Factors influencing the evolution of social behaviour in Australian crab spiders (Araneae: Thomisidae)

Biological Journal of the Linnean Society (1998), 63: 205–219. With 6 figures

Factors influencing the evolution of social behaviour in Australian crab spiders (Araneae: Thomisidae) THEODORE A. EVANS1 Department of Zoology, University of Melbourne, Parkville, Victoria, 3052, Australia Received 27 March 1997; accepted for publication 28 August 1997

The social Diaea are non-territorial, periodically-social spiders that do not weave a snare web, a factor considered to be important in spider sociality. Maternal care and heritable retreats are factors common to most group living animals, including social Diaea; suggesting that they are important factors in the evolution of spider sociality. A 4 year survey, along with field and laboratory experiments revealed that mother spiders provided crucial care in the form of a protective Eucalyptus leaf nest and large prey for their offspring. After the mother’s death, the nest was inherited and expanded by the offspring. Larger groups built larger, more protective nests, but expended less individual effort doing so, and so survived better than smaller groups.  1998 The Linnean Society of London

ADDITIONAL KEY WORDS:—communal behaviour – maternal care – protective retreat. CONTENTS

Introduction . . . . Material and methods Survey data . . Experimental data Results . . . . . Survey data . . Discussion . . . . Acknowledgements . References . . . .

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INTRODUCTION

The discovery of South American spiders with ‘gregarious habits’ amazed Darwin (1845: 37); since then some form of ‘gregarious habit’ has been found in over 100 spider species in 26 families (see Buskirk, 1981; D’Andrea, 1987; Avile´s, 1997 for reviews). These social behaviours have been categorized using territorial aggression 1

Present address: CSIRO Division of Entomology, Canberra, ACT, 2601, Australia. Email: theo. [email protected] 0024–4066/98/020205+15 $25.00/0/bj970179

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and the duration of the group; those considered to have the most derived social state are non-territorial and permanently-social. Spiders that fall into this category typically display social behaviours such as cooperative prey capture, communal feeding and communal nest construction, and have some form of irregular snare web (Shear, 1970; Burgess, 1976; Buskirk, 1981; D’Andrea, 1987; Avile´s, 1997). Unlike social spiders from other families, Diaea socialis Main (Main, 1988), D. ergandros Evans and D. megagyna Evans (Thomisidae) (Evans, 1995) and Delena cancerides Walckenaer (Sparassidae) (Rowell & Avile´s, 1995) are from families that do not weave any form of snare web, a behaviour considered to be important in the evolution of sociality in spiders (see above reviews). The absence of snare web weaving in Diaea and Delena suggests that the adaptive significance of their social behaviour cannot be explained easily in the existing classifications of social spider behaviour. This study deals with Diaea ergandros, an annual species that inhabits Eucalyptus forests in south-eastern Australia (Evans, 1997). In early summer, gravid females migrate alone from their home nest to built a brood chamber from several Eucalyptus leaves, in which they lay a single eggsac of c. 45 (range 15–80) eggs. The female continues to attach leaves to this brood chamber after her eggs hatch, and, in autumn, is eaten by her offspring who then inherit her nest (Evans, 1995; Evans, Wallis & Elgar 1995). The offspring remain together in the nest, continuing to add leaves onto the nest, and foraging cooperatively from entrances on its surface. The following spring, the offspring mature, and after mating, migrate to reproduce. Avile´s (1997) classified Diaea as ‘technically’ ‘non-territorial periodical-social’ because the groups eventually disintegrate after mating as female spiders migrate alone to found new nests. However, she noted that because Diaea groups persist after the spiders mature that these species “appear to be at the transition point between periodic- and permanent-sociality”. Therefore, the social Diaea are well placed for investigation into factors that promote extended group living in a typically solitary family. Work in other diverse taxa has produced three factors suggested to favour the evolution of sociality: gradual development, parental care and inhabiting a permanent, expandable and protective retreat (Andersson, 1984; Alexander, Noonan & Crespi, 1991; Seger, 1991; Crespi, 1992; Duffy 1996; see also Seger & Moran, 1996). The maternal care exhibited by thomisids may preadapt them to sociality. Female crab spiders often enclose their eggsacs with vegetation and remain with them until they hatch (Bristowe, 1958). This behaviour is exhibited by solitary Diaea and their close relatives Xysticus and Cymbacha L. Koch that curl a single eucalypt leaf as a brood chamber (Main, 1988). However, the social Diaea invest more energy in pre-oviposition preparation by using four or more leaves, creating an incipient nest, before laying their single eggsac, and post-oviposition care, behaviour not seen in solitary crab spiders. The delayed (adult) dispersal seen in the social Diaea offspring, well past the lifespan of the mother, suggests that other benefits must accrue to group living in addition to those provided by the mother. One benefit might be the nest itself, as a protective structure or foraging area. The nest attracts attention from potential predators, including vertebrates such as birds and possums, and may be invaded by commensals and other spiders such as Clubionia robusta L. Koch, a common, barkdwelling spider. The constituent leaves are held together tightly, and their haphazard arrangement is labyrinthine inside, features that may guard against such predators. Here, I report on a long-term survey, two field and two laboratory experiments that describe and investigate aspects of the life history of D. ergandros. In particular,

