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Spinning behaviour and morphology of the spinning glands in male and femaleAposthonia ceylonica (Enderlein, 1912) (Embioptera: Oligotomidae)

Janice S. Edgerlya, Sebastian Büsseb,∗, Thomas Hörnschemeyerb

a Department of Biology, Santa Clara University, Santa Clara, CA, USAb Johann-Friedrich-Blumenbach-Institute of Zoology and Anthropology, Department of Morphology, Systematics and Evolutionary Biology,Georg-August-University, Göttingen, Germany

a r t i c l e i n f o

Article history:Received 25 March 2011Received in revised form29 November 2011Accepted 23 December 2011

Keywords:Synchrotron radiation micro computedtomography (SR�CT)Sexual dimorphismSpinning apparatusSilk spinning behaviourEmbiidina

a b s t r a c t

Embioptera (webspinners) are unique among insects in that juvenile and adults of both sexes spin silk.They possess spinning apparatuses in the basitarsomeres of their prothoracic legs, which they use tobuild galleries as habitat and protection. Embioptera are primitively social and cooperate in building thegalleries. They also show sexual dimorphism that comprises modifications of the mandibles in males, thewinglessness of the females and differences in the morphology of the forelegs. In the present investigationwe address the correlation of spinning behaviour and sexual dimorphism in the spinning apparatus ofAposthonia ceylonica (Enderlein, 1912). To analyse spinning behaviour we conducted video observationsof Ap. ceylonica in artificial habitats. We observed females and males alone as well as female–male pairsto cover possible effects of interactions between sexes. The morphology of the spinning apparatus wasanalysed and reconstructed using high resolution X-ray computed tomography (SR�CT). The observationsshow that during trials of 24 h adult males and females produce similar amounts of silk per body weight,despite the fact that adult males do not feed, perhaps due to modifications of their mandibles related tocourtship that interfere with feeding. Spinning glands in males are distinctly smaller than in females inabsolute values, which reflect the general size difference in females and males. Despite their smaller bodysize, the volumes of reservoirs of spinning glands are larger in males in relative as well as in absolutevalues. Together with spinning behaviour and the amount of silk production, this indicates that malesproduce and store gland secretions in the large reservoirs prior to their final moult for later use.

© 2012 Elsevier GmbH. All rights reserved.

1. Introduction

Embioptera are a small group of primitively social, poly-neopteran insects, comprising about 300 described species andperhaps 2000 existing worldwide (Ross, 2009). They have a slen-der and elongate body shape (Fig. 1) with body lengths rangingbetween 8 mm and 25 mm (Günther, 2005). Webspinners havetheir highest diversity in tropical and subtropical regions but arealso represented in the Mediterranean and other semi-arid regions(Ross, 1966, 2000).

The monophyly of Embioptera is supported by both morpho-logical (Hennig, 1969; Boudreaux, 1979; Kristensen, 1991, 1995;Beutel, 2004; Grimaldi and Engel, 2005; Willmann, 2005; Beuteland Gorb, 2001, 2006; Szumik et al., 2008) and molecular anal-yses (Wheeler et al., 2001; Kjer, 2004; Terry and Whiting, 2005;Kjer et al., 2006; Szumik et al., 2008). The most distinct autapo-morphy of Embioptera is the silk-spinning apparatus located in the

∗ Corresponding author. Tel.: +49 (0)551 395451; fax: +49 (0)551 395579.E-mail address: Sebastian.Buesse@biologie.uni-goettingen.de (S. Büsse).

basitarsomeres of the prothoracic legs (e.g. Hennig, 1969, 1994).Apart from Embioptera, a spinning apparatus in the basitarsomereis only known in males of Hilarini (Diptera: Empididae) (Young andMerritt, 2003). Embioptera use silk to build galleries, which serveas both habitat and as protection against predators (Edgerly, 1988,1994; Ross, 2000; Edgerly et al., 2006). Sociality in Embiopteraas summarized by Edgerly (1997) is established by their parentalbehaviour and the mutual building and inhabiting of silk galleriesin many species. They are generally classified as colonial and/orsubsocial.

