Newly-Discovered Sociality in the Neotropical Spider Aebutina binotata Simon (Dictynidae?)

10 Pages·1993·9.9 MB·English

by  Leticia Avilés

1993. TheJournal ofArachnology 21:184-193
NEWLY-DISCOVERED SOCIALITY IN THE NEOTROPICAL
SPIDER AEBUTINA BINOTATA SIMON (DICTYNIDAE?)
Leticia Aviles': Museum ofComparative Zoology, Harvard University, Cambridge,
Massachusetts 02138 USA.
ABSTRACT. The neotropical spider Aebutina binotata Simon (Dictynidae?), previously known from a few
museum specimens, was discovered to live in colonies and to exhibit highly cooperative behaviorsthat would
classify it as non-territorial, permanently social. Colonies ofthis species, that contained from 14-106 adult
femalesplustheiroffspring,wereobservedinatropicalrainforestsiteinEasternEcuador.Thespidersoccupied
communal nests in which they cooperated in prey capture and fed communally on the prey. Large prey items
were moved to the feeding site by the coordinated effort oftwo or three individuals. The spiders periodically
carried out web maintenance activities; but when widespread damage to the nest occurred, they moved as a
grouptoanewlocation. Careofthebroodappearedtobecommunalsincetheoffspringfromdifferentmothers
intermixed in the colonies and were all cared for by a decreasing number ofsurviving females. Adult females
participated most heavily in all the activities ofthe colonies, with no apparent division oflabor among them.
Inparticular,noreproductivedivisionoflaborwasobserved: alladultfemalesincoloniesobservedthroughout
the egg-layingperiod apparently laid a single egg sac each.
The most advanced form of social behavior operative behavior in spiders, although rare rel-
known for spiders involves cooperation among ativetothetotalnumberofspiderspecies(about
membersofacolonyinbuildingandmaintaining 40,000describedspecies), isperhapsmorewide-
a communal nest, capturing prey on which to spread than previously believed.
feedcommunally,andtakingcareoftheoffspring Here, I report on newly discovered sociality
(Buskirk 1981; D’Andrea 1987). Thesetasksare in the neotropical cribellate spider Aebutina bi-
performed by members ofthe same generation notataSimon(Simon 1892).Thisspecieswasde-
without any apparent division of labor among scribed by Simon at the end ofthe last century
them (Darchen & Delange-Darchen 1986). In from a few female specimens collected in the
particular, unlikewhatoccursinthe mosthighly Brazilian Amazonas Province (Simon 1892).
social insects (Wilson 1971), no specialized re- From a systematic point of view, Aebutina bi-
productive castes are present in social spiders: notata has proven to be an enigmatic species
most, ifnot all (Vollrath 1986), individuals in a whose placement in any of the currently de-
social spider colony apparently bear offspring. scribedspiderfamiliesisnotfullyresolved(Leh-
This form of social behavior, labeled as non- tinen 1967). The genus Aebutina, of which A.
territorial permanently social (D’Andrea 1987) binotata is the type species, is temporarily as-
or quasisocial (Wilson 1971), has arisen inde- signed to the Dictynidae (Petrunkevitch 1928;
pendentlyinatleastsix spiderfamilies. Todate, Millot 1933), afterhavingbeen originallyplaced
a total of 14 species in eight genera have been by Simon (1892) in the Uloboridae. Previous to
described as possessingthe traits that would de- thepresentstudy, noinformationon thelife his-
finethemashavingattainedthislevelofsociality tory or behavior ofA. binotata was available.
(forpartial listsseeBuskirk 1981;and D’Andrea The observationsthat I reporthere indicate that
1987; for species not included in these partial this species is colonial and that it exhibits the
lists, seeMain 1988, Rypstra&Tirey 1989;Avi- stronglycooperativebehaviorscommonto non-
les in press 1, in press 2). Ten ofthese species territorial permanently social spiders.