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I investigated the influences of the mother on the survival and growth of her offspring early in the life cycle, and also investigated the influence of group size on survival and growth of group members, and on nest construction later in the life cycle (after the mother’s death), so as to better understand the adaptive value of their social behaviour. MATERIAL AND METHODS

Survey data I surveyed the demography of D. ergandros from nests collected from five sites in Victoria, Australia, from March 1991 to August 1994. I chose the sites at Mt Disappointment State Forest, Kinglake National Park, Yan Yean Water Catchment, Brisbane Ranges State Park and Otways Ranges State Forest, due to their similarity in floristic diversity, proximity, ease of access, low canopy (10–20 m) and low human activity. I surveyed the field sites each month during this time, except for March–June 1993. I collected a total of 505 nests by either climbing the tree, or using secateurs on an extending pole. These were bagged individually and transported to the laboratory for dissection. Animals found in the nest were separated into three categories: D. ergandros, commensals and predators. The 11 521 (including 267 mothers) D. ergandros collected were weighed and categorized by sex and developmental stage (adult, subadult and juvenile). Adults were distinguished by their colour and developed genitalia (sixth instar females and fifth instar males). Subadults could be determined from partially formed genitalia (fourth and fifth instar females, and fourth instar males). Juveniles were third and earlier instars, and it was not possible to determine their sex. I defined commensals as those animals that D. ergandros neither attacked nor avoided for 48 hours when contained together in jars in the laboratory. I identified animals as predators when they were seen to eat D. ergandros, either in the field, or in the laboratory. I found oophagous larvae or pupae in some eggsacs, and identified them after the adults eclosed. I measured dry leaf weight (one week at RT, then 6 h at 150°C) of 374 nests. I compared group size (excluding the mother if present), mean spider weight (mean weight of spiders found in the nest, excluding the mother if present), adult and subadult sex ratio (number of males divided by the total number of spiders), nest size (dry weight of leaves), and construction activity (proportion of green leaves in the nest) with time of year and each other using Spearman’s correlation and multiple regression analyses, the residual values (i.e. the deviation from the predicted value) from the regressions were compared with t-tests to consider maternal or predator effects. I performed the analyses on Systat for windows (Systat Inc., Evanston IL, USA). Variables were transformed logarithmically where necessary, to achieve normal distribution and increase homogeneity of variances in order to meet parametric analysis assumptions (Sokal & Rohlf, 1981). Experimental data Effects of maternal care I examined the influence of the mother and spiderling group size on the survival and growth of spiderlings in a field experiment. I collected 35 nests from Yan Yean