Due to previous studies of the embiopteran prothoracic leg(Melander, 1902; Enderlein, 1909, 1912; Rimsky-Korsakov, 1914;Mukerji, 1927; Barth, 1954; Alberti and Storch, 1976; Nagashimaet al., 1991; Ross, 2000) the general morphology of the basitarsalspinning apparatus is relatively well known. However, these inves-tigations did not combine comparative morphological, ecologicaland behavioural aspects, a feature of our project. An embiopteransilk gland basically consists of a reservoir surrounded by gland tis-sue. The latter secretes the silk into the reservoir where it is storeduntil needed for spinning. The glands are arranged above, amongand next to each other in the basitarsi of the forelegs (Mukerji,

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Fig. 1. Male and female Aposthonia ceylonica (Oligotomidae) showing the typicalsexual dimorphism of embiopterans.

1927; Barth, 1954; Alberti and Storch, 1976; Nagashima et al.,1991). Spinning bristles are located on the ventral surfaces of thefirst and second tarsomeres (Melander, 1902; Rimsky-Korsakov,1914; Barth, 1954; Alberti and Storch, 1976; Nagashima et al., 1991;Dubitzky and Melzer, 1999). Spinning secretion is emitted fromeach of the reservoirs through a single duct that is connected toa spinning bristle (Mukerji, 1927; Barth, 1954; Alberti and Storch,1976; Nagashima et al., 1991).

Sexual dimorphism in Embioptera has been mentioned in theliterature for several morphological characters. For instance, whilenymphs and adult females show typical orthopteroid mandibles,those of adult males have undergone morphological changes. Thespecialized male mandibles, which are often elongate and forceps-like, are used for clinging to the female prior to and duringcopulation (Friedrichs, 1934; Lacombe, 1958; Ross, 1966, 2000;Günther, 2005). The compound eyes are oval to reniform. In themale, they are of moderate size, more strongly curved and largerthan in the female. Sexual dimorphism in eye shape may be cor-related with neoteny in the females, a characteristic of all extantEmbioptera (e.g. Ross, 2000; Günther, 2005). There are also specieswith apterous males, for instance, in Australembiidae (Kaltenbach,1967; Ross, 2000); some males are neotenic, resembling nymphsand females, while others have enlarged heads and mandiblesand these morphs probably engage in fighting and mate-guarding(unpublished observations, JSE). Due to neoteny, the female geni-talia are strongly simplified (Ross, 2000; Klass and Ulbricht, 2009)

Fig. 2. Prothoracic basitarsus of female Aposthonia ceylonica reconstructed from SR�CT-data. Glt – gland tissue; res – reservoir. (A) 3D-resonstruction. (B and C) 3D-reconstruction with crosscut and single cross section, respectively, showing gland tissue and reservoirs. (D and F) 3D-reconstruction with longitudinal cut and section.

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whereas the male terminalia are complex and asymmetrical. In themale, the ninth and tenth sternites are fused and form the sub-genital plate. The supra-anal plate is usually notched or cleft andthe proximal left cercomere is modified to serve for clinging tothe female during copulation (Verhoeff, 1904; Krauss, 1911; Beier,1955; Ross, 1963, 2000; Klass and Ulbricht, 2009).

Published data discussing the prothoracic legs with regard tosexual dimorphism only refer to the shape of the basitarsomeres incross-section. It has been shown that it is narrower and more cylin-drical in male Oligotoma nigra Hagen, 1866, whereas it is almosttriangular in the females (Ross, 2000). The intersexual differencesof the spinning apparatus, especially the structure of the glands,have not been studied so far. The goal of this investigation is to seeka better understanding of the spinning apparatus and its functionalcorrelation to spinning behaviour.