havebeendescribedortheirsocialitydiscovered In this paper I describe the structure of the
in the last 30 years, indicating that highly co- nests and colonies and report on behavioral as-
pectsofthesocialityofthisspecies: observations
'Presentaddress: Dept, ofEcologyandEvolutionary involvingcooperation on web maintenance and
Biology, Univ. ofArizona, Tucson, Arizona 85721 repair,colonyrelocation,preycaptureandtrans-
USA port, food sharing, communal brood care, and
184
AVILES~=SOCIALITY INAEBUTINA BINOTATA SIMON 185
2
Figures 1, 2.~Aebutina binotatagenitalia: 1. Male; 2 Female. Drawings from computerdigitized images by
W. Maddison. Scale bars correspondto 0.1 mm.
tolerance to members of other colonies. Even binotata in a tropical rainforest site by the Tar-
though not directly related to sociality, I also apuy Riverin Eastern Ecuador, Sucumbios (for-
present observations on courtship and mating. merly Napo) Province (0° 08' S, 76° 16' W, 210
m
METHODS abovesealevel).Ifirstdiscoveredtwocolonies
in January 1983 and then an additional one in
Thespider.—Liveadultfemalesof binotata, February 1984. From this date to September
mm =
which measure about 5 in length {x 4.7, 1984, I visited the area on six occasions and
n = 7), have a diamond-shaped, bright yellow identified a total of44 colonies (24 up to a July
abdomen with a black spot on each side (there- visit and 20, either new or previously recorded
mm
from the name binotata). The males (3.4 colonies that had relocated themselves, in the
when adult, n = 2) and early-instar individuals September visit).
are also yellow, of less intense coloration and For each colony observed, I recorded its po-
with somewhat less clearly marked spots. Males sition,distancefromtheground,andstructureof
are adult in the 1‘^ instarwhile femalesare adult the nest, includingthe size ofthe leaf(or leaves)
in the 8'^ instar (Aviles 1992). The egg sacs are supportingthecolonyandthe percentoftheleaf
mm
spherical and measure around 3.5 in di- (orleaves) occupied. I countedthe total number
ameter. Theyconsistofa mesh ofwhitesilkthat ofadult females, egg sacs, and males present in
surrounds the yellow-colored embryos. the colonies, and estimated the number ofju-
One ofthe reasons for the uncertainty in the veniles ofdifferent size classes (i.e., instars).
systematicplacementofA. binotatahasbeenthe Iconductedbehavioralobservationsonanop-
lackofmale specimens (Lehtinen 1967). Incon- portunistic basis. Observations were conducted
nection with the present study I collected males during the day, usually between 900 and 1800
which have been deposited in the collections of h, on one occasion from 700 h. Activities in-
the Museum ofComparative Zoology, Harvard volving nest maintenance and repair were ob-
University. In this study I do not address the served regularly in the colonies. I observed one
systematic placement ofA. binotata. However, completeshort-distancecolonyrelocationevent,
I provide drawings ofmale and female genitalia aportionofanother,anda 13-hourperiod(1200-
for future reference (Figs. 1, 2). Initial determi- 1800 h and 700-1400 h ofthe day after) ofone
nation ofthe female specimens was done by J. long-distance relocation event. I also obtained
HunteroftheMCZandlaterconfirmedbycom- indirect evidence ofsix other relocation events.
parison with the types by H. W. Levi. I recorded complete sequences ofprey capture,
The observations.—I studied colonies of A. including prey transportation and initiation of
186 THEJOURNALOFARACHNOLOGY
feeding, on 13 occasions,andon fiveothersafter cm^ and included either a portion ofa large leaf
feeding had been initiated. Four cases of prey or several small leaves. Nests were found be-
m m
rejection were observed. Other prey capture or tween0.5 and4,5 fromtheground,although
feeding events were observed on a more casual nests occurring higher than this would have
basis. I observed courtship and mating for a pe- probably been missed. The nests appeared in-
riod of 100 min (1215-1400, August4, 1984) in visiblewhen seen from aboveandcouldonlybe
one colony containing fouradult males and 106 located by looking for the spiders underneath
females. I observed a total of four copulations leaves. From below, the appearance ofthe col-
and 16 unsuccessfulmountingattempts. Onead- onies was striking because ofthe bright yellow
ditional copulation was observed at an earlier coloration ofthe adult females and spiderlings
dateinadifferentcolony(colony5,May20, 1155 and the whiteness ofthe egg sacs.
