Morphology of antennal sensilla of the brown spruce longhorn beetle, Tetropium fuscum (Fabr.) (Coleoptera: Cerambycidae)

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Arthropod Structure & Development

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Morphology of antennal sensilla of the brown spruce longhorn beetle,Tetropium fuscum (Fabr.) (Coleoptera: Cerambycidae)

Colin A. MacKay a,b,*, Jon D. Sweeney b, N. Kirk Hillier a

aDepartment of Biology, Acadia University, 33 Westwood Ave., Wolfville, Nova Scotia B4P 2R6, CanadabNatural Resources Canada, Canadian Forest Service-Atlantic Forestry Centre, PO Box 4000, Fredericton, New Brunswick E3B 5P7, Canada

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Article history:Received 22 December 2013Received in revised form11 April 2014Accepted 11 April 2014

Keywords:UltrastructureMorphologyOlfactoryGustatorySensillum

Abbreviations: EAG, electroantennogram; GP, groobasiconica; ORN, olfactory receptor neuron; SC, senselectron microscopy/micrograph; SSR, single sensillucodea; TEM, transmission electron microscopy/micsensilla.* Corresponding author. Department of Biology, Aca

Ave., Wolfville, Nova Scotia B4P 2R6, Canada.E-mail address: [email protected] (C.A. MacKa

http://dx.doi.org/10.1016/j.asd.2014.04.0051467-8039/� 2014 Published by Elsevier Ltd.

Please cite this article in press as: MacKay, C(Fabr.) (Coleoptera: Cerambycidae), Arthrop

a b s t r a c t

The antennal sensilla of the brown spruce longhorn beetle, Tetropium fuscum (Fabr.) (Coleoptera:Cerambycidae) were examined with particular focus on the sensilla present on the apical flagellomere.T. fuscum antennae are composed of 11 segments, namely the scape, pedicel, and nine flagellomeres. Ninetypes of sensilla were observed: three types of sensilla chaetica, sensilla trichodea, two types of sensillabasiconica, grooved peg sensilla, thick-walled sensilla, and Böhm bristles. Seven of these types werepresent on the apical flagellomere, the exceptions were sensilla chaetica type 3 and Böhm bristles. Therewere no significant differences in the distribution or density of sensilla present on the ninth flagellomereof males and females, except that males had significantly more sensilla chaetica type 1, which are putforward as the putative contact chemoreceptors for T. fuscum.

� 2014 Published by Elsevier Ltd.

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1. Introduction

Longhorn beetles respond to olfactory cues such as plant vola-tiles (Ikeda et al., 1980; Chénier and Philogène, 1989; Hanks, 1999;Allison et al., 2004; Pajares et al., 2004) and pheromones (Laceyet al., 2004, 2007; Ray et al., 2009; Pajares et al., 2010; Hanks andMillar, 2013) when foraging for food, oviposition sites or mates.Tetropium fuscum (F.), as well as congeners Tetropium cinnamopte-rum Kirby in Richardson and Tetropium castaneum (L.), are attractedto a synthetic blend of spruce volatiles (racemic a-pinene, (�)-b-pinene, (þ)-3-carene, (þ)-limonene, and a-terpinolene), and thatattraction is synergized by the addition of ethanol, which is emittedat greater rates from stressed trees (Sweeney et al., 2004, 2006,2010). Males of T. fuscum and T. cinnamopterum emit the same ag-gregation pheromone (fuscumol) that, when combinedwith sprucevolatiles, synergizes attraction of both males and females of thosetwo species (Silk et al., 2007) and T. castaneum (Sweeney et al.,2010). Fuscumol was the first homoterpenoid alcohol to be

ved peg sensilla; SB, sensillailla chaetica; SEM, scanningm recording; ST, sensilla tri-rograph; TW, thick walled

dia University, 33 Westwood

y).

