Oligomerization and other evolutionary changes in the antennae of tetrastichine wasps (Hymenoptera, Eulophidae)

ISSN 0013-8738, Entomological Review, 2013, Vol. 93, No. 6, pp. 725–736. © Pleiades Publishing, Inc., 2013. Original Russian Text © O.V. Kosheleva, 2013, published in Entomologicheskoe Obozrenie, 2013, Vol. 92, No. 1, pp. 70–84.

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Oligomerization and Other Evolutionary Changes in the Antennae of Tetrastichine Wasps

(Hymenoptera, Eulophidae) O. V. Kosheleva

All-Russia Institute for Biological Plant Protection, Krasnodar, Russia e-mail: [email protected]

Received June 12, 2012

Abstract—The following trends in oligomerization of the male and female antennae in Tetrastichinae were deter-mined. (1) Reduction (shortening) of the basal segment of the funicle. (2) Reduction (shortening) of the middle segments of the funicle (not recorded in chalcids before). (3) Loss of homonomy by the funicle. (4) Fusion of the claval segments. (5) Reduction (shortening) of the apical claval segment. (6) Incorporation of the apical funicular segment into the clava (not recorded in eulophids before). Sex dimorphism in the antenna structure apparently re-sults from different rates of the evolution of parts of antennae in males and females, in particular from different rates of oligomerization and appearance of novel structures in the male antennae. DOI: 10.1134/S0013873813060067

The subfamily Tetrastichinae is the largest in the family Eulophidae of the superfamily Chalcidoidea. The world fauna of tetrastichine wasps presently com-prises about 100 genera and over 1800 species. The subfamily has a cosmopolitan distribution, its Palae-arctic fauna including about 1000 species in 40 genera (Noyes, 2010).

Tetrastichine wasps are about 2 mm long; their morphology is quite uniform. Identification of many species is difficult due to the insufficient knowledge of the fauna, morphology, and taxonomy of the group as well as poor knowledge of the weight and variation of individual characters. The taxonomy of the subfamily is mainly based on chaetotaxy, structure of the ap-pendages, and sculpture of various body parts. The morphology of the antennae is of great significance for determining the phylogenetic relations in the subfam-ily. This communication considers the ways of oli-gomerization and other evolutionary transformations of the antennae, with the view of revealing new taxo-nomic characters and possibly improving the classifi-cation of the subfamily.

Analysis was carried out using the author’s collec-tion material and the available publications on the Palaearctic fauna of the subfamily (Domenichini, 1956; Bouček, 1971; Kostjukov, 1977a, 1977b, 1978, 1989, 1995; Graham, 1987, 1991; Yegorenkova et al., 2007). This work continues and broadens the evolu-

tionary studies of the antennae in Eulophidae, and in particular in Tetrastichinae, started by the earlier re-searchers (Domenichini, 1956; Bouček, 1971; Kostju-kov, 1977a, 1977b; Graham, 1987, 1991; Storozheva, 1991; Yefremova, 2002; Yegorenkova and Yefremova, 2007).

The insect antenna consists of three main parts: the basal segment or scape, the pedicel, and the flagellum. The flagellum of a generalized antenna includes sev-eral, sometimes many segments of approximately uni-form structure (see, e.g., Schwanwitsch, 1949). The first two antennal segments in parasitic hymenopterans are never reduced or merged with other segments; all the processes leading to changes in the number of seg-ments include only the flagellum. The evolution of the flagellum in Tetrastichinae, as well as in chalcid wasps in general, includes oligomerization which is manifested by differentiation of the flagellum into the anelli, the funicle, and the clava. The trend toward reduction of the number of antennal segments, charac-teristic of all the chalcid wasps, is also typical of Tet-rastichinae (Trjapitzin, 1977; Kostjukov, 1977a, 1977b, 1978; Graham, 1987, 1991). The greatest num-ber of antennal segments in the ancestors of tetras-tichine wasps, and possibly of representatives of other chalcid families, was 13 (Trjapitzin, 1977; Kostjukov, 1978; Rasnitsyn, 1980).

Not only the shape and size of organs but also their number may change considerably in the course of evo-

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lution. A gradual decrease in the number of homolo-gous organs was termed oligomerization by Dogiel (1954). According to the cited author, this process may be realized by different mechanisms. (1) In the most common variant, some of the homologous organs be-come aborted. This way of oligomerization was later referred to as “litation” (Podlipaev et al., 1974), to reflect the idea of some elements of the system being “sacrificed” to the benefit of the whole. (2) In some rare cases, oligomerization proceeds by merging of homologous (homodynamic) elements, such as gan-glia. (3) One more way of oligomerization consists in exclusion of some homologous or homodynamic or-gans as the result of acquiring by them new functions.