h). A pilot test of tolerance to conspecihcs in- Webconstructionandrepair.—Thenestswere
volvedthe introduction ofoneadultfemaleinto constructedand maintained cooperatively. Web
a foreign nest. Additionally, for ten weeks I maintenanceactivitiesinvolved: (a)periodically
maintained in the laboratory two colonies on adding cribellate silk to maintain the stickiness
which I conducted casual behavioral observa- ofthe web, (b) removingandreplacingdamaged
tions. web following destruction by the rain, (c) re-
pairing holes left by ensnared prey, and (d)
RESULTS
throwingoutdebris. Alltheseactivitiescouldbe
The nest and colonies.—The nests of A. bi- simultaneouslyperformedbyseveralindividuals
notata are basically two-dimensional structures in different areas ofthe nest. For instance, in a
consistingofone or a few contiguous leaves and nest of 13 adults and around 190juveniles, web
their connecting branches covered on both sur- repairafterarainstormrequiredabouttwohours
faces by a continuous layer ofsilk (Fig. 3). The during which 16-26 spiders working at a time
outer surface ofthe web is covered by cribellate removed the damaged silk, added new silk lines
silkthatcauseinsectstogetentangledwhenland- acrossthesurfaceoftheleaves,andaddedafinal
ing on it. The sheet ofsilk on the underside of layer ofcribellate silk.
the leaves (the lower web) is not attached to the Unlike other social spiders where regularweb
leafblade, but separated by an open space that reinforcementactivitiestakeplaceexclusivelyat
is used by the spiders as refuge. Egg sacs and sunset(e. g., inAnelosimuseximius, Tapia & De
spiderlingsoccupythisspace, sittingontheinner Vries 1980),A. binotata spidersaddedcribellate
surface of this lower web and congregated to- silkperiodicallythroughouttheday. Thespiders
wardsthecenter.Openingsallowspiderstomove were inactivein colonies seen earlyinthemorn-
freely from one surface ofthe web to the other. ing (around 700-730 h) and all activity seemed
Adultspiderssitontheoutersurfaceofthelower to have ceasedin two colonies observedat 1800
web, lined up along the edges ofthe leaf(Fig. 4) h. At this time the spiders retreated to the un-
inapositionthatallowsthem rapidaccesstothe dersideofthe leavesandpresumablycarriedout
topoftheleafwhereinsectsusuallygetentangled. only prey capture activities until the following
Major perturbations to the nest, such as exper- day.
imental shaking, cause the larger individuals in Colony relocation.—Extensive destruction of
a colony to drop to the ground on silk draglines the web resulted in colonies abandoning their
alongwhichtheyreturnoncethedisturbancehas original nest. One ofthe colonies, for instance,
stopped. moved to a leaf 12 cm from its original location
Thesizeofthecolonies,measuredasthenum- after the leafthat supported its nest dried out.
ber ofadult females present in a colony around Atlaterdates,thisandthreeothercoloniesmoved
m
the time the eggs were being laid, ranged from from0.3-4.5 afteraheavyraindestroyedtheir
14-106 {n = 19, median = 40, mean = 46.7 ± nests. Colony relocation involvingmuchgreater
1 1.8, 95%conf int., Fig. 5). Colonieswithyoung distances(>> 5m),independentofwebdestruc-
spiderlings could contain up to eight hundred tion, apparently took place prior to mating and
individuals; but, because ofthe smaller size of egg laying (Aviles 1992).
theyoung,thetotalbiomasswasprobablywithin Just prior to colony relocation, a fraction of
a similarrange. Thearea occupiedbyanestwas the spiders in a colony could be seen initiating
found to be proportional to the numberofadult the production ofairborne silk lines by hanging
females present (Fig. 6); it ranged from 74-200 down2-5 cmfromtheirnest(seeEberhard 1987
AVILES-SOCIALITY INAEBUTINA BINOTATA SIMON 187
Figures3,4.-~AebutinabinotatacoloniesinTarapuy,Ecuador: 3. Wholenest;4. Adultfemaleslinedupalong
the edge oftheirnest.
188 THEJOURNALOFARACHNOLOGY
number of adult females
Figures 5, 6.—-5. Numberofadultfemales(and/oreggsacs, whicheveris larger)presentinAebutina binotata
coloniesseen sometimeimmediatelybeforeorduringtheegglayingperiod; 6. Correlationbetweenthenumber
ofadult females in a colony and the surface occupied by theirnest (measured in cm^).