2. Materials and methods

2.1. Morphological investigations

Synchrotron radiation micro computed tomography (SR�CT)was applied to generate data for the three-dimensional reconstruc-tion of the structures of interest. Function and construction of asynchrotron were described by Betz et al. (2007). The data for thepresent study were generated in the following facilities: DeutschesElektronen Synchrotron (DESY) in Hamburg (Germany), using the

tomography beamline at Petra III (Proposal no. I-20090102, Aug.2009); Swiss Light Source (SLS), Villingen (Switzerland), beamlineTomcat (Proposal no. 20080794, May 2009) and HelmholtzzentrumBerlin (BESSY II), Berlin (Germany), beamline BW2 (Proposal no.2009 90372, Aug. 2009).

This study focuses on males and females of Aposthonia ceylonica(Enderlein, 1912) (Oligotomidae, Fig. 1). For purpose of comparison,we examined the prothoracic legs of male and female Aposthoniajaponica (Okajima, 1926) and Antipaluria urichi Saussure, 1896 aswell as prothoracic legs of female Aposthonia borneensis (Hagen,1885), O. nigra Hagen, 1866 and Haploembia solieri (Rambur, 1842)(sexuals and asexuals). Details of these specimens will be presentedelsewhere. The specimen of Ap. japonica originated from the collec-tion of the Sugadaira Montane Research Center of the University ofTsukuba in Ueda, Nagano, Japan. All other specimens originatedfrom breeding cultures of the Department of Biology, Santa ClaraUniversity, California, USA.

The specimens were fixed in Duboscq-Brasil solution (Romeis,1987). After this initial fixation specimens were stored in 70%ethanol, dehydrated in a graded ethanol series, dried with a critical-point dryer (Balzer CPD 030 Critical Point Dryer) and mounted onspecific synchrotron facility holders. This combination of fixationand drying unfortunately caused the detachment of the epidermisand muscles from the cuticle (Figs. 2 and 3).

Three-dimensional reconstruction (processing and visualiza-tion) of the data was done with Mercury Amira® 5.2 (Visage

Fig. 3. Prothoracic basitarsus of male Aposthonia ceylonica reconstructed from SR�CT-data. Glt – gland tissue; res – reservoir. (A) 3D-resonstructions. (B and C) 3D-reconstruction with crosscut and single cross section, respectively, showing gland tissue and reservoirs. (D and F) 3D-reconstruction with longitudinal cut and section.

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Imaging, Richmond, Australia). Measurements were also donewith Amira®, which allows a highly accurate assessment of allvolumes and lengths (Ashburner and Friston, 1997; Gaser et al.,1999; Woermann et al., 1999). The output unit of the program(in this case volume-pixel = voxel) was converted into cubicmicrometre (�m3), cubic millimetre (mm3) and millimetre (mm)as appropriate. A voxel is a volume pixel with discrete values ofXYZ-coordinates within the dataset. The spatial resolution wasdetermined from the beamline parameters and the magnificationof the lens applied during data acquisition. Images of the recon-structions were subsequently processed in Photoshop CS3 (AdobeSystem Inc., San José, USA).

The number of glands in each prothoracic basitarsus was deter-mined by counting the gland’s reservoirs. The volumes of individualreservoirs and the standard deviations were calculated. Addition-ally, maximum lengths and widths of the female and male spinningtarsi were measured.

2.2. Analysis of behaviour of Ap. ceylonica

Individual adult females and males (five of each) and fivefemale–male pairs were placed in a burrow (0.3 cm wide × 0.5 cmdeep × 5.8 cm long) drilled into a block of plywood to resemblea crevice in bark, a natural habitat for this species (originallyfrom India). Each individual or pair was filmed with a solid statevideo-camera with an 18 mm to 108 mm F 2.5 TV-style zoomlens (Javelin Electronics, Torrance, California, USA) for 1 h andrecorded onto a DVD to allow playback. Their behavioural actswere recorded using Observer software (version 5.0, by NoldusInformation Technology, Wageningen, the Netherlands) as anevent recorder. We scored the following acts and computed timespent in each over the hour: sit still, travel, flip over, U-turn,groom, spin silk, bite substrate, bite silk and antennate other (thelatter only for males with the females). From this information weconstructed time budgets for females, males alone and males withfemales, including probability of transition from one behaviour toanother and total duration in seconds. In addition, we determinedthe distinctiveness of these male and female behaviours by con-ducting a discriminant analysis using JMP Statistics (version 7, SASInstitute Inc., Cary, North Carolina, USA).

