1 7 FIG.I.DiagramofsomecervicothoracicnervesinDissosteiracarolina(fromSchmitt). Dissosteira,somedorsalpronotalmusclesof theheadreceiveinnervationfromapurely suboesophagealnerve. Schmitt(1962)reviewedtheliteratureonthepatternofinnervationofthesecond (posterior)cervicalnerveinvariousinsectsthatwerestudiedbyHolste( 1910),Maki (1936),Marquardt(1939), Nliesch(1954), Wittig(1955), Pipa and Cook(1959),and Schmitt(1959).Schmittconcludedthatthesecondcervicalnervegenerallyprovides innervationto thepronotalmusclesof thehead,andinsomeinsectsthefirstandsecond tergalmuscles(op-t1,2)receiveinnervationfrom,boththesecondcervicalmuscleand theprothoracic dorsalnerve. Thesefactssuggestthatthedorsalmusclesoftheheadareatleastpartly labialin ongm.Heretheneurologicalfactsandtheembryologicalfactspertainingtotheorigin oftheexternaldorsalareaandsomedorsalmusclesareingeneralagreement.Mostof theother neck musclesareprobablyof prothoracicorigin,althoughpossiblytheventral longitudinalmuscle(s1or2)islabialinorigin,asWiesmann(1926)foundembryo-logicallyinCarausius.Theseneurologicalandembryologicalfactsindicatefurtherthat thecervicalareaisacompositearearesultingfromthefusionofthedorsallabialand ventral prothoracic segments.As shown in Part II of thiswork,however,thisgeneraliza-tiondoesnotapplytotheentognathousApterygota,inwhichtheqorsalpartofthe neckregionisprobably a part of the prothorax. Theoriginandevolutionofthecervicalsclerites:Theoriginofvariouscervical sclerites,whethertheyarelabialorprothoracicinderivation,hasbeendisputed.The lateralcervicalsclerite,whichoccursinmostorders,isconsideredfirst. InLepismachilis(Fig.35),asBarlet( 1967)hasshown,whatwascalledthe processuspostgenalevidentlycorrespondstothecondylethatarticulateswiththe anterior end of thelateralcervicalsclerite,becauseit iscontinuouswiththelateral endof thetentorium.TheplatethatBarletcalledscleriteanapleuralsuperieurarticulateswith the postgenal process.In Lepismachilis thereforeit is clearly the pleuron thatarticulates withthepostgenalprocess. The postgenal processhasanother processthatprojectsbackward(Fig.35),which Barlet( 1967)called scleritepostlabiale.Two musclesof Barlet,50and54,areinserted onthispostlabialscleriteandthepostgenalprocess;judgingfromtheirpositions,these musclesareprobablyhomologouswitht-cv2andt-cv3inthePterygota,whichare insertedontheanteriorendofthelateralcervicalsclerite. Henry(1958)ignoredthispossibilityofdoubleinnervationofamuscle. 8MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA Theabovesimilarityinpositionandintheassociatedmusculaturebetweenthe scleriteanapleuralsuperieurandthelateralcervicalscleriteleadsustosuspectstrongly thatthescleriteanapleuralsuperieurinLepismachilisandthelateralcervicalscleritein somepterygotesare probably homologous.The nextquestion is,"What part of the pro-pleuron correspondstothe lateral cervicalsclerite,andinwhatinsects?" InSchizocerus(Fig.155B)andotherHymenopterathereisnoisolatedlateral cervicalsclerite.Instead,theanterior endof thepreepisternumarticulatesdirectlywith the head,just asinLepismachilis.Therefore,thelateral cervicalscleriteinother insects canbeadetachedpreepisternum. InMicropteryx(Fig.144A)thepositionofthelateralcervicalscleriterelativeto theneighboringplatesisthesameasthatofthepreepisternuminthepterothorax.In Trichoptera(Fig.141A)thelateralcervicalscleriteissoobviouslyseriallyhomologous withthepreepisternuminthepterothorax,thatTindall(1965)hadnohesitationin identifyingthetwostructures.InSialis(Fig.118A), again,thelateralcervicalsclerite isobviouslyamodifiedpreepisternum.Mutuura(1961)alsohomologizedthelateral cervical sclerite inMecoptera with the prothoracic episternum.InOdonata,which share anumberof plesiomorphiccharacterswithMachilidae,thelateralcervicalscleriteisof preepisternalorigin( p.386).Inhigherholometabolousinsects,suchasDipteraand Lepidoptera(Monotrysia,Ditrysia),the preepisternalorigin of the lateral cervicalsclerite hasbeengenerally obscured. InallhemimetabolousordersandinColeopterathelateralcervicalscleriteisof presternalorigin.In Lepisma(Fig.37),aswellasinLepismachilis(Fig.35),thepro-thoracicpresternumextendslaterallyanteriortotheepisternum.Inthemesothoraxof Plecoptera(Fig.48)thepresternumoccupiesacorrespondingpositiontothosein LepismaandLepismachilis;itisfragmentedintomedianandpairedlateralpieces,and the latter isoften calledthe interpleurite.In theneckregionof Capnia(Fig.46A) ,the lateralcervicalscleriteandthepresternumclearlycorrespondtothethreepresternal sclerites in the mesothorax of Perla(Fig.48).These correspondencesinpositionof the scleritesinquestionleadustoconcludethatthelateralcervicalscleriteinCapniamay wellbeofpresternalorigin.Further,thefactthatinPlecopterathelateralcervical scleritearticulateswiththeanepisternumandthepreepisternumisfusedwiththebasi-sternum,excludesthepossibilitythatthelateralcervicalscleritecanbeofpleural(pre-episternal)derivation.-Similarly,inthedermapterousgenusAllostethus(Fig.79A)thelateralcervical scleriteclearlyarticulateswiththereducedpreepisternum;inBlattaria(Fig.54A)the lateralcervicalscleritearticulateswith the precoxalbridge,whichislargely preepisternal; inother related ordersand in Coleoptera also,the lateralcervicalscleritearticulateswith thepreepisternalarea.Thesefactssuggestthatthelateralcervicalscleriteinthese ordersispresternalinorigin,asinPlecoptera.Fragmentationofthelateralcervical scleriteintoanteriorandposteriorpiecesiscommoninlowerhemimetabolousorders. In most Hemiptera thelateral cervical scleritehasbeenlost. InMegalopterathepreepisternumandthebasisternumarefirmlyunited,and togethertheyextendforward;anadditionalventrolateralcervicalscleriteofpresternal origin liesanterior tothe preepisterno-basisternum(p.285). The above discussion on the origin of the lateralcervical sclerite leads ustoconclude thatatleastfourdistincttypesoflateralcervicalsclerites(undetachedpreepisternal, detachedpreepisternal,presternal,preepisterno-presternal)occurinthePterygota. Withregard tothe origin of thedorsalcervicalsclerite,Mutuura(1961)contended thatinPanorpathedorsalcervicalscleriterepresentstheantecosta,becauseadorsal longitudinalmuscle[cv(d)-t 1]from thefirstphragma isattachedtoit.Alicata(1962b) cameindependentlytoaverysimilarconclusion;hethoughtthatthedorsalcervical scleriteinorthopterousgeneraDolichopodaandTfoglophilusrepresentstheacrotergite, because the dorsal longitudinal muscle [cv(d)-t 1]isattached to it. MATSUDA:THEINSECTTHORAX9 Themusclecv(d)-t1isseriallyhomologouswitht14,whichextendsbetweenthe phragmata.Onthebasisof associatedmusculature,therefore,theseconclusionsappear tobevalid.. However,theevolutionofthemusclesandtheintegumenttowhichthe m u s c l e ~areattachedcouldhaveproceededindependently,bothinphylogenyand ontogeny,asMatsuda( 1965)discussedfully.'Therefore,thedorsalcervicalsclerite is notnecessarilythedetachedacrotergiteofthephragma,astheseauthorsbelieved.It appearsmoreprobablethatthedorsalcervicalscleriteispurelyasecondarysclerite produced fortheattachment of themusclecv(d)-t1.Thisideaof thesecondaryorigin ofthescleriteexplainsbetterthereasonwhythedorsalcervicalscleritevariessomuch bothinshapeandpositioninvariousinsects.Tobefullyacceptable,theideaofthe acrotergal origin of the dorsal cervical sclerite requires more evidence,such astransitional stagesshowinggradualdetachmentof thedorsalcervicalscleritefromthepronotumin phylogeny,andthisgradualdetachmentshouldbecomparablewiththegradualdetach-mentofthepreepisternum(lateralcervicalsclerite)inthephylogenyofsomeholo-metabolousinsects. The ventralcervicalscleriteismerelyasecondarysclerotization.It mayberepre-sentedbyoneormoretransversescleritesbetweenthelateralcervicalsclerites,and usually occurs inorders in which the dorsal cervicalscleriteispresent.In Hemipteroidea andin' mostholometabolousinsects,nowell-formedventralcervicalscleritehas been found. DuPorte( 1960,1962)proposed that the gula originated in the prognathoushead by incorporation of the ventralcervicalsclerites that had become united with thepostocciput andthesubgena.Insupportofhiscontention,DuPorteadducedtheconditionofthe neckregion in Anisolabis,in which theanteriorventralcervicalscleriteisdefinitively the gula that isunited with thesubmenturn,but isseparated fromthepostocciput.DuPorte believedthatthisconditionrepresentsatransitionalstagetowardthemoresolidified gularareainColeoptera.DuPorte(1962)admitted,however,thatincertainlarval Coleoptera,asSnodgrass( 1960)showed,thedefinitivegulaisapparentlyformedby the mesalgrowthofpostgenae.EssentiallyinagreementwithDuPorte,Akbar( 1965) admitted,inthedefinitivegulaof someColeoptera,thatthesclerotizedneckmembrane isincluded. Theories:Verhoef!(1902-1904),asacorollaryofhisboldassumptionthatthe intersegmentalareasof the thoracic segmentsrepresent independent segments,contended thattheneckregionalsorepresentsanindependentsegment,themicrothorax.This theory isentirely inconsistent with embryology, which hasconsistently shown no evidence that supportsthe presenceof asegmentbetween the headandtheprothorax.Sincethe theory hasbeenrepeatedlydiscredited overthe years,it isnolongernecessarytodiscuss it indetail. Crampton(1917a,1926a)wasrightinnegatingthepresenceofadistinctsegment inthisareaandalsoinrefuting the interpretationof Riley( 1904)thatthecervicalarea iswholly labial in origin.But Crampton went too far when he denied,in hisreinterpreta-tionofRiley'swork,thepossibleinclusionofthelabialsegmentintheneckregionof termites(Holmgren1909).ItappearsthattothemindofCramptontheneckregion wasaspecificareawhichdoesnotincludeeithertheprothoracicorthelabialelement. Essentially,therefore,Crampton'sconcept oftheintersegmentalarea fortheneckregion wasequaltothemicrothoraxofVerhoef!,althoughCramptondidnotcalltheareaa segment.Adefinitiveintersegmentalareashouldbelongtoasegmentthatiseither anteriororposteriortotheareainquestion,orshouldbelongtoboth,asCrampton ( 1914b)thought. Cramptonhomologizedhis(definitive)intersegmentalpleuralscleriteinthe posterior thoracicsegmentsof Eosentomonand Japyxwiththe lateralcervicalscleritein Capnia(Fig.2).TheinterpleuriteofCramptonwasthereforepartlybasedonthe Seealsop.85inthiswork. 