The first two mechanisms of oligomerization have played an important role in the evolutionary transfor-mations of the antennae in tetrastichine wasps. Reduc-tion of the funicular segments has been the most fre-quent event. Oligomerization of the clava usually re-sults from merging of some of its segments, but no case of such merging has been demonstrated in the funicle.

The process of oligomerization of the antennal fla-gellum in another chalcid group, the family Pteromali-dae, was described by Dzhanokmen (1994); it also occurs by reduction and merging of segments. The cited author noted that reduction events prevailed in the proximal part of the flagellum, whereas merging usually occurred in the distal part. The loss of homon-omy was shown to be more expressed in females of Pteromalidae, which may be associated with a greater significance of the apical antennal segments for search and choosing the host (Dzhanokmen, 1994).

According to Rasnitsyn (1967), the adaptive signifi-cance of oligomerization may be related to higher effi-ciency of complex systems: the energy cost of building and managing a system of complex organs may be lower than that of a system of simpler and less effi-cient organs, which have to be more numerous to pro-vide the same level of functioning.

K.B. Gorodkov, who had published a number of pa-pers on the phenomenon of oligomerization, consid-ered reduction of a system as one of the simplest ways of lowering the cost of its functioning (and building), not by modifying the elements but merely by decreas-ing their number (Gorodkov, 1985).

The antennae of tetrastichine wasps are complex systems performing an important sensory function; this is the obvious reason for their gradual improve-

ment by oligomerization (merging of some elements and differentiation of the antenna into distinct parts).

The morphology of tetrastichine antennae reveals well-expressed sex dimorphism resulting from differ-ent rates of evolution of individual structures, which are usually higher in females, and in particular from the development of a novel structure in males, termed the sensory plaque by Yegorenkova (2007); this organ probably performs a sensory function and is found among Eulophidae only in Tetrastichinae. The females have antennae with a 3-segmented funicle (except for species of the genus Hyperteles; see Figs. 21, 22), whereas the males of most genera have a 4-segmented funicle.

The Female Antennae

The scape. Most tetrastichine wasps have an elon-gate, cylindrical basal segment. The females manifest two principal structural modifications of this segment and a wide range of intermediate states.

(1) Considerable elongation of the scape: Tetrasti-chomyia clisiocampae Ashmead (Fig. 14), Aprosto-cetus sibiricus Kostjukov (Fig. 1).

(2) Plate-like dilation of the scape along its entire length (Syntomosphyrum calamarius Graham, Fig. 11), in the distal portion (Crataepus marbis Walker, Fig. 12), or in the middle (Aprostocetus taxi Graham, Fig. 7).

Dilation of the basal segment is to be found in other chalcid wasps as well. In the opinion of Trjapitzin (1977), plate-like dilation of the scape in Encyrtidae performs a protective function when the antenna is in repose to make the body more compact and stream-lined in flight. According to Storozheva (1991), dila-tion of the scape in another chalcid group, Eulophinae, cannot be accounted for by its protective function, since representatives of Encyrtidae often possess a dilated scape subject to considerable specialization not found in Eulophinae. Both the cited authors reject the assumption that the surface of the scape may ex-pand to provide space for numerous sensilla since this segment does not usually bear any special sensilla in Eulophinae and Encyrtidae. V.A. Trjapitzin also ar-gues against the idea that enlargement of the scape may be related to the presence of powerful muscles operating the remaining parts of the antenna; however, according to N.A. Storozheva, this explanation may be partly true for Eulophinae which have long outgrowths on the funicular segments.

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The pedicel. The structure of this segment is uni-form; its elongation may be correlated with elongation of the whole antenna or the basal funicular segment. In females of Aprostocetus phillyreae Domenichini

(Fig. 5) and Tetrastichomyia clisiocampae Ashmead (Fig. 14) elongation of the pedicel is accompanied by slight elongation of the basal funicular segment which in this case loses a considerable part of its sensilla.