Silk spinning behaviour of these individuals was recorded indetail using methods developed in a recent larger study of the orderin the Santa Clara University lab and will be briefly summarizedherein. Each embiid that spins places its front tarsi in particularpositions, planting its silk ejectors and then pulling back its footwhile releasing silk. We identified a number of standard steppingpositions, during our investigations of dozens of species, to includenear the head, reach, cross, side, back, overback and paw (see Fig. 6for details). The embiid also spins with the venter down against thesubstrate or flipped over to face the newly formed silk covering.These positions are named “dorsal” and “ventral” respectively. Allspinning steps were quantified for each individual using Observer.For consistency, the same person called out the stepping whileanother person entered the steps into the event recorder. Spinningis very quick and hence, playbacks were conducted in half or quarterspeed. Kinematic diagrams were drawn to display the proportion ofstepping in each foot position. Kinematic diagrams of other speciesare included to allow wider comparisons of males and females toeach other and to other species. We included species with a similararboreal lifestyle from the same family (Oligotomidae: Oligotomasaundersii Westwood, 1837, Ap. borneensis) as well as other oligoto-mids that live within leaf litter (O. nigra, H. solieri, sexual form andan asexual form called H. “tarsalis” herein) and two species froman outgroup (Australembiidae: Metoligotoma pentanesiana Davis,1936, Australembia incompta Ross, 1963).

Transitions from one step to another are informative as aphylogenetic character (D. M. McMillan, pers. comm. 2011) andtherefore we include that information here as we explore the dif-ferences between males and females. We applied a non-parametricmultivariate method (using a Bray–Curtis Similarity Matrix basedon rank similarities between individuals) and cluster analysis todetect similarity among spinning individuals (PrimerE softwareversion 5, Plymouth Routines in Multivariate Ecological Research,Plymouth, UK, 2001; K. R. Clarke and R. N. Gorley). PrimerE also hasa module called BVSTEP, a method resembling stepwise multipleregression, used to identify the most informative behaviours thatgive rise to a non-metric MDS ordination. We were able to reducethe large dataset of 83 step transitions to a smaller, more diagnosticset of behaviours; 32 transitions were ultimately used to generatea cluster diagram showing % similarity among spinning individualsfor eight species, including the male and female Ap. ceylonica.

2.3. Silk production by males and females

In order to determine the silk output of males and females,four 24 h trials of four individuals of each sex were placed into anacrylic plastic arena (10 cm diameter × 4 cm tall) with a flat-blacktextured base to promote silk spinning as well as create contrastfor photographing the translucent white silk. We carved 8 circu-lar divots (20 mm diameter wide and 4 mm deep) into the base togive the embiids access to crevices where they could attach theirsilk. The entire base was photographed after the 24 h trial with adigital camera (Canon Eos Digital Rebel). The image was importedinto ImageJ64 Freeware downloaded from the National Institutesof Health website (http://rsbweb.nih.gov/ij) and the area of silk wasmeasured by selecting all patches and adding up the total area cov-ered with silk. Integrated Density was also measured as a relativemeasure of thickness of silk which varied from thin and whispy(more grey because the background shows through) to more cloth-like (closer to white). For these trials, silk production was comparedin terms of total weight (g) of the four embiids per trial. Because ofsmall sample sizes, medians were used instead of averages for allcomparisons.