10 MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA FIG.2.VentralregionoftheprothoraxofCapnia,themesothoraxofEosentomon,andthe metathoraxof Japyx;drawnasthoughspreadout inoneplane.Shadedareasrepresentinter-segmentalareasofCrampton(fromCrampton). contemplationofstructuresonEosentomonandperhapsinotherProtura,inwhich fragmentationofsclerites frequentlyoccurs(Fig.28). Nevertheless,Crampton'shomologizationofthelateralcervicalscleritewiththe interpleurite, which often can beadetached presternal sclerite(see p.24), wasessentially correct.Such ahomologizationisvalid,however,only forhemimetabolousinsectsand Coleoptera.Cramptonwas,therefore,notjustifiedinextendinghishomologizationto other orders. Snodgrass'( 1935)theoryontheoriginoftheneckregionstatesthatthisregion containsaninvisibleprimaryintersegmentallinebetweenthelabialandtheprothoracic segments.Itfurtherstatesthattheprincipaldorsallongitudinalmuscle(op-t3or cv( d) -t 1), which connects theback of the head and the antecosta of the mesonotum,and the ventrallongitudinalmusclebetweenthe posteriortentoriumandtheprofurca(s1or 2)are fusionproductsof themusclesof the labialand prothoracic segments,whichwere originallyattachedtothissupposedprimaryintersegmentalline(ortotheantecosta). Snodgrass'theory iscorrect ismaintainingacompositenatureoftheneckregion.The musclesin question,however,areseriallyhomologouswith the segmentalmusclesinthe pterothoraxandcannotbefusionproducts,althoughtheymayreceivedouble innervations. Someearlier workersincludingRiley(1904)thoughttheneckregionwaslabialin ongm.RecentlyWada(1966)cametoaconclusionsimilartoRiley's.Hisstudyof the resultsof artificially produced defectsand other teratologicalmaterialsin the embryos MATSUDA:THEINSECTTHORAX11 ofTachycinesindicatedthepresenceof morphogeneticalunits,whichWadacorrelated withsegmentalcompositionofthehead,includingtheneck.Hisfindingsledhimto conclude that the largest part of theneckwasof labialorigin,the dorsalsideoftheneck immediatelybehindthepostocciputwasmaxiiiary,andonlytheverynarrowhindmost stripwasprothoracicinorigin.Inthiskindofexperimentation,however,thedefect-producing operationsaremade at acertaintimeortimesduringthecourseof embryonic development.Aftertheoperations,theregulativeaspectsofdevelopmentwouldbe eliminatedoratleastdisturbed.Consequently,theresultsmayrefertoartificially produced mosaicsof embryonicparts,andisnot validfortheidentificationof asegment. THETERGUM Divisionofthetergum:InLepisma(Fig.37)atergumconsists,fromthefront backward,oftheantenotum,theprescutum,thescutum,andthepostnotum.Inthe Pterygotaatypicalpterothoracictergumconsistsoftheacrotergite,theprescutum,the scutum,thescutellum,andthepostnotum.Besidesthedivisionsofthescutumand scutellum,whichwillbediscussedlater,misinterpretationshavedevelopedconcerning thehomologiesoftheanteriorandposteriorpartsoftheterguminThysanuraandthe Pterygota.Theconfusionsarosefromthefactthatawell-formedmesothoracicpost-notumisabsentinsuchlowerpterygoteordersasBlattaria,Mantodea,andOrthoptera, andthatthepostnotumisoftenunrecognizableordifficulttorecognizeinimmature insects. ThefactthatthepostnotumisclearlypresentinEphemeropteraandPlecoptera, which exhibit somany primitivefeatures,however,supportstheideathat thepresenceof thepostnotumrepresentsaprimitivecondition.Thepostnotumintheseordersisprob-ablyhomologouswiththepostnotuminLepisma(Fig.37)andOncojapyx(Fig.32), which,accordingtoBarlet(1951)andBarletandCarpentier(1962),descendstomeet thedorsalposteriorpartofthepleuronasinpterygotes.Further,thepostnotumin Lepisma and Oncojapyxisdorsallyhiddenbeneath thetergallobeasinpterygotes. Inthemesothoraxof Blattaria andOrthoptera,towhichSnodgrass(1927)attached importanceinpostulatinghistheory(tobediscussedbelow),thereducedpostnotumis hiddenbeneaththeposteriorareaofthemesonotum(Figs.54B,70B),although Snodgrassfailedtorecognizeit.InsomehigherorderssuchasDipteraandHemiptera thepostnotumhasbeengreatlyenlarged.Evidently,therefore,theevolutionofthe mesothoracicpostnotumhasbeeneithertowardreduction,ortowardenlargement. When thepostnotumisreducedlaterallythepostalarbridgeisnotformed. Snodgrass(1909,1927,1935)believedthatthepostnotumisasecondarydevelop-ment thatoccurredwithacquisitionof thewing.He( 1927)attributeditsorigintothe prolongationof theacrotergitethathasdevelopedalongthea'ltecosta,orthetrueinter-segmentalridge(Fig.7 A).AccordingtoSnodgrass( 1927,1935),thepostnotum (Fig.7C ).consistsofthegreatlyenlargedacrotergiteandtheantecostabearingthe phragma; thepostnotum may containanarrowstrip,theposttergite,immediatelybehind theantecosta;thephragmaisasinglewalledstructurethatarisesfromtheantecosta, despitehisearlier( 1909)contentionthatthephragmaarisesfromtheposteriorendof thepostnotum(thepostphragma)andalsofromtheantecosta(theprephragma)ofthe succeeding segment.Weber(1924b)alsoemphasizedthesecondarynatureof thepost-notum,butthoughtthephragmawasprimarilydoublelayeredasSnodgrass(1909) thought.Weberattributedthedevelopmentofthephragmatomuscleaction. Itshouldbepointedout,however,thatinEphemeroptera,Plecoptera,andsome otherordersthesecondphragmaisdoublewalled,whichclearlyshowsthatboththe postnotumandtheantecostacanproducethephragma,andthisfactdecisivelydisproves thetheoryofSnodgrass(1927,1935)ontheoriginofthepostnotum.Clearly, Snodgrass'theory wasbasedonthepreconceptionthat thephragma arisesonlyfromthe antecosta;andthispreconceptionapparentlyforcedhimtostretchhisimaginationand postulatethatthepostnotumisanenlargedacrotergite.Inanycase,theformationof 12MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA PRESCUTOSCUTALSUTUPH RAGMAPRESCUTUMGITE ANTE COST PARAPSIDAL SUTURE ANTEROLATERAL SCUT ALSUTURE TRANSSCUTAL SUTURE TERGALFISSURE) AXILLARY CORD LATERALPARAPSIDAL SUTURE ARM r---ANTERIOR NOTAL WINGPROCESS AL POSTMEDIAN NOTALWING PROCESS ERIOR NOTALWING PROCESS SCUTOSCUTELLARSUTURE RECURRENTSCUTOSCUTELLARSUTURE FIG.3.Diagramof tergalsuturesandstructuresinthepterothorax(original). thedoublewalledphragma,whichfrequentlyoccursinpterygotes,isdifficulttoexplain in termsof thetheoryof Snodgrass.If thistheoryisfollowed,onemustassumethat thephragmainvaginatesfromtheantecosta,thenturnssharplyupwardtobecomethe postnotum(anterior wallof the phragma),whichisindeeddifficult toconceive. Theacrotergite,whichSnodgrassconsidered tobetheforerunnerof thepostnotum, ismerelyanarrowmarginalstripanteriortotheantecosta,andhenceishomologous with theantenotum inLepisma.The acrotergitecannot developintoalargepostnotum. Anintratergalphragma(pseudophragma)inDiplura,Thysanura,andOdonataisdis-cussedin latersections(pp.97,388). Tergalsuturesof thepterothorax(Fig.3):Asaconsequenceoftheacquisitionof wings,varioussutureshavedeveloped on the tergumof wing-bearingsegments.On the tergum,therefore,twokindsofsuturesoccur:thoseinheritedfromtheThysanura-like ancestor,and thosedevelopedafteracquisitionof wings. The anteriormostsuture inLepismaistheantecostathatdemarcatestheantenotum and gives rise to the endonotum internally.In the Pterygota theantecosta demarcates the acrotergite and internally givesrise to the phragma.Obviously,therefore,theacrotergite ishomologous withtheantenotum,and the endonotum ishomologouswith thephragma. Thehistologicaldifferencebetweentheendonotumandthephragmacanbecompared with the tentoriumof epidermaloriginin ThysanuraandthetentoriuminthePterygota, whichisproducedbytheectodermalinvagination. The prescutoscutalsuture in Lepisma(Fig. 37)isanearly straight transversesuture runningacrossthetergumnearitsanteriormarginandbehindtheantecostalsuture. The prescutum,demarcated posteriorlyby thissuture,isthereforearather narrowtrans-verseplate.InthePterygotathehomologoussutureistheanteriorventralnotalridge ofSnodgrass(1909)andtheprescutoscutalsutureofSnodgrass(1927,1935),which agreesinposition withtheprescutoscutalsutureinLepisma.Theprescutoscutalsuture ispresentinEphemeroptera,Plecoptera,Blattaria,Mantodea,Dermaptera,Orthoptera, MATSUDA:THE THORAX13 andsomeotherorders.Whenthissutureisretainedlaterally,itreachestheposterior limit of the base of the prealar arm(Fig.3). Typically,thescutoscutellarsutureinthePterygotaisananteromesallyproduced suturethatariseslaterallyfromtheareaanteriortotheaxillarycordoneachside.It occurscommonlyinholometabolousinsectsandlessfrequentlyinhemimetabolous insects.ThefactthatitisabsentinEphemeropteraandPlecoptera,asinThysanura, makesitdoubtfulthatthissuturehasbeeninheritedfromtheThysanura-likeancestor, despitetheoccurrenceofadoubtfulscutoscutellarsutureinthepseudofetusofDilta (Bin Fig.36A), whichisdiscussedon p.104. Severalothertergalsuturesdiscussedbelowhaveevidentlydevelopednewlyinthe pterothorax.Whiletheir functionalsignificanceisoftenunclear,someof thesesutures, especiallytheparapsidalandthelateralparapsidalsutures,recurfrequently,regardless of the phylogenetic relationshipsof thegroupsinwhichtheyoccur.Anextensivestudy of these newly acquired sutures would yielda wealth of data for the study of convergence. Theanterolateralscuta!suturecutstheanterolateralangleof thescutumthatbears the anterior and theantemedian notal wing processes;thisarea iscalledthe suralare.In Pterondrcys(Fig.46B),Stagmomantis(Fig.62),and inmanyothersthissutureiscon-tinuouswiththeprescutoscutalsuture.The