Figs. 1–14. Antenna of a female [(1, 4, 6–11, 13) left; (2, 3, 5, 12, 14) right] (after Graham, 1987, 1991; Kostjukov, 1995): (1) Aprosto-cetus sibiricus Kostjukov (right view); (2) A. humilis Graham (right view); (3) A. escherichi Szelényi (right view); (4) A. hofferi Kostju-kov (left view); (5) A. phillyreae Domenichini (right view); (6) A. ciliatus Nees (right view); (7) A. taxi Graham (left view); (8) A. microfuniculus Kostjukov (right view); (9) A. gratus Giraud (right view); (10) A. marinikius Kostjukov (left view); (11) Synto-mosphyrum calamarius Graham (left view); (12) Crataepus marbis Walker (right view); (13) Quadrastichodella eucalypti Timberlake (left view); (14) Tetrastichomyia clisiocampae Ashmead (right view).

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Opposite trends can also be observed: for example, females of the Australian species Quadrastichodella eucalypti Timberlake (Fig. 13) have a long and robust pedicel but transverse funicular segments; on the other hand, females of Aprostocetus gratus Giraud (Fig. 9) have an elongate basal funicular segment and a no-ticeably diminished pedicel. Females of Aprostocetus escherichi Szelényi (Fig. 3) have a wide pedicel and a relatively narrow basal funicular segment, whereas A. humilis Graham (Fig. 2) is characterized by a small pedicel and a wide basal funicular segment. The pedi-cel of A. escherichi Szelényi (Fig. 3) is densely covered with setae and may share the sensory function with the basal funicular segment, which is narrow and lacking some of its rod-like sensilla. The pedicel of A. humilis Graham (Fig. 2) is relatively narrow and bears noticeably fewer setae, whereas the basal funicular segment has the complete set of rod-like sensilla.

The anelli. Graham (1987, 1991) distinguished 5 types of anelli in tetrastichine wasps based on their length to width ratio: laminar, or the thinnest ones (the 1st anellus in Tetrastichomyia clisiocampae Ashmead, Fig. 16), discoid (the 1st and 2nd anelli in Kolopterna salina Graham, Fig. 15), subdiscoid (the 3rd anellus in Aprostocetus gratus Giraud, Fig. 17), transverse (the 1st anellus in Tamarixia monesus Walker, Fig. 20),

and quadrate, with subequal length and width (the 3rd anellus in Kolopterna salina Graham, Fig. 15). The presence or absence of hairs was also taken into ac-count: the first 3 types of anelli lack hairs, the last 2 types usually bear hairs. The number of anelli in females of tetrastichine wasps varies; for example, species of the genus Aprostocetus usually have 4 anelli (group lycidas), less frequently 3 or 2, their shape varying from subdiscoid to laminar (Figs. 17–19). The antennae of females of the genus Minotetrastichus usually have 2 or 3 anelli but M. loxotoma Graham has 4 anelli, the first of them being subdiscoid and the rest, laminar. The 3rd anellus of Kolopterna salina Graham is quadrate or slightly transverse and bears several hairs whereas the first two anelli in this species are discoid (Fig. 15). Two species of the genus Synto-mosphyrum differ in the shape of their anelli which are laminar in S. calamarius Graham and subdiscoid in S. apama Walker. The least number of anelli is ob-served in species of the genus Tamarixia: the first anellus is transverse, and the second one may be pre-sent in the rudimentary state (Fig. 20). Species of the genus Puklina also have one transverse anellus; the second anellus, if present at all, is laminar. According to Graham (1987), the plesiomorphic antennae in the females of tetrastichine wasps comprised 3 anelli and 4 funicular segments.

Figs. 15–20. Right antenna of a female (anellus), right view (after Graham, 1987, 1991): (15) Kolopterna salina Graham; (16) Tetrasti-chomyia clisiocampae Ashmead; (17) Aprostocetus gratus Giraud; (18) A. orithyia Walker; (19) A. apiculatus Graham; (20) Tamarixia monesus Walker.



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The funicle. The greatest number of funicular seg-ments (4) was observed in females of the genus Hy-perteles. Most species of tetrastichine wasps have a 3-segmented funicle, with the basal segment aborted. Gradual reduction of the basal funicular seg-ment can be observed in species of the genus Hyperteles. In particular, H. luteus Ratzeburg has a 4-segmented funicle in which the 1st segment is not shorter than the others and bears a complete set of sensilla (Fig. 21); in H. collega Ratzeburg the 1st segment is slightly shorter than the others and already missing the rod-like sensilla (Fig. 22). The relative length of the 1st segment further decreases in H. elongatus Förster, which is accompanied by loss of rod-like and mushroom-shaped sensilla (Fig. 23).