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.A., et al., Morphology of anteod Structure & Development

discovered from cerambycids and the first pheromone to bedescribed from the subfamily Spondylidinae (Silk et al., 2007).Traps baited with fuscumol and spruce volatiles have been used inoperational surveys to determine the distribution of T. fuscum inNova Scotia (NS), Canada for regulatory purposes since 2007(Sweeney et al., 2010), and pheromone-based tactics such as masstrapping and mating disruption may have potential use for sup-pression of T. fuscum populations.

The more fundamentally we understand how insects, particu-larly longhorn beetles such as T. fuscum, use smell, taste and othersenses to locate hosts and mates, the more likely we will developeffective semiochemical-based methods for managing those spe-cies that are pests. To this end, our objectives were to describe themorphology of antennal sensilla on T. fuscum, following themethods of Crook et al. (2008a,b) in their examination of theantennal sensilla of the woodwasp Sirex noctilio Fabr. (Hymenop-tera: Siricidae), and the emerald ash borer, Agrilus planipennisFairmaire. We describe the external and internal structure ofdifferent types of antennal sensilla observed on T. fuscum, andcompare the density of each type of sensillum on the ninth flag-ellomere of male and female T. fuscum for evidence of sexualdimorphism. We predicted that males would have more contactchemoreceptors at the tips of their antennae than females becausemales of many cerambycid species, including T. fuscum, recognizeconspecific females by antennal contact with specific cuticularhydrocarbons on the surface of the female elytra (Ginzel and Hanks,2003; Ginzel et al., 2003, 2006; Silk et al., 2011). We also predicted

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nnal sensilla of the brown spruce longhorn beetle, Tetropium fuscum(2014), http://dx.doi.org/10.1016/j.asd.2014.04.005

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that females would possess a greater number of olfactory sensillathan males because, although both sexes are synergisticallyattracted to the combination of host volatiles and fuscumol (themale-emitted pheromone), there is often a significant female biasin trap catches (Sweeney et al., 2010).

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2. Methods

2.1. Beetles

Sections of the main stem (i.e., bolts about 35 cm long by 18e25 cm diam.) were cut from red spruce trees in Hemlock RavinePark, Bedford, NS (44.690278�N, 63.668056�W) in the winter of2009e2010, incubated at 20e22 �C, 40e65% RH, in Plexiglas� cagesin containment facilities at the Atlantic Forestry Centre, NaturalResources Canada e Canadian Forest Service, Fredericton, NewBrunswick (NB), Canada, and checked 5 d per week for emergenceof adult T. fuscum. Adults that emerged in the spring of 2010 wereexamined for antennal deformities before selection, and the largestpieces of frass and other debris were removed with a paintbrush.

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2.2. Scanning electron microscopy

Ten beetles (five males and five females) were killed by placingthem in a �18 �C freezer for 4e8 h. Beetles were soaked overnightin a 4% Triton X-100 solution to remove debris and lipids and thendehydrated for 5 min each in 70% ethanol and 95% ethanol (Rayet al., 2006). Following dehydration, specimens were soaked inhexane for 2 h, sonicated (ultrasonic bath) (Branson 5200, BransonUltrasonic Corp., Danbury, CT) for 30 s in hexane to remove anyremaining lipids, and then allowed to air dry.

Using a clean, new razor blade, the heads were removed and cutin half. Both halves of each head were mounted on the samealuminum stub using double-sided carbon tape so that antennaewere as perpendicular as possible to the stub (Fig. 1a). They werethen fixed in place with conductive colloidal graphite with an iso-propanol base, coated with carbon using carbon evaporation(Edwards E12E carbon evaporation unit, Crawley, England) andsputter coated with gold (Edwards S150A Sputter coating unit,Crawley, England). Scanning electron microscopy (SEM) was per-formed using a scanning electron microscope (JEOL JSL 6400,

Fig. 1. aeb. Scanning electron micrographs of (a) mounted male Tetropium fuscum head, latflagellomeres with arrowhead indicating pseudo-segment on distal ninth flagellomere. Ori

Please cite this article in press as: MacKay, C.A., et al., Morphology of ante(Fabr.) (Coleoptera: Cerambycidae), Arthropod Structure & Development

University of New Brunswick (UNB), Fredericton, NB) at 5 kV. Foreach specimen, at least three micrographs of each antennalsegment were taken around the circumference of the left antenna(right halves reserved as spares) to enable a full 360-degree view.When subsequent examination of images revealed gaps in thecircumferential view of a given specimen, additional SEM wasperformed in the Acadia Centre for Microstructural Analysis(ACMA) lab at Acadia University using a JEOL JSM 5900 LV at 25 kV.