3. Results

3.1. Comparison of male and female gut

Male embiids tend not to feed after their final moult. To ver-ify that Ap. ceylonica males displayed this quality, we dissected10 specimens. No food particles were present in any one of thesemales, and most had very little fat. For comparison one adult femalewas dissected and its gut was found to be filled with food. Shediffered from all 10 males by having a large crop, conspicuousproventriculous and large stores of fat.

3.2. Comparison of male and female silk glands in Ap. ceylonica

Even with the naked eye, differences in the prothoracic legs offemales and males are easily recognizable (Figs. 2 and 3; Table 1).These differences become even more obvious when the reser-voir numbers of the glands (corresponding to the numbers ofthe glands), the reservoir volumes (mean of 20 reservoirs perbasitarsomer of foreleg) and the overall spinning tarsus (thickenedbasitarsomer of the forelegs) volumes are compared. In the femalesthe values are larger than in the males for body length and weight,volume and width of spinning tarsus, reservoir volume and num-ber of glands when the tarsus is viewed in cross-section. Males areapproximately equal to the females in spinning tarsus length andnumber of reservoirs when counted in a longitudinal section of thetarsomere. The male exceeds the female in single reservoir volume

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and in number of reservoirs per spinning tarsus volume. This is alsoreflected in the relations of reservoir quantity per mm3 and overallreservoir volume between the sexes. The gland tissue surround-ing each reservoir was readily recognizable. It is conspicuous that

the gland tissue in the males is thin in contrast to the voluminousgland tissue in the females (Figs. 2 and 3). The structures investi-gated in Ap. ceylonica show the same characteristics as in the otherEmbioptera used for comparison (see Section 2).

Fig. 4. Ethograms of behaviours exhibited by females, males or males placed with females of Aposthonia ceylonica during 1 h trials filmed and scored during playback withan event recorder. Size of each box represents the mean time spent in a behaviour and the outer dashed line shows the extent of the standard error of the mean. Arrowsreflect the mean probability of transition from one behaviour (at the start of the arrow) to another at its tip. Sample sizes are shown in parentheses.

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Table 1Morphometrics of silk glands and productivity of Aposthonia ceylonica.

Females Males Male/Female (%)

Dimensions of the insectsWeight (g) of embiids in silk trials (median)a 0.0185 0.006 32Length of fixed individual (mm) 7.5 7.0 93

Silk gland characteristics of fixed individualsVolume of basitarsomere (mm3) 0.0223 0.0059 26.5Basitarsomere volume/body length 0.00305 0.00085 27.9Basitarsomere width (mm) 0.0865 0.0466 53.9Basitarsomere length (mm) 0.143 0.145 101.4Number of glands 53 31 58.5Number of reservoirs visible in cross section 8 5 62.5Number of reservoirs visible in longitudinal section 14 16 114.3Single reservoir volume (mm3) 1.7 × 10−5 3.7 × 10−5 218.6Reservoirs per mm3 2314 5200 224.7

Silk productiona

Silk area (median) 78,767 11,161 14.2Integrated density (ID) 6,516,802 2,173,142 33.3Area of silk per body wt (g) (median)b 3,673,181 3,368,000 91.7ID per body wt (g) (median)b 3.88 × 108 3.62 × 108 93.3

a Each of 4 trials per gender included four individuals held for 24 h; total weight of the 4 individuals was used herein; area and integrated density of silk were measuredwith ImageJ64 freeware.

b One trial where males did not spin in 24 h was omitted from this summary statistic.

3.3. General behaviour of Ap. ceylonica

The ethograms and discriminant analysis of time spent invarious behavioural acts reveal that males and females to be dis-tinguished from each other (Figs. 4 and 5). Sit and spin were themore common behaviours for males; females showed more tran-sitions among sit, travel, bite substrate and spin. Grooming wasparticularly noticeable in males alone who groomed the ventralsurface of their basitarsomeres, probably cleaning their silk ejec-tors. Discriminant analysis revealed that individuals tended to bemore like others in the same treatment group. However, one femalewas miscategorised as being more like males with females.