sutureisstronglyridgedinLepidoptera;in themetathoraxof someHomopteraandHymenoptera,thecorrespondingareaismem-branousand the suralarebecomesanisolatedsclerite. Thepairedparapsidalsutures(Fig.3)ariseanteriorlyfromtheprescutoscutal suture.Oftenthesuturesonthetwosidesbecomeconfluentposteromesally.Many earlierworkers,includingSnodgrass(1909),Crampton(1914a),andsomemodern workers,wronglydesignatedtheareaboundedmediallybytheparapsidalsuturesasthe prescutuminHymenoptera,Diptera,Hemiptera,Coleoptera,etc.Inthemesothorax ofEphemeroptera,Hymenoptera,Bittacus,andDipteratheprescutalareaextends posteriorly alongtheanterolateralmarginof thegreatlyelongatedmesonotum.Inthese formstheparapsidalsutureshaveshiftedtheirpositionsconsiderablycaudad,butthey maintainthe connections withtheprescutoscutalsuture,asinotherorders. The lateral parapsidal sutures inPlecoptera(Figs.46B,47A)and other ordersarise fromthelateraloftheprescutoscutalsutureoneachside,andextendposteriorly towards the median longitudinalaxis,where they may become confluent.Weber( 1924b, 1933)mistookthesesuturesfortheparapsidalsutures,andcalledthemedianarea boundedbythemtheprescutuminerror.Thelateralparapsidalsuturealwaysstarts from the anterior limit of the lateral scuta!marginor fromtheposterior(or lateral)end oftheprescutumdelimitedbytheprescutoscutalsuture.Itisimportanttopointout thattheanteriorendof theanterolateralscuta!suturealsousuallyextendstothepoint wherethelateralparapsidalsuturestarts(Fig.3).Therefore,thethreesutures(pre-scutoscutalsuture,lateralparapsidalsuture,andanterolateralscuta!suture)typically meetatonepointalongtheanteriorlateralmarginof thetergum;thisfactprovidesan importantlandmarkindeterminingtheborderbetweentheprescutumandthescutum, which isusefuleven whenone or twoof thethreesuturesareabsent. Thetransscutalsutureconnectsthescuta!lateralmarginimmediatelybehindthe anterior(or antemedian)notalwingprocessoneach side.Thetergalfissureoccursin this area in many lower formsof many orders.Most probably, therefore,thetransscutal suture wasproduced by sclerotizationand ridge formationalongthetergalfissure;thisis obvious,forinstance,inHymenoptera.Insawfliesthemembranoustergalfissureis clearly present,but in many higher Hymenoptera thisarea isrepresentedbyaninternally ridgedsuturetraversingtheentirescutumofthemesothorax.Awell-developedtrans-scuta! suture isalso present in many Heteroptera, Diptera, Hymenoptera, andsporadically insome other orders. Theposterolateralscutalsutureisamarginalsuturedemarcatingtheposterolateral angle of the scutum that bearsthe posterior notalwing process.It arisesfromthescutal lateralmarginanteriortotheposteriornotalwingprocess,andrunsobliquelymedic-caudad.Thedegreeofdevelopmentofthissuturevariesconsiderablyindifferent 14MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA insects;insomeitextendsasfarasthescutoscutellarsuture,andinothersitextends evenbeyond(e.g.,Homoptera). Therecurrentscutoscutellarsuturearisesfromornearthemiddleofthehind marginof thescutellum,thendivergesanteriorlyonbothsidesinvaryinglengths.This sutureoccursinEphemeroptera,Plecoptera,Orthoptera,andothers;itliesproximalto theposterolateralscuta!suture. Modificationsof thetergalsurface:Themostmarkedandfrequentmodificationof thetergumistheprolongationof themesothoracic scutellum.Theprolongedscutellum either fitsinspecial conformations of themetanotum,or simply liesabove themetanotum ,andprojectsbeyond.Such asecondaryprolongationof themesothoracicscutellumhas occurredindependentlyatleastinDermaptera,Coleoptera,Diptera,Hymenoptera, Hemiptera(Homoptera,Heteroptera),andperhapsinmanyotherpterygoteorders. Foraccommodationoftheforewing,modificationofthescuta!areaofthemesonotum hasoccurredincertainorders,suchasHemiptera,Coleoptera,andDermaptera.In certaincases,thegrooveformedonthemesonotumforholdingthewingsmaybe associatedwiththeposterolateralscuta!suture.Thelossoftergalsuturesoccursin short-wingedand winglessforms. Thelateralmarginofthepterothoracictergum:Withtheacquisitionofwings, variousarticulatoryprocesseshavedevelopedonthelateralmarginofthepterothoracic tergum.The most anterior of theseistheprealar arm,which isoftenalateralprolonga-tionof theprescutumandextendsbehindthetegula;itisoftencalledtheprescutalarm. Thepresentworkhasshown,however,thattheprealararmcanalsobeacrotergalin origininBlattaria,Mantodea,andsomeotherorders,asLaGreca(1947b)pointedout inOrthoptera.Theprealararmisoftenunitedwiththedorsaloranterodorsalpartof theanepisternum,andtogethertheyformaprealarbridge.Theprealarbridgeis usuallyabsentinthemetathorax,althoughitispresentinsomeNeuropteraandperhaps insomeother lowerorders.InMantodeatheprealararminvaginatesintothethoracic cavity,andinsomeOrthopteraitisdifferentiatedintoanarticularlobe.Inmost Mecoptera theprealararmisabsent,but inDipteraaharm,comparableinpositionand shape withtheprealararm,isformed.InHymenopteraalso,theprealararmisabsent. LaGreca(1947b)andhisstudentscarefullystudiedthescuta!lateralmargin.In severalorderstheydiscoveredfivepointsofarticulationwiththeaxillarysclerites (Fig.3),insteadoftwo(Snodgrass1935),orthree(Weber1933).Thesepointsof articulationareprojectionswithvaryingdegreesofdevelopment.Theanteriormostis theprocessonotaleanteriore(anteriornotalprocess),andinsomecases,theprocesso notalemediaisdifferentiatedimmediatelybehind.Thesetwoprocessestogethercor-respondtotheanteriornotalwingprocessof Snodgrass.Theprocessonotalemediais renamedtheantemediannotalprocess,because of itspositionrelativetothreeprocesses that follow. Themembranoustergalfissureextendsproximally,separatingtheantemediannotal wingprocessfromthemediannotalprocesswhichLaGrecacalledprocessob(hintere TergalhebelofWeber1924b,1933).Anothersmallprocess,vofLaGreca,lies posteriortohisb,whichinthisstudyiscalledthepostmediannotalprocess.The hindmostprocessisthelong-recognizedposteriornotalwingprocess;theposteriornotal processisoftendetachedfromthescutumitself,andwhenthisissoit. isusuallycalled thefourthaxillarysclerite.Typically,thefirstfourprocessesarticulatewiththefirst axillarysclerite,andthelastwiththethirdaxillarysclerite. Of thesenotalwingprocesses,themedianandthepostmedianprocessesoftenare pronouncedinthehigherorders.InOrthoptera,forexample,theantemediannotal processiswelldifferentiatedandliesclearlyseparatedfromtheanteriornotalprocess, whereasinother lower hemimetabolousinsectsitisoftendifficulttodistinguishfromthe anteriornotalprocess.Themedianandpostmedianprocessesareconspicuously developed only ina few orders such asLepidoptera,Diptera,and Hymenoptera;and they areinconspicuousorunrecognizableinBlattaria,Plecoptera,andMantodea.Allof thesefactssuggeststhatprimitivelyonlytheanteriorandposteriornotalprocesseswere MATSUDA:TiiEINSECTTiiORAX 15 c An. A PROXIM.MED.PLATEJUGALFOLD FIG.4.Diagramshowingaprimitivewingbase(adaptedfromSnodgrass). presentandwerearticulatedwiththefirstandthethirdaxillarysclerites,andthatthe other processesdifferentiated later. Thetegulaand theprealar sclerite:Thetegulaisusuallya scalelike,setigerouslobe overlapping the baseof the wing.It hasthereforeintimateassociationwith thehumeral plate.InPlecopteraandDermapterathetegulaoccursbothinthemesothoraxand metathorax;inotherordersitispresentonlyinthemesothorax,andinColeopteraitis absenteveninthemesothorax.InHymenopteraandLepidopterathetegulaisgreatly enlargedandprotectsthebaseoftheforewing;itissupportedbythesubtegula.In Psocoptera the tegula isabsent. Thesmallprealar scleriteoften occurs,inassociationwiththetegula.Thissclerite ispresumablyadetachedprescutalsclerite.Usuallymusclet-p4isattachedtothis sclerite.ThesubtegulainTrichopteraandLepidopteraishomologouswiththeprealar scleriteinotherorders. Theaxillarysclerites:InOdonatatheaxillaryscleritesareunique,exceptforthe firstaxillarysclerite,whichcanbehomologizedwiththatoftheotherpterygotes.The secondandthirdaxillaryscleritesinotherordersarenotseparatelydifferentiatedin Odonata,andthetwoscleritesarerepresentedbyasingle,posteriorarticulatoryplate called theaxillaryplate.However,inanother order,Ephemeroptera,in whichalsothe wingsarenot flexedatrest,allthreeaxillaryscleritesaredifferentiatedandmaintainthe sametopographicrelationshipsamong themselvesasthey doinmany otherorders.An uoiquefeatureinEphemeroptera,however,isthatthemedianplateisnotdividedinto proximalanddistalpartsbythebasalfold. Inlower neopterousorderseachaxillaryscleritehasmoreor lessfixedtopographic relationshipswithotheraxillarysclerites,thescuta!lateralmargin,andwiththewing veins(Fig. 4). Thefirstaxillaryscleritearticulateswiththeanteriornotalwingprocessandwith the scuta!lateralmargin.Oftenthemedianandthepostmedianprocessesalsoprovide 16 MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA pointsofconnectionwiththefirstaxillarysclerite.Laterally,thisscleriteisassociated with the proximalmargin of the second axillary sclerite,and connected withtheproximal endoftheScvein.Itsposteriormarginisoftenconnectedwiththeproximalprocess of thethirdaxillaryscleritebyatendinousligament. Besidesitsassociationwith the firstaxillary sclerite,atypicalsecondaxillarysclerite islaterally joined to theproximalmedianplateandby itsanteriormargintothebaseof theRvein;posteriorly,it articulateswiththethirdaxillarysclerite.The secondaxillary scleriteissclerotizedon theventralsurfaceofthewingbasemembrane,andrestsonor isconnectedby aligament tothepleuralwing process:itisthereforethepivotalsclerite. Theventralsecondaxillaryscleriteisoftenassociated,eitherbyfusionorconnection, with the thirdaxillary scleriteand with the subalare. Thethirdaxillaryscleritehastypicallythreeprocessesorprojections:anterior, proximal,anddistal.Byitsproximalprojectionitarticulateswiththeposteriornotal