Of the 10 beetles that were micrographed using SEM, twoantennae of each sex were completely analyzed in a preliminaryexamination (MacKay, 2010). To increase efficiency and providecomparisons among multiple beetles, data from six other beetles,three male and three female, were collected only from the mostdistal flagellomere (F9).

2.3. Transmission electron microscopy

Antennae from two male and two female T. fuscum were pre-pared for transmission electron microscopy (TEM) by injecting0.1 M glutaraldehyde in 0.05 M Na cacodylate buffer, pH 7.4 with0.1 M sucrose directly into live beetles before antennae wereremoved and submerged in fixative for 3 h at 20 �C (Lucarotti,2000). Antennae were post-fixed in 1% OsO4 in 0.05 M Na caco-dylate buffer pH 7.4, dehydrated and embedded in Epon-Araldite asdescribed by Lucarotti (2000). Serial sections 1 mm thick were cutfrom the tip of one antenna using a Diatome Histo knife on an RMCMT-7000 ultra microtome. Cuts were made prior to sectioning toexpose areas of interest identified from SEM images. Sections wereplaced on clean, glass slides and stained with toluidine blue (EMS)(Lucarotti et al., 2012). Preliminary TEM work was done using aJEOL 2001 STEM (UNB, Fredericton, NB) at 200 kV. Micrographswere taken at three different locations along F9 (tip, “sensillarfield”, midpoint) as well as from the apical face of F8.

Further sectioning and TEM was done using a Phillips 301 TEM(ACMA, Acadia University, Wolfville, NS). Sections were taken fromthe “sensillar field” present on the ninth flagellomere, as well asfrom the distal face of F8.

2.3.1. Sensilla classificationSensilla were classified as chaetic, trichoid, or basiconic types

based on external morphology from SEM images (Altner and

eral view, and (b) female T. fuscum antenna showing scape (S), pedicel (P) and all nineentation bar, A ¼ anterior, V ¼ ventral.

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Table 1Mean numbers (�SD) of different types of sensilla on the ninth flagellomere of fivemale and five female Tetropium fuscum, with statistical comparisons (Welch Two-sample t-test for unequal variances, N ¼ 5 ea. sex), and pooled male and femalemean sensillar lengths (�SD) in mm (N ¼ 20 ea. type). Values in square brackets arenumber of sensilla per mm2 surface area. S. chaetica 3 (SC3) has no values as it wasnot present on F9. Length measurements were taken from sensilla present on F9except for SC3, which were taken from the sensilla on the scape.

Sensilla type Number on F9 Length(sexespooled)

Males Females t-Value df p

S. chaetica 1 77.8 (14.38)[0.39] (0.08)

39.2 (13.16)[0.29] (0.05)

�4.43[�2.36]

7.94[6.92]

0.0022[0.051]

64.8 (6.5)

S. chaetica 2 268.4 (45.94)[1.34] (0.21)

209.4 (66.16)[1.55] (0.38)

�1.64[1.06]

7.13[6.31]

0.14[0.33]

31.6 (4.0)

S. chaetica 3 e e e e e 153.9 (23.7)S. trichodea 89.8 (31.02)

[0.45] (0.16)52.6 (9.40)[0.41] (0.13)

�2.57[�0.43]

4.73[7.85]

0.053[0.68]

34.3 (4.9)

S. basiconica 1 319.4 (16.92)[1.60] (0.14)

255.4 (97.18)[1.83] (0.27)

�1.45[1.70]

4.24[5.81]

0.22[0.14]

9.1 (0.9)

S. basiconica 2 163.8 (36.95)[0.82] (0.21)

139.2 (42.03)[1.03] (0.24)