3.4. Silk spinning by Ap. ceylonica

3.4.1. Step dynamicsThree males in each treatment group and four females spun silk

during the 1-h sessions within the burrow. The total number ofspin steps were on average 1914 (±392 (SE)) for females and 3039(±625.1) for males (2-tailed t-test, assuming unequal variances;t = 2.306, df = 8, p = 0.17). The six males who spun were combined forthe t-test because the number of spin steps did not differ based ontreatment group. The three highest scores were generated by maleswho stepped 5675, 3715 and 3014 times and the least by a femalewith 855. Despite some energetic spinning males, the differences bygender were not significant. The cluster diagram based on percentsimilarity of transitions between spin steps reveals that male andfemale Ap. ceylonica are very similar to each other and to otheroligotomids that share the arboreal lifestyle (Fig. 7). The inclusion ofdata from other species shows that leaf litter and arboreal embiidstend to display different styles of spinning.

Details of spin steps are shown in the kinematic diagrams (Fig. 6)where three other species are included for comparison. All showan overall style of stepping mostly around and near their heads.Subtle differences include greater emphasis of front stepping byleaf litter embiids (M. pentansiana, A. incompta and H. solieri sexual)and more paw and stepping to the side, back and overback by bothmale and female Ap. ceylonica. These steps contribute to silk beingdeposited along the length of the body and lead to construction ofsilk tubes as tight as a glove. Another subtle but possibly importantdifference is that the leaf litter embiids tend to alternate right andleft when stepping near their head whereas Ap. ceylonica tend torepeat the “reach” step again and again depositing more silk in this

Fig. 5. Results of a discriminant analysis (JMP 7 Statistics) sorting 15 individuals ofAposthonia ceylonica based on general behaviours including sit, spin, travel, groom,U turn, flip, antennate females (a male behaviour) and bite substrate. Treatmentgroups (females, males alone and males with females) are shown in grey shadingas distinct zones identified by the statistical analysis. The relative position of eachindividual is indicated by a symbol reflecting gender and/or social status for themales (alone or with a female).

position before moving to another position. This was especially trueof the males. Males with females showed the most asymmetry intheir spin steps apparently because they sat side-by-side with theirfemales, thus restricting the range of movement on one side. Malesmate with females by moving along their right side, and thus theirrelative position is not random.

3.4.2. Silk productionFemales (n = 4) in each of four trials produced more and thicker

silk than did an equal number of males per trial (Fig. 8, Table 1).However, if silk area and density are expressed relative to bodyweight, males and females achieved very similar results, with malesgenerating 91.7 and 93.3% respectively for these measures of silkoutput by females (Table 1). Of note, however, is that the silk outputwas scant with at most two tubes of silk being produced in the 24 hperiod.

4. Discussion

Our observations confirm that males of Ap. ceylonica do not feedafter their final moult. This finding is consistent with Ross (2000)

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Fig. 6. Details of spinning as represented by kinematic diagrams in Aposthonia ceylonica where the diameter of each circle indicates average proportion of steps in a particularposition around the body and near the head. The arrows show the average probability of transitioning from one step to another for those transitions identified by statisticaltests as being informative for separating individuals from each other in an ordination process (details in Section 2). An arrow that leads into a circle indicates the probabilityof repeat stepping in one position whereas an arrow that leads from one circle to another shows the probability of moving from one step to a different one. The number ofindividuals per species contributing to the mean values is shown in parentheses. Species displayed were chosen to highlight interesting differences or similarities.

conclusion based on his examination of thousands of male web-spinners – that is, non-neotenous males do not feed as adults.However, Ap. ceylonica males engaged in spinning just like thefemales. Even the amount of silk produced in 24 h by males wascomparable to that produced by females if set in relation to theirbody mass. However, the inability of the males to feed may putphysiological stresses on them not applicable to females. We sus-pect that silk production incurs costs, especially if the results

gleaned from an investigation on three other species of webspin-ners (Edgerly et al., 2006) can be applied to Ap. ceylonica. In thatstudy, experimental groups of nymphal An. urichi forced to replacetorn silk developed into smaller adults than those from controlgroups with intact silk. In addition, in the short term, adult femaleAn. urichi and Notoligotoma hardyi (Friederichs, 1914) produced,respectively, 1.7 times and 2.0 times more CO2 (as evidence ofmetabolic output) when spinning than when resting.