wingprocess,anditsproximalmarginhasaligamentousconnectionwiththeposterior marginofthefirstaxillarysclerite.ItsdistalendconnectswiththeAveins. Theproximalmedianplateliesdistaltothesecondaxillarysclerite.Typically,its distalendisseparatedbythebasalfoldfromthedistalmedianplate.Thebasalfold formsaprominent convexfoldduring flexionof the wings. Judging from their stable connectionswith particular wingveinsinlower pterygotes, thefirst,second,andthirdaxillaryscleritescanbesafelyregardedasmodifiedand detached basalparts of the wingveins:the firstaxillary scleriteisthebaseof theScvein; the secondisthat of theRvein;and thethirdisthat of theAveins.Thesmallhumeral plate ontheproximalanteriormarginofthewingrepresentsthedetachedbaseoftheC vein;inOdonatathisplateislarge'.InPlecoptera(Fig.47A)andEphemeroptera (Fig.44A)thedistalmedianplateisnotdifferentiated,or poorlydifferentiated,andthe associationof theproximalmedianplatewiththeMandCuveinsisdefiniteanddirect. The proximalmedianplateiscomparable withtheaxillary sclerites,becauseitrepresents thebasalpartoftheM-Cuveins. The formationofthedistalmedianplateisthereforeclearlysecondary,althoughit iscommon in winged insects.The development of thedistalmedianplate asameansof resistingelevationofthefoldduringflexionofthewingmusthaveoccurredsimul-taneouslywiththeformationofthebasalfold;thisfactaccountsfortheabsenceofthe distalmedianplateinmayflies: whichneverfoldtheirwings.InBlattaria(Fig.55 A) other veinsalsohaveadditional detached bases. AmongtheNeopterathemostprimitiverelationshipbetweenaxillaryscleritesand wingbasesisseeninPlecoptera(Fig.47A),whichwasstudiedbyOnesto(1965).To reconstructaprobableprimitive wingbase(Fig.4),therefore,thewingbasesinPlecop-teraandEphemeropterawereseriouslytakenintoaccount.Thediagram(Fig.4)isa modificationofadiagramgivenbySnodgrass(19 3 5) .Thedistalmedianplateisnot/ shown,andthetendinousconnectionextendsfromthethirdtothe firstaxillarysclerites in thisdiagram. AsSharplin( 1963c)showedinLepidoptera,thesclerotizedelementsatthewing baseareunusually flexible.The fuchsinophilmesocuticleatthewingbaseisnotacon-tinuouslayer,butisdividedintoconesandpegs(Fig.5).Thisdiscontinuousmeso-cuticle is subject to distortions,projecting down throughtheendocuticle.The continuous exocuticularlayerandsubdividedmesocuticleconstitutethebendingcuticle.Sharplin distinguishedtwotypesofthebendingcuticle:themesocuticleprojectingintothe endocuticleasaseriesofirregularpegsvaryinginlengthanddiameter;andthelarge, moreregularlyspacedconesofthemesocuticlereachingnearlytotheinnermarginof the endocuticle. Arubberlikeprotein(resilin)thatconstitutestheligamentsconnectingstructuresat the wing base wasrecognizedby Weis-Fogh( 1960).Theseligamentsevidentlyplayan importantroleintheflightmechanism,andhavebeenobservedfromtimetotimeby insect morphologistssinceVoss(1905).LaGreca(1947b),Sharplin(1963a,b),and HomologiesofthewingbasestructuresinOdonatawiththoseinapaleodictypterousspecies,Ostravanigra, arediscussedonp.43. NORMAL EXOCUTICLE . .: .. B MATSUDA;THEINSECTTHOKAX BENDING EXOCUTICLE ENDOCUTICLE 17 FIG.5.A,diagrammaticverticalsectionsofcuticle(fromSharplin);B,diagramofthe actionofbendingcuticle(fromSharplin). Hatch( 1966),amongmorerecentworkers,observedtheligamentousconnections. LaGreca( 1961)madeanelectron-microscopicstudyofthewingbaseinLeucophaea maderae. THESTERNUM Theendosternum,thesternalfurca,andthespina:IntheApterygotatheendo-sternalskeletonsorprocessestakevariousshes,suchasplate,stalk,bridle,etc.A seriesofstudiesbyCarpentierandBarletduringthepasttwodecadeshaveelucidated theseendosternalstructuresinDiplura,Collembola,andThysanurainsufficientdetail; thesestudiesmadethefollowinganalysispossible. Endosternalprocessesareoften represented by muscles.For instance,thepostcoxal endosternalbridgesdandjaremusclesinCampodea(Fig.33)andTetrodontophora (Fig.31A),kinCampodea(Fig.33)andTetrodontophora(Fig.31A),andninthe mesothoraxofTetrodontophora(Fig.31A)arerepresentedbymuscles.Thesefacts parallelthe casesof some headmusclesthat arehomologouswithendoskeletalstructures inThysanura(Matsuda1965). AsCarpentier(1946b,1949),Barlet(1951,1952a,b),CarpentierandBarlet (1951),andBarletandCarpentier(1962)attemptedtoshow,therelativepositionsof endosternalprocessesarefairlyconstantthroughouttheApterygota.Therearesome inconsistencies,however.Forinstance,eisafurcasternalstructureinCtenolepisma (Fig.38A),but it isattachedtotheanteriormarginof thespinasternuminTetrodonto-phora(Fig.31A)duetoreductionofthesternum(inthelatter);inCtenolepisma (Fig.38A)darisesfrombandinTetrodontophora(Fig.31A)itarisesfromg;in Lepisma(Fig.37)thereare twicethenumberof hin the mesothorax; in Thysanura flies ontheanterolateralangleofthefurcasternumandmeetsthedescendingkatapleuron, althoughit liesonthekatapleuroninCollembola.Itisthereforenotsurprisingtofind someinconsistenciesin,forinstance,Carpentier'swork(1949)regardingthehomology ofthepoststernumbasedontheattachmentofprocessesinTetrodontophora(p.92). Aswillbe pointed out again later,overreliance on theendosternalprocessesaslandmarks mustbeavoided.However,thepositionsofcertainprocessesappeartobeusefulin determiningassociatedexternalpartsofthesternum;thus,pisalwaysattachedtothe dorsalmarginofthekatapleuron;aisalwaysattachedtothespinasternumandlis attached to theanteriormarginof the succeedingsegment. 18MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA ThebaseofthesternalfurcainthePterygotaisprobablyhomologouswiththe endosternalprocessfintheApterygota,sinceinthePterygotatliesternalfurcaarises fromanareathatcorrespondstothepositionoff intheApterygota.Thesternalfurcal baseusually hasanexternalpit,andmustthereforearisefromectodermalinvagination. The evolution of theendosternal fand the furca isthuscomparable with the evolutionof thetentorium;theepidermaltentoriumintheApterygotahasbeenreplacedbythe tentoriumproducedby theectodermalinvaginationinthePterygota(Matsuda1965). InNicoletia(Fig.39B)andCtenolepisma(Fig.38A)aisattachedtotheposterior marginof thespinasternum(the poststerniteofCarpentier1946bandofBarlet1952a), andlisattachedtotheanteriorpartofthepresternumofthesucceedingsegmentin Ctenolepisma,andtothecorrespondingarea(theborderbetweenpresternum1and presternum2ofBar!et1952a)inNicoletia(Fig.40).InOncojapyx(Fig.34)alsoa andlarisefromthepositionscorrespondingtothosein NicoletiaandCtenolepisma,viz. afromtheborderbetweenthespinasternumandthepresternum(theintersternumof BarletandCarpentier,1962),andlfromtheborderbetweenthepoststernumandthe presternum(the presternum 2of Denis,1949)of the succeeding segment.In Campodea (Fig.33)too,aand loccur inthe corresponding positions. Inregardtothespina(mofCarpentierandBarlet)intheApterygota,Carpentier (1946b)foundinCteno/episrna(Fig.38A)that it isnot formedexclusivelyfroma,but thatlalsocontributes.InLepisma(Fig.37)thespina(m)isapparentlyassociated with aand /.InCarnpodea(Fig.3 3)the spina( m)issmallerand itslocationissimilar tothatinThysanura.InJapygidaeBarletandCarpentier( 1962)foundthatthe strongly developed cuticular spina arisesanteriorly fromthespinasternum; Barlet(1965) further showed that inOncojapyx the spina isalsoconnected withl. ThesefactsshowthatthelocationofthespinainJapygidae,atleastitsanterior limit,isdifferentfromthatinThysanuraandCampodea.Thisdifference,however,has probably resulted from the fact that inJ apygidae the spina issostronglyinvaginatedthat evenitscuticularpartandthemoreanteriorpartofthesternum(thespinasternum) havebeenaffected.Inthisrespect,thespinaismorespecializedinJ apygidaethanin ThysanuraandCampodea. InthelowerPterygotaalso,thespinalinvaginationinvolvestheexternalcuticule, andinitslocationthespinainthelowerPterygotaismorecomparablewiththatof Japygidaethan withthat of Thysanura.Besidesthe furcalbaseandthespina,thedistal part of thefurcainthePterygotaisassociatedwiththepleuralarm;andthisdistalpart of the furca isprobably homologous with ginApterygota,since p' alwaysconnects,either asamuscle'orasanendoskeletalbridge,gandthedorsalmarginofthekatapleuron, thesiteof thepleuralarminmostPterygota.InOdonatatheprefurcaistheinvagina-tionfromthebasisternallateralmargin,andisprobablyhomologouswithhinthe Apterygota.OtherpartsoftheendosternumintheApterygotahavebeenlostinthe Pterygota. DivisionofthesternumintheApterygota:Inageneralizedthysanurousgenus Nicoletia(Fig.40)andinOncojapyx(Fig.34),eachthoracicsegmentconsistsoffive subdivisionsor apotomes.They are,from the front backwards,the presternum,the basi-sternum,thefurcasternum,thespinasternum,andthepoststernum.Althoughthereis no problem withthefouranteriorapotomesasintegralpartsof the sternum,thequestion hasbeenraisedwhether the part herecalledthe poststernumrepresentsthefrontmostor thehindmostpartof asegment.If the locationof thespinalpit,whichcorrespondsto ainJapygidae,istakenasthe intersegmentalboundary,thisapotomemustbelongtothe succeeding segment.This is,infact,the opinion of Denis( 1949,1963), whocalledthe apotomethepresternumI.However,theconceptof theintersegmentallocationof the spinabyearlierworkersincludingSnodgrass(1935),whichDenisaccepted,wasbased ontheobservationofpterygotes,inwhichthisapotomeandtheassociatedinternal processlhavebeenlost.It ishighlyprobable,therefore,thatthepoststernuminthe Apterygotarepresentsthehindmostpartofasegmentinamoreprimitivesternum. 