�0.98[1.46]

7.87[7.89]

0.36[0.18]

17.0 (3.1)

Thick walled 9.8 (2.77)[0.05] (0.02)

6.2 (3.27)[0.05] (0.02)

�1.88[0]

7.79[7.72]

0.098[1]

2.8 (0.3)

Total 929 (115.4)[4.65] (0.65)

702 (220.7)[5.15] (0.98)

�2.04[0.96]

6.04[6.97]

0.087[0.37]

e

Bifid 1.4 (1.1) 2.6 (1.8) e e e e

Fig. 2. Ninth flagellomere of a female Tetropium fuscum antenna showing the distribution o(SB1, SB2). The high density SB1 sensillar field is outlined in white.

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Prillinger, 1980; Zacharuk, 1980; Keil, 1999), and their possiblefunction (olfactory, gustatory, etc.) was suggested from ultrastruc-ture observed in TEM images and compared with other Coleoptera(Hallberg, 1982; Isidoro et al., 1998; Bartlet et al., 1999; Sen andMitchell, 2001; Lopes et al., 2002; Crook et al., 2008a,b). Based onultrastructure, the three main types are: wall pore sensilla, whichare typically olfactory; tip pore sensilla, which are typically gusta-tory or mechanosensory; and aporous sensilla, which are typicallymechanoreceptors (Altner and Prillinger, 1980).

2.4. Analysis

Scanning electron micrographs were analyzed, and each type ofsensilla counted using the image processing software ImageJ(Rasband, 1997). Micrographs of each segment were first visuallyinspected for points of reference that could be used to delineate thedifferent micrographs so that areas were not counted twice ormissed. Color-coded dots were added to the different types ofsensilla to avoid counting any sensilla more than once. Each time adot was added, the program would save the coordinates of eachpoint. The mean number and density of each sensillar type on F9was compared between male and female T. fuscum using the Welcht-test (R Development Core Team, 2011). Surface area (mm2) of F9was estimated using the formula for a cylinder (pD � L) and a cone

f s. chaetica types 1 and 2 (SC1, SC2), s. trichodea (ST), and s. basiconica types 1 and 2

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Fig. 3. (a) Scanning electron micrograph of several sensilla chaetica type 1 (SC1) at thetip of a female Tetropium fuscum antenna. (b) Transmission electron micrograph of aSC1 from the ninth flagellomere of a female T. fuscum with pore channels indicated bywhite arrowheads. (c) Scanning electron micrograph of two sensilla chaetica type 2(SC2) at the base of a SC1 on the seventh flagellomere of a female T. fuscum (whitearrowheads). (d) Transmission electron micrograph of a SC2 on the ninth flagellomereof a female T. fuscum showing the sensillar lumen (SL) free of dendritic material. (e)Scanning electron micrograph of a sensilla chaetica type 3 (SC3) on the second flag-ellomere of a female T. fuscum.

Fig. 4. (a) Scanning electron micrograph of a sensillum trichodea (ST) (white arrow-head) on the ninth flagellomere of a female Tetropium fuscum. (b) Transmission elec-tron micrograph of a ST from the ninth flagellomere of a female T. fuscum showing porechannels (black arrowheads).


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(pr � S), where D ¼ diameter, L ¼ length, r ¼ radius, and S ¼ lengthof cone side.

3. Results

T. fuscum antennae have 11 segments: scape, pedicel, and nineflagellomeres. F9 appeared on initial inspection to be divided intotwo different sections (pseudo-segments) (Fig. 1b), but upon SEMexamination, what appeared to be a division between flagellomereswas determined to be a grouping of chemoreceptors.

Nine differentmorphological types of sensillawere found on theantennae of T. fuscum. On average, males had more of each type ofsensilla than females, but the differencewas significant only for SC1and ST (Table 1). When sensillar density was expressed per mm2 ofsurface area to account for flagellomere size, results were similar,but only SC1 density was significantly different between males andfemales (Table 1). Mean surface area of F9 was greater on malesthan females but not significantly (Welch’s t ¼ �2.59, p ¼ 0.057).Statistical analyses, however, were limited by small sample size.