formethod ofairborne line production). Once a in a few hours or in consecutive days. For in-
dragline became attached to the nearby vegeta- stance, a colony with eight adult females and
tion it was first followed by the spiderthat orig- more than 200 juveniles (mostly 4th-instar)
inateditandthenbyotherspiders. Evenifthere moved to a location 30 cm away in the span of
were draglines attaching in different directions, 5-”6 hours. Another colony that contained eight
all spiderseventuallymovedalongasingledrag- adult females and approximately 350 3rd-"5th
line since the spiders followingan isolatedroute instarjuveniles moved first to an intermediate
m
returned tojoin the majority. This method was stop 1.8 from its original location, remained
repeatedfromonestoptothenextuntiltheeven- there for a period ofa day and then continued
tual settlement of the colony. In the cases ob- on for an additional meter. Colonies migrating
served, all individuals in a nest moved to the previous to matingand egg layingappearto mi-
new location. grate for longer periods. For instance, a colony
Therelocationofacolonycouldbecompleted with adult males and females that I followed for
AVILES-SOCIALITY INAEBUTINA BINOTATA SIMON 189
13 hours had not settled when I stopped the ob- edge and onto the other side of the nest, and
servations after a day and a half and 50 m of another three minutes to move it one cm into
group migration (Aviles 1992). itsfinalpositionwherefeedingwasinitiated.Two
Prey capture and feeding,—Prey capture in- other group transport events observed required
volvedthesimultaneousparticipationof1-6 in- two and three individuals to transport an ho-
dividuals, apparently depending on the size of mopteran and a fly, respectively.
the prey and the efforts it made to free itself. If Communal brood care.—While most of the
present in sufficient numbers, only the adult fe- adultfemaleslinedupalongtheedgesofthenest
malesparticipatedin preycapture, thoughlater- ready to participate in prey capture, a number
instar spiderlings would participate when there ofthem mountedguard by the egg sacs and spi-
were relatively few adult females in the nest or derlings. Besides sac guarding, parental care in-
the prey were small. The females placed along cludedcatching prey forthe spiderlings and ini-
theedgeofthenestclosesttoanstrugglinginsect tiating the enzymatic digestion of the prey.
wouldrushtowardsitandattackitbyhrstbiting Regurgitation feeding was not observed during
its appendagesandthen otherparts ofthebody. the study period, though specific studies would
Once completely overcome, prey items were be required to confirm its absence.
movedtotheundersideoftheleaveswherefeed- Two lines of evidence suggest that parental
ing took place. Adult spiders initiated prey di- care is communal: (1) the spiderlings in a nest
gestion (Fig. 7) and later left the prey to theju- intermix freely, in a way that it does not seem
veniles. Prey trapped and consumed included possible for a mother to discriminate between
wasps, mosquitoes, cockroaches, one large ant, her own and other mother’s offspring; and, (2)
some beetles. A small coccinelid and another the number ofadult females present in the col-
small beetle were rejected. Very large insects, onies drops continuously (most likely due to
such as a 4 cm long moth and a 2 cm cetonine mortality) during the incubation and emergence
beetle, were ignored. periods,whilealltheeggsacsandoffspringpres-
Group transport.—Transportation of items ent in the colonies continue to be cared for. For
around the nest, either prey to be consumed or instance, half the adult females of one of the
debris to be thrown out, were also among the coloniesobservedthroughoutmostofitslifecy-
activities regularly carried out by the spiders. cle were already goneby the timetheiroffspring
Small itemswere handledby individual spiders. had onlyreachedtheirthirdorfourth instar. All
The transportation oflarge objects, on the other theoffspring, however, continuedtobecaredfor
hand, required a group effort that was particu- bytheremainingfemaleswhosenumbercontin-
larly challenging given the sticky nature ofthe ued to decrease until none were left by the time
entire surface oftheweb. Completegrouptrans- the offspringhad reachedtheirsixth instar. This
porteventswere observedonthree occasions. A indicates that a large majority of the offspring
large ant moved from the upper to the lower were raised by females other than their mother.