15pisprobablyhomologouswithmusclep-s1,whichconnectsthefurcalarmandthepleuralarminthe Pterygota. MATSUDA:TilEINSECTTIIORAX19 Barlet and Carpentier's( 1962)proposalof the term the "intersternum"forthisplateand thefor itsdorsal counterpart has been unfortunate, sincesuch terminologies areconfusing(seep.94andp.99). In Oncojapyx(Fig.34)the Yridgeor lineispresentonthemiddleof the sternum. Its lateral end articulates with thecoxa near the endosternal tendon f,and posteriorlyit is continuouswiththespina.InthemesothoraxofCampodea(Fig.33)alsothelateral endoftheYridgereachesthebaseoff.IntheprothoraxofCampodeatheYline apparentlybecomesaT-shaped linewhoselateralendsconnecttheinnercoxalmargins. IntheprothoraxofEosentomon(Fig.29)theanteriortransverselineliesinthearea correspondingtothehorizontallineinCampodeaandthetwolinesareprobably homologous.InthemetathoraxofEosentomon(Fig.29)boththeYlineandthe horizontal line connecting the anterior end of the Yare present.Among Thysanura only theY lineispresent inthemesothoraxof Ctenolepisma(Fig.38A),andtheY lineand the horizontal lineareapparently present inPetrobius(Fig.38B); inNicoletia(Fig.40) onlythehorizontallineispresent.Thehorizontallineisassociatedlaterallywithfin Nicoletia(Fig.40)andprobablyalsoinPetrobius(Fig.38B).Thishorizontallineis thereforethesternacostaofWeber( 1933),whichisassociatedwiththefurcalbase. The sternacostamayor may notformacondyle,whicharticulateswiththecoxa inthe Apterygota. Thesefactssuggestthat boththeYlineandtheanteriorline(thesternacosta)are primary partsofthesternum,andthattheabsenceofeitherof themisdueprobablyto loss.BarletandCarpentier(1962)havesuggestedthatthetriangularspacebetween thebranchesoftheYlineinDipluraisbasisternal,inthebeliefthattheundulation formedby branches of the Ycorrespondstothe sternacosta.Acloseexaminationofthe factsmentionedabove,however,showsthatthetwostructuresmayhaveoriginated independently.It thereforeremainsarbitrarytoregardeithertheYlineorthesterna-costaasdemarcatingthefurcasternumfromthebasisternum.Inthepresentwork Weber'sdefinitionofthesternacostaisacceptedanditisusedtomarkthelineof boundarybetweenthefurcasternumandthebasisternum.Snodgrass( 1935)alsoused the horizontalstern acostaasthelinebetweenthe furcasternumandthebasisternum. Divisionof thesternuminthePterygota:InthePterygotatheoriginaldivisionof thesternumhasbecomeobscuredbecauseoffusion,loss,andmodificationofparts. Thepresternuminpterygoteinsectsisdifficulttorecognize,duetolossoftheline separatingitfromthebasisternum.Thetruepresternumhomologouswiththeone intheApterygotais,however,presentatleastinsomePlecoptera.InPerla(Fig.48) the median presternal area iscontinuous withthebasisternum; the lateralisolatedplateor plateslyinganteriortothepleuron(interpleurites)probablyrepresentthetrueisolated presternum.This interpretation issupported by the fact that inThysanura(Figs.35,37, 38,40)andDiplura(Fig.34)thepresternumextendslaterallyanteriortothepleuron. Crampton(1909)alsorecognizedthepresternuminNemouraandPteronarcys. Althoughthelinedemarcatingthepresternumfromthebasisternumappearstobe presentinNico/etia(Fig.40)andPetrobius(Fig.38B),thislinehasbecomelostin Ctenolepisma(Fig.38A)andLepisma(Fig.37).Itmaybesaid,therefore,thatthe suture between thepresternum andthebasisternumwaslostearly intheevolutionof the Thysanura-Pterygota complex. According to Snodgrass(1935),themarginalnarrowpresternumisseparatedfrom thebasisternumbytheinternallyreinforcedsutureinsomeinsects.However,thepre-sternalridgeor sutureof Snodgrassoccursinhigherinsects,suchasCalliphora,andis absentinlowerinsects.Therefore,thissutureisnottheprimaryborderbetweenthe presternumandthebasisternum.Mostprobablytheridgeisasecondarilyproduced reinforcing ridgeandtheseparatedareaistheprepectus(orhypopteron),whichoccurs alongtheanteriormarginofthepleuronandthesternuminhigherinsects.Weber's ( 1924b)assertion that the presternum inpterygotesisa plate secondarilyseparatedfrom thebasisternum isalsounfounded. 20MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA Theseparationofthebasisternumfromthefurcasternumisusuallyobscurely definedin the Pterygota.In somepterygotes,however,thesternacostaandtheYridge arepresentandaremostprobablyhomologouswiththosepresentintheApterygota. AsWeber( 1933)showed,inPlecopteratheYridgeisconnectedanteriorlybythe horizontalsternacosta,whichextendstothefurcalbaseoneachside;andtheYreaches posteriorlyasfarasthespina.Grandi(1948)alsoshowedtheY ridgeinthelarvaof Perla.Among other pterygotes,the sternacosta andthe Yridgearefoundalsoin larval Sialis(Weber1928a).Yet,inmany pterygotesonlythehorizontallineconnectingthe furcalbasesispresent,andthislineprobablyrepresentsthesternacosta,whichdelimits the furcasternum fromthe basisternum.In mostadult holometabolousinsects,in which thefurcalbasesareapproximatedtowardsthemedianlongitudinalaxis,thesternacosta islost. Delamare-Deboutteville( 1952)definedthesternellumintheBlattopteroideaasthe medianelongateplatethatextendsintothebasisternalareaandbearsthefurcaonits posterolateralangles.IfDelamare-Debouttevilleusedthetermsternellumforthe furcasternumasitisgenerallyunderstood,thisdefinitionofthesternellum( furca-sternum)isunacceptable,since thearea in question liesanteriortothefurcalbase.The sternellumof Delamare-DebouttevilleispurelyaspecializationinBlattopteroideaandit cannot be homologized with the Yridge,which lies posterior tothe furcalbase.Laterally the basisternummeets,or isunitedwith,thepreepisternum inlowerpterygotes,asinthe Apterygota. Crampton( 1909)showedpairedplatesbehindthefurcasternum( furcisternumof Crampton)andinfrontofthefirstspinainNemoura,andcalledthemthepostfurcal sclerites.Weber(1924b)thoughtthatthepostfurcasternite(postfurcalscleriteof Crampton)behindthefurcasternuminlowerPtergotaarosefromthemembranous regionbetweentwoneighboringsternalareas,perhapsasaresultofthemuscleattach-ment( t-s2) ,and that therelationshipbetweenthepostfurcalsterniteandthespinathat followsisanalogoustotherelationshipbetweenthepostnotumandthephragma.He further thought that the spina isonly adetached part of thepostfurcasternum;Snodgrass (1927)acceptedtheinterpretationofWeber.ForWeberandSnodgrassthepost-furcasternumwasasecondarydevelopmentjustas,intheiropinion,thepostnotumwas secondary. Thelocationofthepostfurcasternumanditsassociationwiththespinaexactly reflecttherelationbetweenthespinasternumandthespinaintheJapygidae.It seems highlyprobable,then,that thepostfurcasternuminsomeinsectsisinfacttheremnant of thespinasternum,andnotasecondarily formedarea.Inmanyhigherpterygotathe postfurcasternumorthespinasternumhasbecomegreatlyreducedorvirtuallylost. Increasedassociationofthespinawiththespinasternum,inturn,musthavebeena / contributingfactorinthereductionofthissternalpart.Atthesametime,thespina has lost itsconnection with the endosternalprocessI,whicharisesfromthepoststermim; consequently,thepoststernumhasbecomefunctionallymeaninglessandaltogetherdis-appearedalongwithI.Asaresultofthelossofthepoststernum,further,thespina hascometooccupythedefinitiveintersegmentalpositioninthePterygota.Itis possible,however,that thispoststernalplatehasbecomeindistinguishablyfusedwiththe presternumofthesucceedingsegmentincertainprimitiveinsects,suchasPerla (Fig.48). Inhigherinsectsthereductionoftheposteriorpartsofthesternumhasinvolved generally the spinasternumand the furcasternum.In themesothoraxof Orthoptera,for instance,the spina hascome to occupy a position betweenfurcalbases,asaresultof loss of theseposterior sternalareas. Inmanyhigherorders,suchasHomoptera,Diptera,Hymenoptera,Coleoptera, Odonata,andothers,eventhemesothoracicspinahasbeenlost,andthemesothoracic furcasternum has greatly reduced; consequently,the anterior part of themetasternum has cometooccupytheareaimmediatelybehindthemesothoracicfurcalbases.Inthese MATSUDA:THEINSECTTHORAX21 forms,therefore,theventralborderbetweenthemesothoraxandmetathoraxhasbecome obliterated.Thisincreasedobliterationof theventralsegmentalborder isin turndirectlyresponsiblefortheformationof thesynthoraxasafunctionalunit,which hasoccurred in varying degreesinvarious higherorders. The ideaof theinvaginated sternum(cryptosternum)in holometabolousinsectswas firstenunciatedbyWeber( 1928a),whoshowedaprobablesequenceofthemedian longitudinal invagination of the sternum in holometabolousinsects.Weber,starting with thesternuminPlecoptera,whichlackssuchaninvagination,wasabletoshowthatthe furcaealsohavemigratedmesally,accompanyingthemedianlongitudinalinvagination. AsWeber showed,thefurcaelieontopof theposteriorendoftheinvaginatedsternum inNeuroptera,Hymenoptera,andLepidoptera.Kelsey's( 1957)study,whichtraced thedevelopmentalchangesofthoracicpartsinCorydalus,amplysupportedtheconcept ofthecryptosternum,ortheinvaginatedsternum. Daly's( 1964)study on the postembryonicdevelopmentof thethoraxinA pis,how-ever,showedthatthemedianlongitudinalridge(lamella)inthebasisternalareais producedbyamigrationorproliferationofcellsfromalowmidventralridge;andthis suggestedthatthebasisternalmedianlongitudinalelevationisnotformedby _invagina-tion.Furthermore,thereareholometabolousinsectsinwhichthepleurosternalsuture actuallyexists,aspointedoutbyMatsuda( 1960b)forGnophomyia(Diptera)and Phassus(Lepidoptera).Intheseformsthemedianlongitudinalsutureispresent,in additiontothepleurosternalsuture,andthelegbasesareunusuallywidelyseparated. Thisfactindicatesclearlythatthemedianlongitudinalsuturecanbeformedin-dependentlyof thepleurosternalsuture,andhencethesuturedoesnotrepresenttheline of unionof thepresternaasFerrismaintained.Theseapparentdiscrepanciesregarding theoriginof themedianlongitudinalsutureorthecryptosternum,however,canstillbe accounted for,asdiscussedbelow. InSialis(Weber1928b),Corydalus(Kelsey1957),Apis(Daly1964),and Cicadidae(p.258),inwhichthedevelopmentof thesternumhasbeentraced,legbases areinvariablyapproximatedtowardsthemedianlongitudinalaxisintheadult,thereby contributingtowardformation,oratleasttdwardtheelevation,oftheposteriorpart of themedian longitudinalridge.Thismechanicalpressure,however,probably doesnot extend toofar anteriorly into the basisternalarea of the greatly enlargedmesothorax such astheonepresentinApis,whichDalystudied.Hence,thekindofdevelopmental mechanismfortheproductionof themedianlongitudinalridgeonthebasisternumDaly (1964)foundmayoftenbeatwork.Withthepresumedinvaginationof thesternum, thepleurosternal border(suture)hasoften become obliterated in holometabolousinsects, aswillbediscussed later(p.37). THEINTERSEGMENTALRELATIONSHIP Inconsideringtherelationshipamongthoracicsegments,itshouldberemembered that:ventrally,boththeanteriorandtheposteriorendsofasegmenthaveundergone reductioninvaryingdegreesinvariousgroupsofthepterygota;anddorsally,boththe hindmarginofthepostnotumandtheantecostaproducethephragma,anqbothare primary partsof athoracictergum. InFig.6,ArepresentstheconditioninThysanurainwhichnoreductionofparts hasoccurred.Theinconspicuousendonotum,with whichthephragmainotherinsects ishomologous,arisesfromtheantecosta.Ventrally,theendosternalprocessesaandI arepresent." In A,B,C,andD,thefirstphragmaalwaysarisesfromtheantecostaof themeso-notum.Intheabsenceoftheprothoracicpostnotum,thisistheonlypointwherethe firstphragmacanbeformed.Ventrally,thepresternumhasbecomereducedor lostin B,C,and D. InB,thesecondphragmaisformedbytheunionof invaginationsfromboththe hindmarginof thepostnotumandfromthemetathoracicantecosta.Thisconditionis 22MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA A B PRC.SC.