3.1. Sensilla chaetica type 1 (SC1)

SC1 were long, robust, bristle-like sensilla, 64.8 � 6.5 mm inlength (Table 1) that extended roughly perpendicular to the surfaceof the antennal body except at the tip, where they extended beyondthe end of the antenna roughly parallel to the antennal body. On F9,SC1 were distributed sporadically around the circumference prox-imally, but were most concentrated at the distal tip of the flag-ellomere (Figs. 2 and 3a). SC1 had pore channels and moreinnervations than puremechanoreceptors (Fig. 3b), suggesting theymay have some gustatory function as well.


SC2 were, 31.6 � 4.0 mm long, robust, bristle-like sensilla(Table 1) and extended from the antenna at roughly a 20� angle(Fig. 3c). On F9, SC2 were distributed evenly around the circum-ference (Fig. 2). SC2 had no evidence of pore channels and limitedinnervation (Fig. 3d), suggesting they may be mechanoreceptors.


SC3 were 153.9 � 23.7 mm long, robust, bristle-like sensilla(Fig. 3e, Table 1). Based on preliminary data, SC3were present on allantennomeres except F9, concentrated on the scape, pedicel anddorsal side (MacKay, 2010). The internal ultrastructure of SC3 wasnot examined.

3.4. Sensilla trichodea (ST)

ST were 34.3 � 4.9 mm long, hair-like sensilla that were pointedapically (Table 1), and lay relatively parallel to the body of the an-tenna (Figs. 2 and 4a). The STweremultiporous (Fig. 4b), suggestingthey may be olfactory receptors.

3.5. Sensilla basiconica type 1 (SB1)

SB1 were short, peg-like sensilla, 9.1 � 0.9 mm in length(Table 1). On F9, SB1 were patchily distributed over the surface butwere concentrated on the “sensillar field” (Figs. 2 and 5a), a band ofSB1 that appeared to create a pseudo-segment (Fig. 1b) asmentioned above. Based on preliminary data, SB1 were present onF1 and F3e9 and were typically concentrated on the distal portionsof the ventral side of the segments (MacKay, 2010). During thepreliminary morphological survey, SB1 were originally believed to


be peg-like contact chemoreceptors like those found on theantennae of the emerald ash borer (Crook et al., 2008a). Internalultrastructural analysis of the sensillar field of the eighth flag-ellomere revealed that they were in fact multiporous olfactorysensilla (Fig. 5b).


Fig. 5. (a) Scanning electron micrograph of a field of sensilla basiconica type 1 (SB1) on the ninth flagellomere of a female Tetropium fuscum. (b) Transmission electron micrograph ofa sensillum basiconica on the eighth flagellomere of a female T. fuscum with pore channels clearly visible around the circumference (black arrowheads). (c) Scanning electronmicrograph of two sensilla basiconica type 2 (SB2) on the ninth flagellomere of a female T. fuscum.

Fig. 6. (a) Transmission electron micrograph of a grooved peg sensillum on the eighthflagellomere of a female Tetropium fuscum. (b) Scanning electron micrograph of agrooved peg sensillum on the ninth flagellomere of a female T. fuscum; (c) Trans-mission electron micrograph of a thick-walled sensillum on the eighth flagellomere ofa female T. fuscum. (d) Scanning electron micrograph of the candidate for thick-walledsensillum on the ninth flagellomere of a female T. fuscum.


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3.6. Sensilla basiconica type 2 (SB2)

SB2 were slightly longer (17.0 � 3.1 mm) than SB1 and less bluntat the tip (Table 1). Based on preliminary data, SB2 were present onF3e9 and were typically concentrated on the distal portions of theventral side of the segments (MacKay, 2010). On F9, SB2 werepatchily distributed, with fewer at the very tip of the flagellomere(Figs. 2 and 5c). SB2 have a similar highly branched dendritic ul-trastructure and multiporous walls to SB1, suggesting they are alsoolfactory sensilla (Fig. 5b).