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Fig. 7. A cluster analysis based on rank similarity of spinning behaviours of individuals (see Section 2 for details of statistics). Each individual spinner is indicated by itsspecies name and labelled in the cluster diagram as L (leaf litter) or A (arboreal) depending on the typical microhabitat of natural populations. Asexual Haploembia solierifemales are designated as H. “tarsalis” to distinguish them from their sexual counterparts. Genus names are abbreviated as follows: M. = Metoligotoma, A. = Australembia (bothAustralembiidae), O. = Oligotoma, Ap. = Aposthonia (both Oligotomidae).

Despite the similar spinning behaviours, the silk glands in Ap.ceylonica are sexually dimorphic. The most significant differenceis the reservoir volume in relation to the general volume of thetarsomere or the glands. In males, the reservoir is distinctly largerthan in females. This suggests that males produce and store thesecretions of the glands in the reservoirs before their final moultwhile they are still able to feed. Stored secretions are later used forspinning. Probably fat reserves are used to produce more silk afterthe final moult, but since resources are limited, males might haveto stop spinning after some time. We noted in a footnote in Table 1that one replicate of males did not produce any silk during the 24 htrial. Upon looking back at details in our notes, we discovered thatthe males in this replicate weighed less than the males in the othergroups, which did spin some silk. This observation, along with otheranecdotal evidence, suggests to us that as males use up their fatreserves, they stop spinning. The relationship among feeding andstarvation, development and aging, and silk spinning in males iscertainly worth a closer look.

Ours is not the first study to examine silk spinning and sexualdimorphism in insects although the theme is apparently rare as sug-gested by the paucity of papers that we detected in a search of theliterature. Two intriguing investigations reported opposite findingsfor male versus female spinners. Underwood and Shapiro (1999)discovered that male caterpillars contribute more silk to nests inthe social caterpillar, Eucheira socialis westwoodi Beutelspacher,1984 (Lepidoptera: Pieridae). In addition, male-biased nests wereheavier and the silk was thicker. Male caterpillars spent more timespinning than did females, and they were more active in general.

Fig. 8. Area of silk produced over a 24 h period plotted as a function of body weightfor all individuals in a trial, which were four males or females of Aposthonia ceylonica(four trials for each gender). Symbols represent individual trials; the medians of eachof the four trials are noted by an X.

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The authors suspected that females are under stronger selection toachieve a heavier adult body weight than males and that colonieswith more male caterpillars, which may be under selection tocontribute to the survival of the colony, did better. The nests innature are male-biased and kin-selection was invoked as a prob-able driver. Wilson and Hölldobler (1980) also found a division oflabour between male and female weaver ant larvae but, in con-trast to the caterpillars, the female larvae produced more silk thanthe males and their silk glands were larger. Wilson and Hölldobler(1980) proposed a kin selection argument to explain the smallersilk glands and reduced contribution by males to the nest buildingdictated by the worker ants (their adult sisters).