+ SCL. PN.AN. PHR.l-PHR.2 (EN.2) (EN.!) SP.1 PHR.1 SP.1 POSTS. PREST.BS.FS. PRC.sc.SCL.PN.ACRT. J FS.SPS. SCL. ____::.:::..._ ____2 '1 SP.2 FS.SPS. SCL.PN. ____ Z FS.

FIG.6.Diagramshowingevolutionofintersegmentalrelationshipsininsects(original). ACRT,acrotergite;AN,antenotum;BS,basisternum;EN,endonotum;FS,furcasternum;PHR, phragma;PN,postnotum;POSTS,poststernum;PRC,prescutum;PREST,presternum;SC, scutum;SCL,scutellum;SP,spina;SPS,spinasternum;aand/,endosternalprocesses.See textforexplanationofA,B,C,D. seeninEphemeroptera,Plecoptera,andothers.Ventrally,thesecondspinaarises whollyfromtheectodermofthespinasternum.InC,thesecondphragmaarisesfrom themetanotalantecosta,themesothoracicpostnotumhavingbecomegreatlyreducedor lost.ThisconditionexistsinBlattaria,lsoptera,Mantodea,andothers.Thedorsal longitudinalmuscleextendsbetweenthephragmatathatarisefromtheantecostaofthe twoneighboringsegments.Inthisrespect,Blattaria,Isoptera,andMantodearesemble Thysanura,but they differfromThysanurainthegreatreductionofthepostnotum.In D,thesecondphragmaisformedonlyfromt!Iepostnotum.Thisconditionexistsin higherHymenoptera,Diptera,Hemiptera,andothers,inwhichforewingsaretheprin-cipalflightorgans,andthepostnotumiswelldevelopedinthemesothorax.Herethe metathoracicdorsallongitudinalmuscleisattachedanteriorlytothemesothoracic phragma;hence,themuscleextendsbeyondtheoriginalsegmentalborder.Thesingle-walled second phragma in Neuroptera and Mecoptera may alsobe the extensionfromthe mesothoracic postnotum. In theorderswherethehindwingsaretheprincipalflightorgans,suchasOrthop-tera,Coleoptera,and Dermaptera,themesopostnotumisgreatlyreducedor lost,and the MATSUDA;THEINSECTTHORAX \ \ \: '' ': "l = INTERSEGMENTAL IFOLO I 7 A SECONDARY, ---;- 1 - 'I I: I MEMBRANE SECONDARY/ SEGMENT/ PHRAGMAHRAGMA B c 23 FIG.7.Snodgrass'diagramsshowingintersegmentalrelationships.Seetextforexplanation of A,B,C. second phragma isformedonlyfromthemetathoracicantecosta,andthethirdphragma isformedfrom themetathoracic postnotum.InTrichopteraandLepidoptera themeta-postnatalthird phragma isdetached fromtherestof themetanotumand isfirmlyunited withtheanteriormarginofthefirstabdominaltergite. Ventrally,theprimaryintersegmentalrelationshiphasoftenbeencompletely obliterated,owingtolossofthespina,arrdtothereductionorlossoftheanteriorand posterior partsof thesternum.In BandC,thereductioninvolvesonlythepresternum. In D,however,even themesothoracicspinahasbecome lost.Asalreadynoted(p.20), in someorders(Diptera,Coleoptera,Hymenoptera,Homoptera,andOdonata)eventhe spinasternumandthefurcasternumhavebecomereducedorlost.Thelostareas,both on theanteriorand theposterior endsof thesternum(shadedinB,C,D),areoccupied bytheposteriorandanteriorendsoftheneighboringsterna(thepresternumandthe metasternum),andtheunionof thetwothoracicsegmentsareincreasinglystrengthened (synthorax).Dorsally,such afirmunionorfusionof segmentsoccursonly rarely(e.g., Mallophaga,Anoplura). Theories:According toSnodgrass( 1927,1935), insoft-bodiedsegmentedanimals, suchasannelidsandlarvalinsects,thesegmentedregionsofthebodyareseparatedby cuticularconstrictionsoftheintegument.Internally,theintersegmentalgroovesform folds,and on thesefoldsthefibersof theprincipal longitudinalbandsof somaticmuscles areattached(Fig.7 A).Snodgrasscalledsuchasystemtheprimarysegmentation. Withsclerotizationofthebodywall,however,membranousintersegmentalareas wereleftouttoallowflexibilityinthemovementofbodysegments,andadefinitive sclerotizedsegment(Fig.7B)inmanyarthropodsisnolongercoincidentalwiththe primaryintersegmentalfolds,sinceitdoesnotcovertheposteriorpartoftheoriginal segmentandmayextendanteriorlybeforetheintersegmentalgroove,onwhichthe dorsallongitudinalmuscleisattached.Thisnewwayofdividingthebodyisclearly secondary,andconstitutesasecondarysegmentation. 6Herethewordintersegmentalisusedinthefunctionalsense.Typically.theareabelongsmorpho1ogica11yto the. segmentthatprecedesthetrueintersegmentalfold,asSnodgrass(1927)stated.. 24 MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA Inthethoracicsegments,accordingtoSnodgrass,thesecondarysegmentation prevails.Snodgrassthought that thepostnotalplate wasactuallyanintersegmentalplate comprisingtwosegmentalelements(Fig.7C),althoughthedorsallongitudinalmuscles areattachedtothephragmathatarisesfromtheantecosta.Snodgrassalsoexplained the secondary ventralintersegmentalrelationships,byassumingtheproductionofaplate analogoustothe dorsalpostnotalplate. Asalreadyconcluded( p.11), Snodgrass'theoryontheformationofthepostnotum isnot supported byfacts,and hisideaof thesecondary segmentationcannot beaccepted. Thefactsshowclearlythattheintersegmentalrelationship'ismuchsimplerthan Snodgrassconsideredittobe.Hiscomplicatedtheorywasclearlybasedonhispre-conceptionsthataphragmaarisesonlyfromtheantecostaandthatthepostnotumisa secondarydevelopment. Finally,itshouldbepointedoutthatinbothSnodgrass'andWeber'sdiagrams (1924b,1933)ofthehypotheticalprimitiveintersegmentalrelationship(Fig.7B),the dorsalmusclesareshownattachedbetweentheantecostae.Suchdorsalmusculature, however,certainly isnottypicalinThysanura.DorsalmusclesinLepisma(Figs.19-21) andinMachilidae(Barlet1950a,1953,1954,1967)extendfreelybeyondsegmental borders.ItisonlyinthePterygota,withalreadywell-developedphragmata,thatthe distribution of dorsaland other musclesislargely confinedwithineachsegment. Chadwick( 1957)proposedthe concept of the intersegmentallaterosternitesforsites ofmuscleattachmentsthatlieatthelateralextremitiesofventralintersegmentalfolds. Heinferredthepresenceof suchanareafromacomparisonwiththeanterolateralarea ofeachabdominalsternite,whichservesasanattachmentpointforapparentlyserially homologousmuscles.Onthethoracicpleurosternalwalltheprimaryintersegmental lineisobliterated.It is,therefore,difficulttoknowtowhichsegmentascleriteonthe definitiveintersegmentalfoldactuallybelongs.Often,however,thesescleritesareprob-ablytheisolatedpresternum,asalreadydiscussed(p.19). Theseisolatedplateshavebeenrecognizedpreviously,forexample,byCrampton ( 1926a),whocalledthemthefurcillae;butmoreoftentheyhavebeencalledtheinter-segmentalpleuritesor theinterpleurite.Theterminterpleuriteisretainedinthiswork,' withtheunderstandingthattheplatemaywellbeofthesternal(presternal)origin. BarletandCarpentier's(1962)andBarlet's(1967)conceptoftheintersegmental relationshipintheApterygotaisdiscussedonp.94. THEORIGINOFTIIEPLEURON SincetheembryologicalworkonHemipterabyHeymons( 1899),thethoracic pleuronininsectshasgenerallybeenconsideredtooweitsorigintothebasalpart ofthe coxaorthesubcoxa,whichbecomesseparatedandimbeddedintothelateralwallof thethoraxbetween thetergumandthe sternum.It hasalsobeenimplicitinmostworks thatthesamedevelopmentalmechanismastheonefoundinHemipteraisatworkto producethepleuralwallthroughouttheArthropoda.Recently,however,Snodgrass (1952,1958),Bekker(I 960),FUller(1963b),andSharov(1966)havedisputedthis theory of thesubcoxaloriginof thepleuronintheArthropoda.Itseemshighlyappro-priate,therefore,to reexaminecloselythepertinent facts. RegardingtheoriginofthepleuronintheCrustacea,Snodgrass( 1952)thought thatwhatHansen(1925)consideredtobethesubcoxainAnaspides(Fig.8D)was actuallythelaterotergiteorthelateralsubdivisionofthetergite,becauseofthepassage ofthebodymusclesthroughthisareaandtheirinsertiononthecoxa,asinother arthropods.InvariousgroupsoftheIsopoda,however,Gruner( 1954)wasableto showvaryingdegreesofincorporationofthecoxaintothelateralwallofthethorax. He showed alsoembryologically that inPorcelliothecoxabecomesincorporatedintothe 'l'0iscussiononasimilarlycomplexventralsecondaryintersegmentalrelationshipproposedbySnodgrass isomitted. BThetermproposedbyChadwickisconfusing,sincetheIaterosteroitehasbeeninuseforapartof thepleuron. MATSUDA:TilEINSECTTIIORAX COXOPODITE PREISCHIOPODITE ISCHIOPODI A PRECOXOPODITE EPIPODITE D 8 EPIPODITE EXOPODITE ENDOPODITE 25 Fm.8.A-C, Basal partof rightmaxillipedofAnaspidestasmaniaeThompson,posterior view. A,fromHansen;B,fromSnodgrass;C,fromBarletandCarpentier;D,thirdleftpereiopod andadjoiningpartofterguminAnaspidestasmaniaeThompson(fromSnodgrass). lateralwallandfuseswiththetergum,andherethecoxaisthedefinitivepleuron,or epimeronincarcinology,andnotthelaterotergite.Snodgrass(1958)subsequently