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Water Catchment in November 1991, dissected them and counted and weighed the spiders. The mother was present in 25 of the groups, and absent in 10. Group size ranged from 12 to 64 spiderlings (mean±SE, 28.8±2.7). I placed the groups (group size not manipulated) into individual plastic vials (6.5×4.5 cm diameter) with an artificial nest (four pieces of paper shaped like Eucalyptus leaves enclosed within green plastic shade-cloth ). The spiders were left in the plastic vials with the artificial nest for one week to encourage weaving (after Evans & Main, 1993). I returned the plastic vials to the collection site, removed the artificial nest without dislodging the spiders, and attached it to a branchlet with several leaves using a twist tie. I monitored the artificial nests during the day and night for activity, especially weaving, foraging or dispersal, for 2 weeks. The experiment ran for 2 months, after which I collected the nests, counted and weighed the surviving animals, and recorded data on nest construction. Survival, growth and nest construction were compared using Spearman’s correlation and multiple regression. I examined the influence of the mother and of prey size on the survival and growth of spiderlings in a laboratory experiment. I collected nests from Yan Yean in March 1992, from which I assembled 48 replicate groups of 20 sibling spiderlings. These were housed in plastic containers (20×10 cm diameter) with two pieces of paper shaped like Eucalyptus leaves. Mothers, which were removed during nest dissection, were returned to their offspring in 24 groups; the remainder did not have a mother spider. Half the groups were fed small flies, Drosophila melanogaster, which are smaller than spiderlings, the other 24 groups were fed large flies, Lucilia cuprina, which are larger than spiderlings. Spiders were fed twice a week with 52 D. melanogaster or two L. cuprina. This was considered to be equivalent available biomass as 26 D. melanogaster (0.86±0.01 mg, n=411) were not significantly heavier from one L. cuprina (20.1±0.18 mg, n=263) (t261=1.95, P>0.05), but 27 D. melanogaster were significantly heavier (t261=2.15, P<0.05). The upper limit was used in an attempt to reduce any effects of a larger amount of inedible exoskeleton in D. melanogaster (after Rypstra, 1993). I recorded prey capture, and removed all dead and live flies before new flies were added. The experiment ended after 2 months when I counted and weighed surviving animals, and calculated the volume of the retreat constructed. I compared survival, growth and retreat size using two factor ANCOVA, with mother presence and prey type as the two factors, and with initial mean spiderling weight as the co-variate. Effects of group size I examined the influence of group size on survival, growth and nest construction behaviour of D. ergandros in a field experiment. I collected nests from Healesville Corrandirk Reserve in May 1991, dissected them, and counted and weighed the spiders. The size of these natural groups (comprised mostly of third instar juveniles and subadults) was not manipulated. Group size ranged from 6 to 70 spiders (24.9±1.9, n=52). I followed the same experimental procedure as for the field experiment above. I returned the artificial nests to the collection site, and monitored them during the day and night for activity for 2 weeks. A severe storm occurred during this time, so I noted damage to the nests, and re-checked them for signs of activity. The experiment ran for 3 months, after which I collected the nests, counted and weighed the surviving animals, and recorded the data on nest construction. Survival, growth and nest construction were compared using multiple regression against initial group size and initial mean spider weight.

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Nests as protective retreats I examined the influence of predators on subadult spiders in a small laboratory experiment. I collected 10 large nests from Yan Yean in October, 1992, and removed 30 spiders from each. I placed three of these groups into plastic containers (30×15 cm diameter), with eight pieces of malleable, transparent, plastic shaped like Eucalyptus leaves attached on the inside top surface. The other seven groups were held in containers without nest-building material. After 1 week, spiders had constructed nests using all plastic ‘leaves’. The spiders were clearly visible, often grouped together, inside the nest. I fed all groups D. melanogaster and L. cuprina, and observed prey capture similar to that seen in the field. I placed one Clubiona robusta (ca. 40 g) into each container after 3 weeks. The containers were watched intensively for 48 hours (including time-lapse video), then periodic observations followed. The experiment ended after 4 weeks when I counted surviving animals.