3.7. Grooved peg sensilla (GP)

The distribution of GP sensilla is unknown, but they wereconfirmed on F8 and F9 from TEM sections (Fig. 6a). Only one GPwas observed in the SEM micrographs (Fig. 6b) out of tenspecimens.

3.8. Thick walled sensilla (TW)

The distribution of TW sensilla is unknown, but they wereobserved on F8 in TEM sections (Fig 6c). TEM images could not besatisfactorily correlated with the SEM images, but a short(2.8 � 0.3 mm (Table 1)) nub-like candidate for TW sensilla wasfound on F9 (Fig 6d).

3.9. Other sensilla

Böhm bristles (Fig. 7a) were found in the preliminary exami-nation of the entire antenna at the junctions between the body andscape, and the scape and pedicel (MacKay, 2010), but they were notcounted nor was their ultrastructure examined. Y-shaped bifidsensilla (Fig. 7b) were found on F9 of all females and four out of fivemales examined. Most specimens had only one or two of thesesensilla, but one male had three relatively close together (Fig. 7c)and one female had five (Table 1). Due to their low frequency ofoccurrence, their unknown distribution on F3eF8, and becausetheir ultrastructure was not determined, they are not included asone of the sensillar types described.

4. Discussion

We observed nine different types of sensilla on the antenna ofT. fuscum, based on morphology and ultrastructure from SEM andTEM imagery, respectively. Compared with other Cerambycidae,T. fuscum had more types of sensilla than the soybean stem borer,Dectes texanus texanus LeConte (which had five) (Crook et al., 2003),


the same number as Leptura arcuata Panzer and Leptura aethiopsPoda (Zhang et al., 2011), and fewer than the yellow longicorn,Phoracantha recurvaNewman,whichhad 12 types (Faucheux, 2011).

Slightly more than half of all sensilla present on F9 appeared tobe olfactory in nature (SB1, SB2, ST) in both females (64%) andmales (62%). The proportion of all sensilla over the entire antennathat are olfactory may be lower because preliminary observationsindicated that densities of olfactory sensilla were highest on thedistal flagellomeres, specifically F9. Zhang et al. (2011) observedgreater numbers of sensilla basiconica on apical than on basalflagellomeres in L. aethiops. T. fuscum had fewer of each type ofsensilla than L. aethiops, but their apical flagellomeres are muchshorter than those of L. aethiops.

For each type of sensilla observed on F9 of T. fuscum, males had1.2e2.0 times as many as females, but the differencewas significant


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Fig. 7. Scanning electron micrographs of (a) Böhm bristles (BB) present at the junction between the scape (S) and pedicel (P) of a female Tetropium fuscum; (b) Y-shaped bifidsensilla present on the ninth flagellomere of a female T. fuscum; and (c) three bifid sensilla (white arrows) present on the ninth flagellomere of a male T. fuscum.


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only for SC1 and ST. When numbers of sensilla were expressed permm2 surface area, the differencewas significant only for SC1. As SC1appear to be gustatory in function, this supports our hypothesisthat males have more gustatory receptors than females. Thesesensilla may play a role in mate recognition and perception ofspecific hydrocarbons on the cuticle of female T. fuscum that elicitcourtship behavior in T. fuscum (Silk et al., 2011). This type ofsensilla chaetica has been observed in other beetles such as thecabbage stem flea beetle, Psylliodes chrysocephala L. (Coleoptera:Chrysomelidae) (Isidoro et al., 1998). SC1 may also be sensitive tohost plant compounds, as is the case with P. chrysocephala. SC2mayserve a protective function, as was suggested by Crook et al. (2003)in their examination of the antennae of D. texanus texanus, as wellas the typical mechanosensory function of sensilla chaetica (Zhanget al., 2011).

Females did not possess significantly more olfactory sensillathan males, contrary to our prediction based on a female bias intraps baited with fuscumol and host volatiles. This suggests thatfemale T. fuscummay differ frommales in the number or sensitivityof fuscumol-sensitive olfactory receptor neurons (ORNs) present inthe olfactory sensilla or that the sexes differ in theway they processsignals from fuscumol and/or host volatile ORNs.