A similar basal segment, still more shortened and bear-ing only hairs, can be found in some species from dif-ferent genera, for example, Tetrastichomyia clisio-campae Ashmead (Fig. 14) or Kolopterna salina Gra-ham (Fig. 31). Such segments (regarded as anelli) dif-fer from the homologous 1st funicular segment of Hy-perteles elongatus Förster only in their length to width ratio. Thus, the anelli of tetrastichine wasps are re-duced segments of the funicle. The process of reduc-tion also involves the 1st segment of a 3-segmented funicle, which is homologous to the 2nd segment of a 4-segmented funicle of Hyperteles. In particular, the basal funicular segment of Aprostocetus ciliatus Nees (Fig. 6) is considerably reduced but is not yet trans-verse, and has no rod-like or mushroom-shaped sen-

Figs. 21–29. Antenna of female [(21–23, 25, 28) left; (24, 26, 27, 29) right] (after Graham, 1987, 1991; Kostjukov, 1995): (21) Hy-perteles luteus Ratzeburg (right view); (22) H. collega Ratzeburg (right view); (23) H. elongatus Förster (right view); (24) Ootetrasti-chus leptocerus Graham (left view); (25) O. zerovae Kostjukov (right view); (26) Baryscapus crassicornis Erdös (right view); (27) B. papaveris Graham (right view); (28) B. embolicus Kostjukov (right view); (29) Aceratoneuromyia polita Graham (left view).

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silla. The funicle of Baryscapus embolicus Kostjukov (Fig. 28) is evidently 2-segmented because its 1st seg-ment is transverse and lacks rod-like or mushroom-shaped sensilla; in other words, the 1st segment is an anellus both morphologically and functionally.

The above examples show that reduction of the number of funicular segments (oligomerization) in tetrastichine wasps starts with diminution of the basal segment. The loss of basal segments may be explained by a smaller sensory load as compared with the apical segments of the funicle.

Oligomerization of the antennal funicle in female tetrastichine wasps may also proceed by reduction of the middle segment. This way was first observed by Kostjukov (1989) in the Algerian species Aprostocetus microfuniculus Kostjukov (Fig. 8), whose biology remains unknown. Reduction of the middle segment manifests itself not only in a considerably smaller size of the segment but also in the absence of rod-like and mushroom-shaped sensilla.

No case of merging of funicular segments (the third way of oligomerization) has been observed in tetras-tichine wasps. However, some species possess very long basal funicular segments with 2 rows of mush-room-shaped sensilla (whereas in most species these sensilla are arranged apically in 1 row) and several rows of rod-like sensilla, which may in fact be the product of merging of two consecutive segments. The homonomous structure of the funicle corresponds to its state in Tetrastichinae ancestors. The loss of homonomy, as well as reduction of segments, is one of the mechanisms of oligomerization. In many tetras-tichine species all the funicular segments are similar in shape and size, usually cylindrical, 1.5 times as long as wide, and bear a complete set of sensilla. The loss of homonomy may be manifested by considerable elonga-tion of the basal funicular segment (in Kolopterna salina Graham, Ootetrastichus leptocerus Graham; Figs. 31, 24) or by changes in the shape of one of the segments (in Aprostocetus gratus Giraud, Fig. 9). Elongation of the basal funicular segments may be explained by the need to protract the apical claval and funicular segments forward, since these segments per-form the sensory function most intensively. However, in some species, e.g., Aprostocetus phillyreae Do-menichini, elongation of the basal segment is not ac-companied by transfer of the sensory load onto the apical segment; instead, the sensory apparatus is con-centrated on the 2nd funicular segment (Fig. 5). Short-

ening of the 3rd funicular segment is often observed, mostly in species with a flattened body, whereas in Syntomosphyrum calamarius Graham the 2nd segment is also shortened (Fig. 11). Females of these species spend most of their life in confined spaces where long antennae would hinder their movements; and shorter antennae reduce the risk of damage to the fla-gellum.