RESULTS

Survey data Solitary gravid females dispersed from their natal nests in late spring–summer and began new nests. Mothers became lighter as autumn proceeded (Fig. 1A), and they were rarely found in nests collected in winter. The mean spider weight increased with time of year peaking in spring, coinciding with maturation (Fig. 1A). Weight differed between sex-classes; adult females (18.8±1.01 mg) were heavier than subadult females (11.9±0.44 mg) (t143=5.98, P<0.001), similarly, adult males (12.0±0.66 mg) were heavier than subadult males (10.1±0.29 mg) (t138=2.55, P <0.02). Females were heavier than males (t283=4.8, P<0.001). The adult sex ratio was extremely variable (range=0.0–1.0), especially in nests containing small adult groups. However, the sex ratio in the younger and larger nests, containing mostly subadult spiders, was close to parity (mean=0.45±0.015, n=132). Group size was highest in the summer after eggs hatched, and was smaller in the later months of the year (Fig. 1B). There was a small, but significant trend for group size to decrease as spider weight increased (Spearman’s correlation r2=0.10, P<0.001) (Fig. 1B). Although nests began as a single leaf (c. 0.12 g), they could eventually contain as many as 62 leaves (9.6 g). Nest size varied over the time of year, the larger nests were found in spring and summer (Fig. 1C). The proportion of green leaves in the nest, a measure of construction activity, varied with time of year; the highest levels were found associated with incipient nests in summer, and with older nests in spring (Fig. 1D). A multiple regression shows that nest size increased with group size (Fig. 2A) and mean spider weight (Fig. 2B) (r=0.75, F2, 372=231.2, P<0.001). The nests with mothers had a significantly lower residual (t373=5.19, P<0.001) (Fig. 2C) suggesting that groups with mothers (of equal group size and mean spider weight) contributed significantly less to nest construction, implicating mothers’ continued contribution to nest building. Nests with predators had a significantly higher residual (t247=2.25, P<0.05) (Fig. 2D), suggesting that spiders were contributing more to nest construction when predators were present. The mean contribution to nest construction per spider (nest weight/group size) decreased with increased group size (r=0.71, F1, 373=370.9, P<0.001) (Fig. 3A),

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Figure 1. The annual variation in natural Diaea ergandros colonies. A, individual spider mass (mothers= filled columns, spiderlings and subadults=open columns). B, number of spiders in the group. C, weight of leaves comprising nest. D, Proportion of green leaves in the nest. E, number of predators found in nest.

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Figure 2. Natural nest size as a function of A, spider size and B, number of spiders in the group (multiple regression: r=0.74, y=0.53 weight+0.30 group size−1.04). The residuals of the multiple regression compared between C, mother presence, and D, predator presence (see text).

Figure 3. Individual nest construction effort of spiders compared with group size from natural groups. A, the nest weight per spider decreases with increasing group size (regression: r=0.66, y=0.47−0.12x). B, the regression residuals of (A) compared between mother presence (see text).

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suggesting that spiders in larger groups expended less effort in nest building. The nests with mothers had a significantly lower residual (t373=3.69, P<0.001) (Fig. 3B) suggesting that mothers lowered significantly the individual effort in nest construction. Most Diaea ergandros nests contained skeletal remains of prey, which could be identified to order as thomisids do not masticate their prey. The most common prey were beetles (Coeloptera), wasps and ants (Hymenoptera), but flies (Diptera), moths (Lepidoptera) and damselflies (Odonata) were also found. Prey were often larger than the spiders; some wasps had a body length five times longer than their captors. Commensals included cockroaches (Blattoidea), psyllids (Hemiptera; Psyloidea), woolly scale insects (Hemiptera; Coccoidea), and the adults and larvae of leaf-eating beetles (Coeloptera; Chrysomelinae). Cockroaches were more common in older, larger nests with mostly dead leaves, whereas the psyllids, scale insects and beetle larvae were common in younger, smaller nests on green leaves. No analyses were performed on these data. I observed three vertebrate predators investigating D. ergandros nests in the field: two birds (Bell Miner, Manorina melanophrys and Yellow-faced Honey-eater, Lichenostomus chrysops), and one mammal (Feather-tailed glider, Acrobates pygmaeus). These species are small enough to reach the spider nest located on the distal end of the branch. They were successful at infiltrating small, incipient nests (1–5 leaves), but not older, larger nests. Laboratory trials I performed with A. pygmaeus were consistent with these field observations. The gliders investigated five large nests ([10 leaves), but did not dislodge any leaves, whereas they successfully tore open five small nests (Ζ5 leaves), and ate the spiders therein. I found three species of oophagous larvae from 34 eggsacs in summer: Acanthostethus sp. (Nyssoninae, Sphecoidea) was predominant, but a pompilid wasp and a fly (species unknown) were found also. I found four species of vagrant spiders in 19% of nests. Over 75% of these were Clubiona robusta (Clubionidae); the remainder were Lampona cylindrata (L. Koch) (Gnaphosidae) or two unidentified salticid species. Predators were most abundant in the warmer months, particularly in older, larger nests with maturing D. ergandros (Fig. 1E). I observed C. robusta eating D. ergandros on the surface of the nest several times in the field. Effects of maternal care In the field experiment, groups with mothers had a lower failure rate: 23 of the 25 groups with mothers persisted whereas only two of the 10 groups (total of four spiders) without mothers persisted (Fisher’s exact <0.00001). Spiderlings in motherless artificial nests were observed abandoning the artificial nests during the first few days after replacement in the field. Consequently, spiderling survival was higher in nests with mothers (mean=56.2±24.9%) than without (mean=1.0±2.1%) (t33=6.94, P <0.001). The proportion surviving was not affected by the initial group size (F1, 31=0.18, NS), the initial mean weight of spiderlings (F1, 31=2.54, NS), or the final mean weight of spiderlings (F1, 23=4.19, NS). Initial mean spiderling weights were not different between with mother (mean=2.21±1.34 mg) and without mother treatments (mean=1.80±0.86 mg) (t33=0.88, NS). Similarly, final mean spiderling weights were not different between with mother (mean=6.85±4.21 mg) and without mother treatments (mean=4.38±1.80 mg) (t23=0.81, NS), although due to the low survival of motherless spiderlings this may not be a meaningful result. Although I observed only mothers attaching leaves onto the artificial nests a multiple regression