Grooved peg sensilla are present in other beetle species(Whitehead, 1981; Hallberg, 1982; Bartlet et al., 1999) and arecomparable to what Zhang et al. (2011) described as SB3 in Lepturaspp. Based on ultrastructure, they are double-walled s. basiconica,which aremost often chemoreceptors, although some also exhibit athermoreceptive function (Altner and Prillinger, 1980; Bartlet et al.,1999). The thick-walled sensilla candidate is comparable in externalmorphology to sensilla described on Leptura spp. and termed SB4by Zhang et al. (2011), but the density of SB4 on Leptura was muchgreater than that of the thick-walled sensilla we observed onT. fuscum. Also, we observed thick-walled sensilla distributedsporadically on F9 on T. fuscum antennae, whereas Zhang et al.(2011) observed SB4 distributed only on F1 on Leptura spp.

A Y-shaped bifid sensillum was observed on the ninth flag-ellomere of nine out of ten specimens in low, varying densities (1e5), and its function is unknown. Saïd et al. (2003) found bifidsensilla on the antennae of the palm weevil, Rhynchophorus pal-marum (L.) (Coleoptera: Curculionidae), but they were striated anddid not show any sign of innervation, leading the authors to spec-ulate a protective function. Bourdais et al. (2006) found morpho-logical alterations, such as bifid sensilla, in individuals of theparasitoid Aphidius rhopalosiphi DeStefani-Peres (Hymenoptera:Braconidae) that were exposed to excessive cold or heat stresseswhile pupating. As the Y-shaped sensilla on T. fuscum typicallyresembled basiconic sensilla more than the striated chaetic or tri-choid sensilla of R. palmarum, and also because these sensilla were


not observed on all samples, and in varying densities when theywere present, it is likely thatmorphological alterations due to stressduring development constitute a plausible explanation for theirpresence and that they are not a distinct sensillar type.

SB1 were the most numerous sensilla present on F9. Along withSB2 and ST, their porous ultrastructure clearly indicates that theyare olfactory receptors. Identification of the olfactory receptors ofthe beetle makes further physiological investigations of these re-ceptorsmore feasible. Futurework using single sensillum recording(SSR) will enable clarification of the role of these olfactory sensillain processing pheromones, host volatiles, and non-host volatiles.Using electroantennograms (EAGs), Silk et al. (2010) proposedseveral relevant host and non-host volatiles that elicited strongantennal responses as potentially useful additions to the currenttrapping lures. Further physiological work should also focus oncontact chemoreception, using elytral cuticular hydrocarbonsidentified by Silk et al. (2011).

Acknowledgments

We would like to thank Natural Science and EngineeringResearch Council, Natural Resources Canada e Canadian ForestService, Canadian Food Inspection Agency, Canada Foundation forInnovation, Atlantic Canada Opportunities Agency e AtlanticInnovation Fund, and Acadia University for their generous funding.Also, the microscopists at University of New Brunswick Fredericton,S. Belfry and S. Cogswell, as well as H. Xu at ACMA for all their helpgetting such fantastic SEM and TEM images. We are also grateful toC. Lucarotti for much help with antennal fixing, embedding,antennal sectioning for preliminary light microscopy and TEM ex-aminations, and comments on an earlier draft of this manuscript.We thank J. Simmons and Halifax Regional Municipality forgranting us permission to fell beetle-infested trees in HemlockRavine Park. Many thanks to W. MacKay, L. Flaherty, C. Hughes, K.van Rooyen, A. Morrison, N. Harn for their assistance rearing bee-tles. Thanks also to P. Silk, T. Smith, A. Redden, and two anonymousreviewers for comments on an earlier draft of this manuscript andto C. Simpson for editorial review.

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Morphology of antennal sensilla of the brown spruce longhorn beetle, Tetropium fuscum (Fabr.) (Coleoptera: Cerambycidae)