showedthat thecorrespondingplatesinamphipodsarefreelysuspendedfromthetergal margins,andstated,"if theseplatesonthelegbasesaretrulycoxal(asGrunerclaims themtobe),thereisnopartoftheskeletoninisopodsandamp hi podsintervening between the tergumand coxa that can be regardedaspleural."Snodgrass(1952,1958) thusdenied the presence,in the Crustacea,of apleuronhomologizablewiththesubcoxa. CarpentierandBarlet's( 1959)detailedcomparativestudyofthelegbaseofthe Malacostraca and the Apterygota,however,has shown that the lateral wallisdividedinto twoparts.Theyshowedthat,inthemaxillipedofAnaspides(Fig.SA,B),what Hansen(1925)and Snodgrass(1952)thought wasthe coxopodite(coxa)wasactuallya segmentdorsaltothetruebutreducedcoxa;andCarpentierandBarletcalleditthe precoxopodite(Fig.SC).WhatGruner(1954)calledthecoxa,therefore,mayalso be the precoxopodite. ThevalidityofCarpentierandBarlet'sidentificationofthereducedcoxopoditeis alsoseen,however,bycomparingFig.9AwithFig.39A.BothinPenaeus(Mala-costraca)andPetrobius(Thysanura)thecoxahasthreepointsofarticulationwiththe precoxopodite(pleuron)andthesternum.TheanteriorarticulationAinPenaeus (Fig.9A)correspondstothetrochantinocoxalarticulationA'inPetrobius(Fig.39A); the posterior articulationBinPenaeuscorrespondstothekatapleurocoxalarticulationC inPetrobius;andtheproximalcoxalmarginhasthearticulationCwiththecoxathat correspondstothebaseoftheendosternalprocessfinPetrobius.Furthermore, 'CarpentierandBarlet(1959)thoughtAwasthepleuralapodeme,butitactuallyisthetrochantinalcondyle asdiscussedonp.106.' 26MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA FURCA COXA B APPENDAGE STERNUM MEMBRANAVENTRALIS FIG.9.A,basalpartof3rdrightpereiopodofPenaeuscaramoteRisso,seenfrominside (fromBar letandCarpentier).BandC,transversesectionofasegmentinembryoof Sco/opendra;Crepresentsalaterstage(fromHeymons). CarpentierandBarlet( 1959)showedintheirfigs.4and5mostoftheendosternal skeletonsinPenaeus(Fig.9A),andtheyareapparentlyhomologouswiththoseinthe Apterygota.BasedonthesefindingsCarpentierandBarletconcludedthatinthese malacostracanstheprecoxopoditeismorphologically equivalent tothekatapleuronin the Apterygota;thelaterotergite,withwhichtheprecoxopoditearticulates,istherefore homologouswiththeanapleuronintheApterygota. The nextquestioniswhether or not theprecoxopoditeintheCrustaceacorresponds to the subcoxa in insects.Carpentier and Bar!et'sstudy now leadsustosuspect that the separationofthecoxaandtheprecoxopoditecanpossiblyoccurduringdevelopment. Whateverthedevelopmentaloriginoftheprecoxopoditemaybe,theareadorsaltothe MATSUDA:TI!BINSECTTI!ORAX EUPLEURON PROCQXA .,..,' '\ -o------"'-"EUCOXA .... ,.., < > < w c i 1"' > w i PROTOPLEURON 27 FIG.10.Verhoeff's(1905)theoryonthedivisionofpleuronandcoxa(fromVerhoeff). truecoxa,i.e.,theprecoxopodite,correspondsbyitslocationtothesubcoxaininsects. The difference isthat intheCrustacea theprecoxopodite isnot imbeddedonto the lateral wallof thethoraxinthewaythesubcoxa becomesimbeddedininsects. Regarding the originof the pleuron in the Chilopoda,Heymons(1898)thoughtthe Pleuren,or B.asalglied,representedafusion( Verwachsung)ofthecoxa withtheappen-dicularsegmentlyingdorsaltoit.Further,inScolopendraHeymons( 1901)found thatboththetergumandthesternumweretripartite;the. tergumhasabroadmedian membranadorsalis,andsimilarlythesternumhasabroadmembranaventralis.The appendagerudimentarisesfromtheareabetweenthelateralpartsofthetergumand the sternum(Fig.98, C).Aspaceisformedabove thebaseof theappendagerudiment andbelowthedorsallyshifted lateralpart ofthetergum;thisnewlyformedspaceisthe pleuron.Thelateralpart ofthetripartitetergumshiftsevenfurtherdorsally,andwith themediantergumformsthedefinitivetergum.Similarly,thelateralpartsofthe sternumandthemediansternumformthedefinitivesternum.Theoriginaltripartite nature of thetergumand thesternum isstillrecognizablein theadult. Verhoetf's(1905)study onthedevelopmentof thecoxopleuroninLithobiusessen-tially agreedwiththestudy of Heymons( 1901)onScolopendra.Verhoetf found,how-ever,thatthelateralappendicularbudeventuallydividesintothecoxaandthepleuron. AccordingtoVerhoeff,thecoxaandthepleuronareformedfromthatpartofhis pleuropoditethatisdorsaltothetelopodite(Fig.10),ofwhichthemoredorsalmem-branousareacarryingthedorsalscleritesandthestigma(spiracle)istheeupleuron. The definitive coxa(pleurocoxa)consistsof four sclerites:thecoxopleuron,theprocoxa, themetacoxa,andtheeucoxa.Ofthese,Verhoeffequatedthecoxopleuronandthe procoxatothesubcoxaof Heymons.Thus,asVerhoeffemphaticallystated,thesesub-coxalplatesare,unlikethesubcoxaininsects,nottheplatesthatareseparatedofffrom thecoxa.InHanseniellaTiegs( 1940)foundthatthepleuralscleritelyingdorsalto the coxa wasonly incompletely separated fromit,and hethought this wasanindication of thepresenceof a subcoxal segment. FortheInsecta,thesubcoxaltheoryisnowsupportedbyfourpiecesofembryo-logicalevidence.Heymons( 1899)firstdiscoveredintheembryoofHeteropterathat the coxa becomesdividedintotwoparts,andthat itsbasalpart,thesubcoxa(Fig.110), eventuallybecomesa part of thepleuron.AsinScolopendra,the appendicularrudiment liesbetweentherudimentsofthetergumandthesternum.Hymonsthoughtthatonly thelowerportionof thedefinitivepleuronimmediatelyabovethecoxainNaucorisand Nepa(Fig.llH)correspondedtotheembryonicsubcoxa.Furthermore,intheyoung nymph ofNepa(Fig.llH)Heymons showedanarrowpleuronbeneath theparatergum. Roonwal(1937)foundinLocustathat,inthe100-hourstage,eachlegconsistsof subcoxa+ coxa + trochanter,femur,tibia,tarsus,andterminaljoint.By the120-hour stage,thebasaljointdividesintotwosegments,theproximalsegmentrepresentingthe 28 MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA subcoxa.Theentiresubcoxaisconvertedintothepleuronand thebasal. articular scleriteofthethoraciclegs.Thetripartitesternumandtwolateraltergalscieritesare present inLocusta,asinHeteroptera.Roonwaldidnot tracejusthowfarthe. subcoxa takespart intheformationof thedefinitivepleuroninlater stagesof development.He simplysaid,"It willthusbeseenthattheentiresubcoxaisconvertedintothepleuron andthebasalarticularscleritesofthethoraciclegs."InTachycinesIbrahim(1958) notedthepresenceof thesubcoxa."Furthermore,Bretfeld(1963),inthreegeneraof Collembola,describedandillustratedthedevelopmentofthetwo-segmentedsubcoxa.11 Thus,theevidenceinfavorof thetheoryof thesubcoxaloriginof thepleuroncontinues to grow,asfar asthe classInsecta isconcerned. Theabovereexaminationofthemoredirectevidenceclearlyshowsthatthe definitivepleuronisdifferentinitsdevelopmentalprocessindifferentgroupsofthe Arthropoda,andthat it isonlyintheInsectathat thepleuronor apartof thepleuronis definitely knownto bederivedby separation fromthedorsalpart of thecoxa(subcoxa), whichbecomesimbedded.Therefore,itistothisspecializeddevelopmentalprocess for theproduction of thedefinitivepleuronintheInsectathatthetheoryof subcoxal origin properlyapplies.Assuming that the pleuralpartsthroughout theCrustacea-Chilopoda-Insecta are homologous,assomescattered evidence indicates,thedifferentdevelopmental processes producing themrepresentacaseof mu!tiphasedmorphogenesisinevolution. InthepresumedevolutionarysequencethroughtheCrustacea-Chilopoda-Insecta thelateralwallofthethoraxhasprobablybecomeenlargedintheChilopoda-Insecta. Thisdorsoventrallyenlargedthoraxthuspavedthewayforeventualaccommodationof thepowerfuldorsoventralflightmusclesininsects. Thesubcoxaandthepleuronininsects:Theextenttowhichthesubcoxalelement ispresent onthedefinitivepleuralwallmustvaryconsiderablyindiffereptinsects.As alreadynoted,Heymons'( 1899)studyonHeteropteraclearlyshowedthatthesubcoxa liesmainlyon the ventralpart ofthe lateralwallofthethorax,andthatthepleuronlies lateraltoit.Crampton(1909),however,pointedoutthatwhatHeymonscalledthe pleuritewasactuallythemesothoracicepimeron.Hereasoned,further,thatthemeso-thoracic episternumandtheepimeron,whichtogetherformthesubcoxaofHeymonsin N epa,wereactuallythemesothoracicepisternum,thelaterale,andperhapsalsothe trochantin.Grandi(1950)alsopointedout inherstudyof thenymphsofPlecoptera thattheepimeralarearemainsmembranouswhiletheepisternalareaissclerotized,and shesuggestedthattheepimeralareamightnotbesubcoxalinorigin.InHeteroptera andthenymphalPlecopteratheepimeralareaoccupiesthedorsalpartofthedefinitive pleuron.It islogicallytobeexpected,therefore,thatthedetachedsubcoxaislargely incorporatedintotheventrallylocatedepisternalarea.InthepseudofoetusofDilta Barlet( 1950b)observed that the anapleuralarea ismembranous.He thereforeassigned asubcoxaloriginonlyto the katapleuron,which issclerotized. Alltheabove-mentionedfactsclearlysuggestthatthedefinitivepleuroninthe Thysanura-Pterygotaconsistsoftheventralsubcoxalareaandthedorsalpleuralarea, whichhavebecomeindistinguishablethroughthesecondarysclerotization.Therefore, Gouin's( 1959)conceptofthepodopleuron( = precoxopodite),whichconsistsofthe anapleuron,12 thekatapleuron,andthetrochantin,maybemoreproperlydefinedas consisting of the trochantin and the katapleuronalone,asBarlet( 1950b)andCarpentier andBarlet( 1959)believed.It shouldberecalled,however,thatinCollembolathe embryonicsubcoxaistwo-segmented.It ispossible,therefore,thattherelativelylarge anapleuralandkatapleuralplatesinthisorderowetheirorigintothistwo-segmented subcoxa. AsMatsuda( 1963a)pointedout,andasdiscussedagainelsewhere(p.68)inthis work,thesubalareistergalinorigin.InthelateembryosofLeucophaeasurinamensis 10Dr.G.MickoleitoftheUniversityofTtibingeninformedmeinprivatecommunicationthatheobservedthe subcoxaduringthecourseofDr.Ibrahim'sstudy. UBretfelddidnottracethedevelopmentofthispeculiarsubcoxafurther.It remainstobeseeniftheclaimed subcoxaisreallycomparablewiththesubcoxainotherinsects. 