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Figure 4. Nest construction from the mother presence and spiderling number field experiment. The number of leaves used in nest construction increases with A, increasing final group size, and B, increasing final spiderling weights (multiple regression: r=0.66, y=0.17 group size+3.30 weight−3.25). Groups with (Ε) and without (Φ) mothers; note that failed groups were not used in analysis (see text).

showed that final group size (Fig. 4A) and final mean weight (Fig. 4B) were correlated with the number of leaves attached (r=0.66, F2, 22=8.43, P<0.005). In the laboratory experiment, all groups survived in the Drosophila melanogaster treatment because both mothers and spiderlings captured D. melanogaster. However, only the mothers could capture Lucilia cuprina; consequently, groups without mothers became cannibalistic after 2 weeks. Therefore, groups with mothers had higher survival rate: 11 of the 12 groups with mothers persisted, compared with only 1 group without mothers (Fisher’s exact <0.001). The number of spiderlings that survived the experiment depended on both the presence of the mother and prey type (interaction F1, 44=8.67, P<0.01). Therefore the effect of the mother was considered separately for each prey type. In the L. cuprina treatment, survival was significantly higher with mothers (t22=5.51, P<0.001), whereas in the D. melanogaster treatment, spiderling survival was not affected by mother presence (t22=1.17, NS) (Fig. 5A). Initial mean spiderling weight was not different between treatments (F1, 44=2.03, NS), but final mean spiderling weights were different between treatments (F1, 20= 10.10, P<0.005, motherless L. cuprina treatment excluded due to low survival). Motherless spiderlings fed D. melanogaster were significantly heavier than spiderlings with mothers fed L. cuprina (t20=3.18, P<0.01). Otherwise, spiderlings with mothers had similar final mean weights in both prey treatments (t20=1.39, NS) and spiderlings fed D. melanogaster grew similar amounts with or without mothers (t22=1.25, NS) (Fig. 5B). Although mothers were of similar initial weight in both prey treatments (t22=0.96, NS), mothers fed D. melanogaster did not change weight over the experiment (paired t9=0.136, NS) whereas mothers fed L. cuprina increased in weight by 50% (paired t5=2.89, P<0.05) (Fig. 5C). Retreats were made by binding the paper ‘leaves’ with silk, which also framed the round entrances. The volume of retreat constructed depended on mother presence (F1, 44=14.8, P <0.001) and the prey type (F1, 44=16.1, P<0.001), and the two effects were additive (interaction NS), as retreats built by groups without mothers

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Figure 5. Mother presence and prey size laboratory experiment. A, spiderling survival. B, growth of spiderlings. C, mother weight change. D, size of retreat woven during the experiment. Initial (Φ) and final (Ε) weight. 0=mother absent, M=mother present, L=Lucilia cuprina, D=Drosophila melanogaster. Different numbers indicate significantly different values (see text).