The clava. Females of most tetrastichine wasps possess a well-delimited 3-segmented clava which is usually wider than the funicle (Figs. 1, 3, 13, 21, 22). The claval segments separated by distinct sutures are typical of most species of the genus Tetrastichus (Fig. 30). All the intermediate states in transition from a 3-segmented to a 1-segmented clava can be observed in different species of tetrastichines. Representatives of the genera Aprostocetus (Fig. 6) and Ootetrastichus (Fig. 25) have the 2-segmented clava, and in some species of Aprostocetus the clava is 1-segmented (Fig. 10). Unlike the funicle, the clava is more often oligomerized by merging of its segments, which is indicated by the presence of 3 rows of rod-like and mushroom-shaped sensilla on a 1-segmented clava (in nearly all the cases) and by barely noticeable traces of sutures in the clava of some species. A 2-segmented clava may be a result either of reduction of the apical segment, or by elimination of the suture between the 2nd and the 3rd segments. Many species demon-strate an initial stage of reduction of the apical segment (Figs. 2, 26–28, 34, 35). In some species the apical claval segment is clearly reduced, lacking rod-like and mushroom-shaped sensilla (Figs. 4, 32, 33). Oligomerization of the antenna in female tet-rastichine wasps may also be manifested by loss of homonomy of the claval segments. In many species these segments vary in size; sometimes the apical segment has no mushroom-shaped sensilla whereas the basal and middle segments possess the complete set of sensilla.

A special way of formation of the clava in females is inclusion of the apical funicular segment into it. In females of Aprostocetus gratus Giraud (Fig. 9), the 1st and 2nd claval segments are merged while the apical segment is reduced; however, the clava remains 2-seg-mented through incorporation of the apical funicular segment. Reduction of claval and funicular segments may be associated with a decreasing sensory load.

Chaetotaxy. The antennal hairs are shorter in fe-males than in males. The antennae of some species,



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such as Tetrastichomyia clisiocampae Ashmead (Fig. 14), have denser and occasionally longer hairs, including the pedicel. Dense hairs on the funicle are characteristic of species of the genus Tetrastichus (Fig. 30). In species of the genus Aceratoneuromyia the apical claval segment bears a very long hair (Fig. 29).

The Antennae of the Male

In most tetrastichine species the antennae are more homonomous in males than in females, and have less differentiated parts of the flagellum (Fig. 53). Males with a 3-segmented clava have a 4-segmented funicle, and those with a 2-segmented clava have a 5-seg-mented funicle, e.g., Aprostocetus marinikius Kostju-kov (Fig. 37). This pattern suggests that the 3-seg-

mented clava was formed by incorporating the apical (5th) funicular segment, first in females and then in males. In females of Aprostocetus gratus Giraud (Fig. 9) the clava includes the 3rd funicular segment which in this species is the apical one. The equally long hairs on the claval and funicular segments, as well as more distinct segmentation of the clava in males, indicate more archaic morphology of the flagel-lum in males as compared with females.

The scape. On the ventral side of the scape in male tetrastichines there is a sensory plaque of varying shape and size; this structure is absent in representa-tives of other subfamilies of Eulophidae. Small, “unilocular” sensory plaques are more often present in males with relatively primitive antennae, for example, in Mischotetrastichus nadezhdae Kostjukov (Fig. 48;

Figs. 30–35. Antenna of a female [(30, 31, 33) left; (32, 34, 35) right] (after Domenichini, 1956; Kostjukov, 1977c; (Graham, 1987, 1991): (30) Tetrastichus coeruleus Nees (right view); (31) Kolopterna salina Graham (right view); (32) Mischotetrastichus nadezhdae Kostjukov (left view); (33) Pronotalia fiorii Domenichini (left view); (34) Melittobia acasta Walker (right view); (35) Pronotalia oro-banchiae Graham (right view).

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the genus Mischotetrastichus is characterized by ar-chaic morphology of the wing and the gaster). This type of a sensory plaque is also characteristic of Apo-tetrastichus postmarginalis Bouček (Fig. 36), a spe-cies with primitive wing venation. Males with ad-vanced antennal morphology have long “multilocular” sensory plaques (Figs. 42, 44, 46, 50, 51). For exam-ple, in species of Tetrastichus the plaque occupies more than half (0.6–0.8) the length of the scape. The position of the sensory plaque varies from the very

apex of the ventral part of the scape (Aprostocetus domenichinii Erdös, Fig. 39) to its basal part (many species of the epicharmus group in the genus Aprosto-cetus, Fig. 38).