12Qouin'sanapleurondoesnotincludethepreepisternuminPerla,asdiscussedonp,40. MATSUDA:TilEINSECTTiiDRAX29 andintheearly stagenymphof Locusta migratoriamanilensis,Maki( 1938)foundthat the subalar-coxalmuscle(t-cx8)wasdorsallyattachedtothe lateralareaof thetergum ortotheventralsurfaceofthewingbase,andthisareaseparatesoffasthesubalare duringthe finalstageof nymphal development.Similar detachmentofthesubalar-coxal musclefromthelateraltergalareahasbeenobserved,orcanbesurmised,inseveral otherorders(p.68). Roonwal( 1937)showed,intheembryoofLocusta,thatthesternumisdivisible intoamedian portion(the primary mediosternite)andtwolateralportions(theprimary laterosternites),andhereportedthatthelatterboreappendagesatitslateraledge.He assertsthathisprimarylaterosterniteisnotthesameasthelaterosterniteasunderstood bymorphologists."Sincetheventropleuritepresumablyformstheventralprocessfor articulationwiththecoxainholometabolousinsects,it ispossiblethatthelaterosternite ofRoonwalandtheventropleuriteof thepresentworkarethesame.If thisinterpreta-tionandtheobservation of Roonwalarecorrect,the ventropleurite,whichisdescribedas a part of thekatapleuroninthiswork,isactually sternalandnot subcoxalinorigin. Aswillbe discussed later,thekatapleuralventropleurite liesbetweentheinnercoxal baseandthefurcalbaseintheadult,andaccordingtoWeber(1928a)thefurcaarises fromthe areathatisboth sternalandsubcoxalinorigin.Onthebasisof theseobserva-tionsinadultinsects,therefore,theventropleuritemaybesubcoxalinorigin.Itis possiblethatinLocusta,wherethesubcoxaandsternumareindistinguishablyfusedin theadult,andtheprimaryborderbetweenthesubcoxaandthesternummaybe obliteratedevenin theembryo.Roonwal'sfinding,therefore,needstobesupportedby similarstudiesonotherinsects,ifthesternaloriginoftheventropleuriteistobefully accepted. Inhigher holometabolousinsectsthe pleurondoesnotarisefromthe subcoxa.For instance,inDrosophila it hasbeenknown sincethework of Chen( 1929)that the thorax and theappendagesof adultDrosophilaarerepresented,attheendofthelarvalperiod, bysixprimordia,orimaginaldiscs;Geigy's(1931)experimentalworkindicatedthe presenceof imaginalcellsof legsintheembryoofDrosophila.Theseembryonicand larvalanlagenoftheimaginallegsandthorax,however,cannotberelatedtothe embryonicand nymphal subcoxa andcoxa in hemimetabolousinsects.Clearly,herethe anlagenreplacement(orsubstitution)inontogenyisinvolvedintheproductionof homologousadult structures." Theories:In thepast,manyauthorsconcernedwiththemorphologyofthepleuron of arthropodsdidnotpay closeattentiontothedifferenceinthedevelopmentaloriginof thepleuronindifferentgroupsoftheArthropoda,andtheyhomologizedthepleural partswiththepresumedsubcoxalcounterpartsofothergroups.Thus,Borner(1903, 1904),inhisstudy of thecomparativeanatomyof variousgroupsofarthropods,found thatthesegmentequivalenttothesubcoxaofHeymons( 1899)ispresentalsointhe CrustaceaandChilopoda,but isnotpresent,orisdifficulttofind,intheChelicerata,the Pantopoda,and theProgoneata;asaresult,heagreedwithHansen( 1893), whothought there wasasegmentproximaltothecoxa intheCrustaceaandInsecta.Borner( 1904) proposedtheconceptofthebasipodite,whichconsistsofthesubcoxaandthecoxa. Verhoeff(1905)criticized the concept of the basipodite,becausethe concept of thecoxa composingit fluctuatesindifferentgroups.Borner(1914)saidthatthesubcoxaasan independent scleritewaspresent inthe Pantopodaandinmanyof theMalacostraca,and intheChelicerataand Hexapodathesubcoxalpleuroncouldsplitintosmallersclerites, which are the epimeron,the episternum,andthetrochantin.Thus,according toBorner, nearly thewholelateralwallof thethoraxissubcoxalinorigin.Ewing( 1928)applied the subcoxaltheorytotheArachnida. "In insectmorphology,theterm laterosternite hasbeenusuallyappliedtotheventropleurite(e.g.,Weber1933). "InLepidopteraKim(1959),Kuskeetal.(1961),andKuske(1963)foundthatthepupalleg,hencethe imaginalleg,isformedfromtheepidermalcellsofthelarvalleg,noimaginaldiscofthelegbeingpresent. Amvngholometabolousinsectsthemselves,therefore,thedevelopmentalprocessofthelegisnotthesame. 30MEMOIRSOFTHEENTOMOLOGICALSOCIETYOFCANADA The theory of the subcoxaloriginof the pleuron elaborated by Snodgrassand Weber hashadconsiderableinfluenceuponlaterworkers.AccordingtoSnodgrass( 1927, Fig.llA-F), thesubcoxawasprimitivelyasingleplatesurroundingthecoxalbaseand articulatingwith the latterbyanteriorand posteriorhinges,fromwhich latertheanterior trochantinaland posterior pleuralarticulations were derived; it wastheventralring of the subcoxa(trochantin)thatborethecondylesthatarticulatewiththecoxa.Withthe acquisitionof the pleural ridgeassociatedwith the development of thewing,thedorsalor anapleuralringsplitupintoafewsclerites;theanteriorscleriteformedtheprecoxal bridge,and theposterior oneformedthepostcoxalbridge.Snodgrassimplicitlyapplied thistheory of the subcoxaloriginof the pleuron toother arthropods. Weber( 1928a)supplementedSnodgrass'theorybystudyingtheventralareaofthe subcoxaanditsrelationshiptothesternumintheinsectthorax.Heproposedthatthe preepisternum(priicoxaleBriicke)andtheventropleurite(Laterosternit)aretheventral limitof thesubcoxathat cameincontactwiththesternum.Inthetheoryof Snodgrass andWeber,nodistinction wasmadebetweentheprimarypleuronandthesubcoxalarea. It wasimplicit that the wholeof the definitivepleuronwassubcoxalinorigin. Ferrisandhisstudentsappliedthesubcoxaltheoryintheirseriesofstudiesofthe insectthorax.Theyalsoacceptedimplicitlytheideathat thewholepleuralwallwasof subcoxalorigin.ThebasicnewconceptstheyintroducedweresummarizedinFerris' paper(1940b)entitled"TheMythof theThoracicSternitesof Insects",inwhichitwas postulatedthatprimitivelythesubcoxaemetontheventralmedianlongitudinalaxisof thethorax,asseeninNeuropteraandmanyotherholometabolousinsects;thislongi-tudinallineofdemarcationwascalledthediscrimen.AccordingtoFerris,then,the exposedsternuminhemimetabolousinsectshasbeenproducedbytheretreatlateradof thesubcoxalelementon eachside.Healsopointedouttheusefulnessoftheparacoxal suture(thepleuralcostaof Ferris)andtheanapleuralsutureaslandmarksinhomolo-gizingpleuralparts. Weber( 1952)acceptedFerris'ideaofthesecondarynatureofthesternumand Carpentier's(1947)ideaofthekatapleuraloriginofthefurca,andproposedamore elaboratetheoryoftheproductionofthesecondarysternuminphylogeny(Fig.12). Since,accordingtoWeber( 1952),thefurcaremainsinthesamerelativeposition,viz., ontheposterior innermarginof thekatapleuralring(Fig.12)duringtheretreat laterad of the pleuroninevolution,theformationof thedefinitivesternumbetweenthe furcaeis dependenton thedegreesofmigrationlateradof thekatapleuralringonbothsides. Sincethepresenceofthesternumasanintegralpartofthethoraxissoevidentin anyinsectembryo,theideaofthesecondarynatureofthesternuminthetheoriesof FerrisandWeberwasunfavorablycriticizedbyMatsuda(1956a),Czihak(1956),and Gouin(1959).Furthermore, aswillbedetailed later, more recent worksontheAptery-gotabyCarpentierandBarlethaveshownthatthefurcadoesnotnecessarilylieonthe pleuron.Earlier,Hansen( 1930)criticizedSnodgrass'diagrams(Figs.llA-F)and claimedthattheywereunrealistic,butthefactsuponwhichhebasedhiscriticismwere mainlyspecializedfeatures,andthereforelargelynotpertinent;inaddition,heignored allembryologicalevidence. Snodgrass( 1952,1958)criticizedthesubcoxaltheoryonwhichhehadearlier elaborated(1927,1935).Heattemptedtoshowquitelegitimatelythattheevidencein favorofthesubcoxaltheoryhadbeendeducedmainlyfromastudyofchilopodsand insects,and thatmistaken significancehadbeenattachedtoHeymons'( 1899)findingof thesubcoxaloriginofthepleuroninsomeinsects,whichledmanyauthorsincluding Snodgrasshimself( 1927)toapplyerroneously theconceptofthesubcoxaloriginofthe pleurontootherarthropods.Snodgrass'( 1952,1958)conclusionwasessentiallythe !\arneastheone reachedinthiswork. Inannelidstheparapodiumisasimpleoutgrowthofthepleuralwall.Therefore, Bekker( 1923,1924,1960)thoughtthatinarthropodsalsothesamestructuralplan mustbemaintained.Thus,whatHeymons( 1901)calledthelateraltergumandthe TERGUM MATSUDA:THEINSECTTHORAX31 SUBCOXA TROCHANTIN ANAPLEURITE TROCHANTINEUPLEURON A 8cD ANAPLEURITETROCHANTIN \ EPISTERNUMPLEURALSUTURE \ EPIMERON TROCHANTIN EF SUBCOXA STERNUM G H PLEURON FIG.11.A-F,diagramshowingoriginandevolutionofpleuralscleritesfromasubcoxal segment;aandbrepresentanteriorandposteriorcoxalarticulations(fromSnodgrass);G, rightmiddlelegof embryoofNaucoris(fromHeymons);H,lasttwothoracicsegmentsand abdomen in nymphalNepacinerea(from Heymons). 32 ANAPLEURON MEMOIRSOFTHEENTOMOLOGICALSOCIE1YOFCANADA KATAPLEURON --H--.; B RCA ANAPLEURON EURON IRCAL @1.. FIG.12.Weber's(1952)theoryontheoriginofsecondarysternum(fromWeber). lateralsternuminScolopendra(Fig.9B,C)are,accordingtoBekker,actuallythe pleuralbasesof theleg.Thedefinitivetergumandthesternumare formedsecondarily fromthepleuron(tergiterudimentandsterniterudiment)bythemedian truetergalandsternalparts.Therefore,what Heymons(1899)discoveredinHeterop-tera can best beexplainedasaprocessof formationof thecoxalsegmentattheexpense ofthepleuron,notasaseparationofabasalpartofthecoxa,andthiswouldbecon-sistent withthe majorevolutionary trendsof the legbaseintheArticulata. ThenoveltyofBekker'stheoryisrefreshingindeed.However,itleavesthe unanswered questionof whetherthekindof tripartite tergumand sternum thatHeymons ( 1901)foundinScolopendra,inwhich themedianmembranousareaissobroad,repre-sentsa primitiveconditionor not.Heymonsattributedthebroadmembranadorsalisto thepresenceoftheabundantyolk.IninsectembryosHeymons(1895,1899)and Roonwal( 1937)foundthatthetergumconsistsoftwolateralplatesthatlaterbecome fused.Ininsects,therefore,itwouldappeartobetripartitewhenthemedianarea be