Embiopterans do not exhibit complex social behaviour as inweaver ants, to be sure, and we suggest that silk spinning byadult males is not altruistic or devoted to supporting colony life,especially because they live for only a week or so after adulteclosion. Very few webspinner species have been studied in thefield, so placing Ap. ceylonica behaviour into a natural contextrequires generalizations pulled from research on another arbo-real webspinner, An. urichi (see references in Edgerly, 1997), inthe plesiomorphic family Clothodidae (Szumik et al., 2008). Labcultures provide clues that Ap. ceylonica colonial behaviour is notdramatically different from An. urichi. In general, colonies developas mother–offspring groups that sometimes coalesce with othersuch groups to form extensive aggregations (see also review inRoss, 2000). Adult females contribute the most to silk productionespecially after their nymphs hatch. Nymphs spin silk but theirproductivity is limited because of their small size. In fact, their suc-cess is compromised if the mother disappears and they are left ontheir own (Edgerly, 1988) for similar results on Anisembia texanaMelander, 1902 (Anisembiidae) (see Choe, 1994). Using An. urichias a model species, we expect adult females to die when the nymphsare half grown, and at that point the silk covering accumulatesholes and tears. The nymphs do not repair these gaps, but insteadmaintain their own narrow galleries within the larger silk struc-ture. They typically disperse before reaching adulthood. Coloniesof An. urichi in Trinidad were never seen to develop over-lappinggenerations. Both males and females displayed reproductive migra-tion, rarely staying in their natal silk. We cannot be sure that Ap.ceylonica behaves the same way in the field, but our lab culturesdisplay similar phenology (our observations are without quanti-tative data). In these, silk builds up as distinct tubes occupied byadult females and their offspring. As they mature, they congregateat the tops of the culture jars, which is unusual for webspinners inour experience (usually they spin silk within the dried leaf matrixor on lichens provided as food). Ap. ceylonica live within silk nestslocated at a distance from the food in their lab cultures. They thentravel back and forth to feed and to rest. Adult males and femalessit in tight silk tubes sometimes alone, sometimes together. Adultmales and females sit in tight silk tubes sometimes alone, some-times together. The relative contribution to colony silk by maleand female webspinners has not been studied, as in the studies byUnderwood and Shapiro (1999) or Wilson and Hölldobler (1980)cited above. Given these researchers’ successes at finding genderdifferences in silk production in immatures, it is worth a closerlook in webspinners. We suspect that the costs of silk spinningmight be relatively high for males and, therefore, immature malesmight withhold silk more and more as they approach maturity, ina similar manner to that seen in weaver ants. Starving male web-spinners are probably less fit as mates: they may be less able to fly,to court, or even to live long enough to find a suitable mate. If malenymphs do indeed synthesize and package silk proteins for lateruse in adulthood, as suggested herein, they would be expected tostop producing silk during their development. Whether and howthey do this is worth exploring not only because of interest in geneexpression in social insects, but also the burgeoning technical and

industrial interest in silk proteins (Hu et al., 2010; Numata et al.,2009).

Morphological methods typically apply to dead specimens. Yet,structures found to be interesting from phylogenetic or functionalangles have to be discussed with regard to their function. The com-bination of behavioural and ecological observations and the mostup-to-date morphological reconstruction methods on which thisstudy is based leads to greater insight into lives of insects. Eco-logical and behavioural studies may often show more obviouslywhich characters can be assumed to be relevant from a view pointof functional morphology. Additionally, collaboration in these quitedifferent fields of research provides insight into, and answers toquestions that could otherwise not be answered from any of thetwo methods alone. The results of this study show very distinctlyhow valuable such cooperation can be and how important the studyof living specimens is whenever morphological questions have tobe addressed.

Acknowledgements

Many thanks to Ryuichiro Machida for providing specimensof Ap. japonica. We are also grateful for the support by the staffat the synchrotron facilities Deutsches Elektronen Synchrotron(DESY) in Hamburg (Germany), Swiss Light Source (SLS) in Villin-gen (Switzerland) and Helmholtzzentrum Berlin (BESSY II) in Berlin(Germany) during our work at these facilities. Frank Wieland, SonjaWedmann, Darren Pollock and an anonymous reviewer offeredhelpful comments on the manuscript. The research at Santa ClaraUniversity was supported in part by a grant from the National Sci-ence Foundation (DEB-0515865), Presidential Research Grants andthe Faculty and Student Research Assistant Program at SCU. Finally,we thank undergraduate research assistants at SCU, especiallyAnnika Meyer and Khaaliq Dejan, for help with scoring behavioursand rearing the insects for this study.

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