The fact that unilocular sensory plaques can be found in species with the most archaic (for Tetra-stichinae) antennal morphology, whereas multilocular plaques occur in all the species with more specialized antennae, suggests the following scenario. The plaque

Figs. 36–44. Antenna of a male [(36–38, 42, 43) left; (39–41, 44) right] (after Bouček, 1971; Graham, 1987, 1991; Kostjukov, 1989): (36) Apotetrastichus postmarginalis Bouček (right view); (37) Aprostocetus marinikius Kostjukov (left view); (38) A. lysippe Walker (right view); (39) A. domenichinii Erdös (right view); (40) A. phillyreae Domenichini (right view); (41) Ootetrastichus rufus Bakkendorf (right view); (42) O. ibericus Graham (right view); (43) Chrysotetrastichus truncatulus Graham (right view); (44) Ch. masculinus Gra-ham (right view).



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originated as a “unilocular” structure; later, as the funicle lost part of its sensilla, the plaque compensated for the weakened sensory function of the funicle by increasing its own surface, i.e., the number of “cells.” An increase in the number of “cells” is certainly a case of polymerization. According to Dogiel (1954), po-lymerization is an evolutionary process of increasing the number of homodynamic organs which, however, is much rarer than a decrease in the number of ho-mologous organs. Besides its original mechanical function, the scape of male tetrastichine wasps ac-quires a sensory function due to the appearance of a novel structure, the sensory plaque.

The various modifications of the antennae consid-ered above for females are also realized in males, and

it is in males that they often reach the most advanced state. In the primitive condition, the scape is rod-shaped. Changes in this structure may involve dilation in its proximal part, as in Ootetrastichus ibericus Gra-ham (Fig. 42) or in its distal part, as in Pronotalia inflata Graham (Fig. 51) and Melittobia acasta Walker; in the latter species dilation is combined with the appearance of a groove in the apical part (Fig. 52). In Oomyzus incertus Ratzeburg (Fig. 46), Baryscapus berhidanus Erdös (Fig. 60), and Aprostocetus phillyreae Domenichini (Fig. 40) the scape is dilated in the shape of a plate or disk.

The pedicel. In males, similarly to females, this segment has quite uniform morphology. In Ootetrasti-chus rufus Bakkendorf the pedicel is elongate and

Figs. 45–52. Antenna of a male [(45–47, 49, 50) right; (48, 51, 52) left] (after Kostjukov, 1977c; Graham, 1991): (45) Oomyzus galeru-civorus Hedqvist (right view); (46) O. incertus Ratzeburg (right view); (47) O. scaposus Thomson (right view); (48) Mischotetrastichus nadezhdae Kostjukov (right view); (49) Crataepus marbis Walker (right view); (50) Pronotalia carlinarum Szelényi et Erdös (right view); (51) P. inflata Graham (left view); (52) Melittobia acasta Walker (right view).

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narrow, and the basal funicular segment is also elon-gate (Fig. 41); in some other species the pedicel is externally quite similar to the funicular segments (Figs. 49, 50). Males and females of the genus Gorio-phagus Graham are characterized by a peculiar sculp-ture of both the scape and the pedicel.

The anelli. The antennae of male tetrastichine wasps include from 1 to 3 anelli, usually one anellus fewer than in females of the same genus. For example, males of the genus Sigmophora have 2 subdiscoid anelli while females of this genus have 3 anelli. Males of species of the lycidas group of the genus Aprosto-cetus have 3 anelli while females usually have 4 anelli.

The funicle. Males of most tetrastichines possess a well-developed 4-segmented funicle. Analysis of the funicle morphology allowed the species to be arranged in a certain morphological series reflecting the process of funicle oligomerization. The number of funicular segments decreases by reduction of the basal segment which starts with a slight decrease in its size, as in

Baryscapus transversalis Graham (Fig. 54). Further shortening of the basal segment is accompanied by loss of mushroom-shaped and rod-like sensilla, whereas the sensory hairs are retained, as in Chryso-tetrastichus truncatulus Graham (Fig. 43) and Oomy-zus scaposus Thomson (Fig. 47). Still further, having lost a considerable part of its sensory function, the basal segment is diminished and transformed into an anellus which initially retains its hairs, as in all the species of the group gallerucae of the genus Oomyzus. Finally, the funicle becomes 3-segmented, as in Oomy-zus galerucivorus Hedqvist (Fig. 45). The process of oligomerization does not stop at the 3-segmented stage but continues with reduction of mushroom-shaped sensilla on the 2nd, 3rd, and 4th funicular segments (Fig. 59) and is completed with the loss of rod-like sensilla (Fig. 55). However, diminution of the basal funicular segment does not always result in loss of sensilla. For example, Baryscapus daira Walker and some other species of this group have a transverse basal funicular segment which still bears rod-like sen-