fed D. melanogaster were the same as retreats built with mothers fed L. cuprina (t22= 0.09, NS) (Fig. 5D). Effects of group size A total of 41 of the 52 groups persisted over the experiment. The proportion of spiderlings that survived was affected by group size. A multiple regression shows that initially larger groups (Fig. 6A) and initially heavier spiders (Fig. 6B) had proportionately more surviving spiders (r=0.64, F2, 48=16.3, P<0.001). Those groups which had attached leaves to their artificial nests endured the storm. The number of attached leaves depended on the proportion of surviving spiders. A multiple regression shows that larger groups (Fig. 6C) and heavier spiders (Fig. 6D) had larger nests (r=0.72, F2, 49=26.8, P<0.001). Nests as protective retreats The nest was crucial to the survival of the D. ergandros. All D. ergandros were eaten by the C. robusta in containers that did not contain a nest (i.e. mortality of 100%). I did not observe any active group defence by the D. ergandros against the C. robusta,

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Figure 6. Group size field experiment. The proportion of spiders surviving increased as a function of A, final number of spiders in the group and B, final spider weight (multiple regression: r=0.64, y= 0.01 group size+0.04 weight−0.28). The size of the nest increased as a function of C, final number of spiders in the group and D, final spider weight (multiple regression: r=0.72, y=0.51 group size+1.14 weight−14.1) (see text).

indeed D. ergandros ignored other group members being eaten. However, 94.6% of D. ergandros survived in containers with nests. The C. robusta made forays into the artificial nests, but they were slowed due to the confined spaces, frequently chewing through silk binding the ‘leaves’. The forays were always unsuccessful; D. ergandros were never captured within their nests as they maintained distance and ‘leaves’ between themselves and the predator. The D. ergandros repaired the damaged silk connections once the C. robusta departed the nest. The D. ergandros were vulnerable when foraging on surface of the nest, which is when the C. robusta made successful captures.

DISCUSSION

The care provided by Diaea ergandros mothers was shown in this study to be very important in spiderling survival. Survey data showed that groups of D. ergandros began with mothers, who lived with their offspring over summer and provided the bulk of the early nest construction. Mothers decreased in weight, perhaps due to matriphagy and disappeared in the late autumn. The field and laboratory experiments

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demonstrated the importance of maternal nest construction and capture of large prey, activities that spiderlings were unable to undertake. In the field experiment, spiderlings without mothers dispersed, which suggested that remaining in a small nest without a mother was more risky than dispersing. Spiderlings have been observed to leave one nest and enter another, presumably to find a group with a higher chance of success. Low survival without mothers may be due to (1) absence of large prey items provided by mothers, (2) hungry siblings becoming cannibalistic (Evans et al., 1995), or (3) a higher predation risk associated with smaller nests. The permanent, expandable and protective nest built by D. ergandros was shown in this study to be very important in the survival of spiders also. Survey data showed that nest size increased with group size and age; maximum nest size was attained when spiders matured, also when predators were most common. The field experiment demonstrated that larger nests were built by larger groups of juveniles and subadults, but these bigger nests required less individual effort. These larger nests conferred the highest level of spider survival. The nest provided defence against inclement weather and predators. The small vertebrate predators that were able to sit on the slender branchlets and leaves that support the nest were not able to penetrate larger nests, whereas invading predacious spiders were foiled by its labyrinthine structure. This was passive rather than active defence, yet it was communal defence because nest construction and repair was performed by many individuals. The two factors found in this study on D. ergandros, i.e. parental care and a permanent, expandable and protective retreat are common to other, diverse social taxa, (Andersson, 1984; Alexander et al., 1991; Seger, 1991). These factors are not limited to thomisids among social spiders; indeed, all other social spiders exhibit these characteristics as well (Shear, 1970; Burgess, 1976; Buskirk, 1981; D’Andrea, 1987; Avile´s, 1997). All social spiders, and closely related solitary and subsocial species, exhibit some form of maternal care. Essential feeding either via regurgitation or by prey provisioning occurs in the well studied social genera Agelena (Agelenidae) (Krafft, 1969; Darchen, 1973), Stegodyphus (Eresidae) (Kullman, 1972; Jacson & Joseph, 1973; Seibt & Wickler, 1988; Schneider, 1995), Anelosimus (Theridiidae) (Kullman, 1972; Brach, 1977; Christenson, 1984), Achaearanea (Theridiidae) (Lubin & Robinson, 1982; Lubin, 1982). Scavenging prey remains may provide a similar function, e.g. Dictyna and Mallos (Dictynidae) ( Jackson, 1978, 1979), Badumna (Amaurobiidae) (Gray, 1983; Downes, 1993) and Cyrtophora (Araneida) (Lubin, 1974), and Tapinillus (Oxyopidae) (Avile´s, 1994). Matriphagy may also be important, as it has been described from three social genera in different families: Diaea (Thomisidae) (Evans et al., 1995); Stegodyphus (Eresidae) (Kullman, 1972; Kullman & Zimmerman, 1975; Seibt & Wickler, 1987; Schneider, 1995) and Theridon and Anelosimus (Theridiidae) (Kullman, 1972; Brach, 1977). All social spiders inhabit permanent, expandable and protective retreats; the most common type is the snare web. All types of retreats are protective and are associated with food supply, and so provide food as well as shelter. All social spider retreats, leaf nests (Diaea spp., Tapinillus sp., Avile´s, 1994), bark retreats (Delena cancerides, Rowell & Avile´s, 1995) or snare webs, can be enlarged as the group increases in size or needs. It is interesting to note that the snare webs and retreats woven by non-territorial social species appear to be analogous to multiple, combined examples woven by their solitary relatives: e.g. Agelenidae (Krafft, 1969; D’Andrea, 1987), Theridiidae (Brach, 1977; Vollrath, 1982; Christenson, 1984; Lubin, 1986), Dictynidae ( Jackson, 1978; Tietjen, 1986) and Eresidae (Seibt & Wickler, 1988; Schneider, 1995).