Figs. 53–60. Antenna of a male [(54–56) left; (57–60) right] (after Graham, 1987, 1991): (53) Hyperteles luteus Ratzeburg; (54) Bary-scapus transversalis Graham (right view); (55) B. endofiticus Domenichini (left view); (56) B. talitzkii Kostjukov (left view); (57) B. papaveris Graham (right view); (58) Puklina amblyteles Graham (right view); (59) Baryscapus crassicornis Erdös (right view); (60) B. berhidanus Erdös (right view).



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silla. It is worthy that all the funicular segments in this species are considerably reduced in size. Males of Melittobia acasta Walker (Fig. 52) possess highly modified antennae: the funicle is subdivided into 2 different parts, the basal funicular segment is hyper-trophied while the remaining 3 segments are short and narrow, still looking as a funicle.

Reduction of the sensory apparatus of the funicle has proceeded in two ways. The first way consists in shifting of the sensilla toward the clava (in Baryscapus papaveris Graham, Fig. 57, B. berhidanus Erdös, Fig. 60, and Puklina amblyteles Graham, Fig. 58) due to the shortening of the segments which become trans-verse. The second way of reduction of the sensory apparatus consists in distribution of very scarce sen-silla over the entire funicle (Figs. 49, 54, 56, 59). Re-duction of the sensory apparatus of the whole funicle is observed in Melittobia acasta Walker (Fig. 52), in which all the funicular segments (1–4th) bear only hairs; in fact, the funicle of this species can be only conventionally considered 4-segmented because its 2nd–4th segments are anelli with hairs (Fig. 52).

The loss of homonomy, as well as reduction of fu-nicular segments, is one of the mechanisms of oligomerization that is typical of many species (Figs. 43, 45, 46, 51, 52).

The clava. Unlike the funicle, the clava in male tet-rastichine wasps is more frequently oligomerized by merging of segments which may be realized in a greater variety of ways than in females. The most oligomerized clava is observed in Aprostocetus marinikius Kostjukov (Fig. 37): its 1st claval segment is separated from the remaining two by a stalk identi-cal to that present between the funicular segments, and is itself more similar to a funicular segment. The scape of this species has a unilocular sensory plaque, indicat-ing the archaic morphology of the whole antenna in which the claval segments are not yet differentiated from the funicular ones. A clava subdivided by only two thin sutures can be found in many species (Figs. 50, 52, 56–59).

Some species have acquired a 2-segmented clava as the result of reduction of its apical segment; this is the second way of clava reduction in males. The initial stage of reduction of the apical segment is marked with the presence of an indistinct suture and the ab-sence of the third row of rod-like sensilla. The clava of Baryscapus papaveris Graham (Fig. 57) and Prono-talia carlinarum Szelényi et Erdös (Fig. 50) is divided

by only one thin stripe and bears only 2 rows of sen-silla, the apical segment being aborted in these spe-cies. On the other hand, some species, such as B. talitzkii Kostjukov (Fig. 56), have a 3-segmented clava with only 2 rows of sensilla but the apical claval segment is not reduced. The most oligomerized clava is that of Pronotalia inflata Graham (Fig. 51), which retains only thin hairs but not rod-like or mushroom-shaped sensilla. A rudimentary segment at the clava apex, described in female tetrastichine wasps, is also typical of males, being more frequently found in spe-cies with archaic antennal morphology.

Thus, evolutionary transformations of antennae in tetrastichine wasps include oligomerization which proceeds by reduction and merging of antennal seg-ments, their modification and loss of homonomy, and polymerization manifested by an increase in the num-ber of cells of the sensory plaque on the scape in males. These processes proceed at different rates in males and females, resulting in sex dimorphism.

ACKNOWLEDGMENTS The author is sincerely grateful to her scientific ad-

visor V.V. Kostjukov for his help during this research, and to A.F. Emeljanov (the Zoological Institute of the Russian Academy of Sciences, St. Petersburg) for his considerable participation in the preparation of this paper.

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Oligomerization and other evolutionary changes in the antennae of tetrastichine wasps (Hymenoptera, Eulophidae)