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There are no experimental studies that consider the importance of the snare web to group survival in other social spider species. However, survey data have shown that large groups make larger webs at lower individual cost for Anelosimus eximius (Vollrath, 1982, 1986), Agelena consociata (Riechert, Roeloffs & Echternacht, 1986), Mallos gregalis (Tietjen, 1986), and Metepeira spinipes F.O. Pickard-Cambridge (Uetz, 1986). As found for D. ergandros, smaller groups or webs are more likely to go extinct for Agelena consociata (Riechert et al., 1986; Roeloffs & Riechert, 1988), S. mimosarum and S. dumicola (Seibt & Wickler, 1988), Anelosimus eximius (Vollrath, 1982; Avile´s, 1986; Venticinque, Fowler & Silva, 1993; Leborgne, Krafft & Pasquet, 1994), Achaearanea wau (Lubin & Robinson, 1982), Metepeira spinipes (Uetz, 1986, 1988), and M. incrassata F.O. Pickard-Cambridge (Uetz & Hieber, 1994). This study on D. ergandros and those of other social spiders support the universality of parental care and permanent, expandable and protective retreats as important factors that influence the evolution of social behaviour in all taxa (Andersson, 1984; Alexander et al., 1991; Seger, 1991). But questions remain to be answered. Maternal care is nepotistic (Hamilton, 1987; Clutton-Brock, 1991), and social spiders groups are kin based, yet social spiders have not yet been reported to have kin recognition. Indeed, the lack of group closure distinguishes spider sociality from that found in other taxa (Wilson, 1971; Buskirk, 1981; Darchen & Delage-Darchen, 1986; Downes 1996). The influence and the adaptive value of these open groups, and the importance of inclusive fitness (Hamilton, 1964a, b), if any, for the evolution and/or maintenance of social behaviour in spiders, has yet to be examined experimentally for social spiders. This group closure and inclusive fitness benefits may prove a fruitful area of investigation offering new insights into the evolution of social behaviour in spiders.

ACKNOWLEDGEMENTS

My thanks to Emily Bolitho, Nathan Evans, and Sally Troy, for their help in the field and laboratory, and to Mark Elgar, Robert Jackson, Michael Lenz, Michael Magrath, Alain Pasquet and Nina Wedell for their discussion and comments that improved this manuscript. Also, special thanks to Simon Ward for allowing me to test nests with his Feather-tail gliders. I am grateful for the financial support provided by the Australian Research Council, Ecological Society of Australia, Royal Society of Victoria, Zoological Society of New South Wales, and the Department of Zoology, University of Melbourne.

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