surface of the nest, for instance, involved the Division of labor.—Adult females tended to
participation ofthree individuals. While one in- participatedisproportionatelyinalltheactivities
dividual cutpieces ofweb to release the ant, the ofthe colony, though later-instarjuveniles par-
second one pulled and the third pushed the ant ticipated to varying degrees. Later instar juve-
in a given direction. Once released, the ant was niles, for instance, handled small prey orpartic-
transported toward the edge ofthe nest. One in- ipatedingroupeffortswhentherelativenumber
dividualliftedtheantfrombelowsoasto main- of adults in the nest was low. Juveniles were
tain it at a distance from the surface ofthe web. relatively more active in web maintenance ac-
As this individual walked towards the edge, the tivities,particularlyinlayingdowncribellatesilk,
other two individuals, one in front and one in though the numbers in which they participated
the back, helped by pulling and pushing in the werenotrepresentativeoftheproportioninwhich
requireddirection. Onceattheedge, theantwas theyoccurredin thecolonies. Forinstance, after
successfully moved to the other side by having a storm had destroyed the upper web ofone of
one spider hold it from above as it walked to- thecoloniesunderstudy,alladultfemales(atotal
wardstheedgewhiletheothertwosupportedthe ofeight) and 20% ofthejuveniles (out ofa total
itemandpulleditfrombelow. Ittookthespiders of 200) were seen removing the damaged web
four minutes to disentangle and bring the ant to andlayingdownnewstrandsofsilk.Inadifferent
theedge, threemoreminutestobringitoverthe colony, which contained 13 adult females and
190 THE JOURNALOFARACHNOLOGY
Figures 7, S.—Aebutina binotata. 1. Group ofadult females feedingon a prey; 8. Matingcouple in Tarapuy,
Ecuador,
AVILES^-SOCIALITY INAEBUTINA BINOTATA SIMON 191
around 190 juveniles, 30-100% ofthe females Table 1.—NumberoffemalesandeggsacslaidinA.
were active at differenttimes during a two-hour binotatacoloniesperiodicallyobservedthroughoutthe
periodfollowingastorm,whileonly4-7%ofthe egglayingperiod.Inadditiontothe45eggsacsshown,
juveniles were. The youngestjuveniles that ap- colony 14 contained newly eclosed juveniles from
peared able to participate in activities such as around fouregg sacs.
laying cribellate silk belonged to the 4th-instar.
Colony
Immature males in the colonies kept in the lab-
oratory were seen laying silk during web rein- 9 22 23 14 25
forcement,whileitwasnotpossibletodetermine Adult females 102 44 30 48-49 52
whether adult males participate in this or any Egg sacs present 102 44 29 45+ 54
otheractivitysincetheywereseeninthecolonies
for only a short period oftime.
There was no evidence of division of labor
amongspidersofagivenagegroup:ataskstarted ed insistently to mount females. Most ofthe at-
by one individual was often completed by an- tempts observed (16 out of 18) were rejected by
otherandthesameindividualcouldbeseencar- the females who either moved away or resisted.
rying out different tasks. Marking experiments, Two resulted in copulations. Two additional
however, are needed to confirm these observa- copulations had already been initiatedwhen the
tions. Regarding reproductive division oflabor, observationsstarted. Copulationstookplacewith
in all the colonies observed throughout the egg the male over the back ofthe female and both
layingperiodthetotalnumberofeggscasespres- facing in oppositedirections (fig. 8; position “c”
entwasthe sameasthenumberofadultfemales in Foelix 1982, p. 195). In one ofthe matings
in the nest (Table 1). Since all the egg sacs in a observed the female remained motionless all
colonywerelaidwithinashorttimespan(Aviles through the mounting. In a second mating ob-
1992); and thus it is unlikely that some fe- served, the male seemed to exert force over the
males may have laid more than one case, it fol- female. The two complete copulations observed
lows that all females in the colonies observed lasted around 4 min. Given the disparity in the
reproduced. Reproductive division of labor, numbers ofmales and females, and the obser-
therefore, appears absent in Aebutina binotata. vation that most or all the females in a colony
Social interactions and tolerance to conspecif- lay eggs (see above), it follows that each male is
ics.—The 14 adults and 74juveniles kept in the able to fertilize a large number offemales.
laboratory in a 40 x 30 x 30 cm terrarium re-
DISCUSSION
mained aggregated throughouta 10-week obser-
vation period. When they relocated their nest The social behavior here described for A. bi-
within the terrarium, all the spiders moved to- notata has strong similarities with that ofother
gether. Encounters between spiders, which were non-territorial permanently social spiders pres-
common during the course oftheir daily activ- ent in the genera Achaearanea (Theridiidae),
ities, involved touching each otherwith the legs Agelena(Agelenidae),Anelosimus(Theridiidae),
and pedipalps. Diaea (Thomisidae), Mallos (Dictynidae), Ste-
Spiders in the field didnot appearto discrim- godyphus(Eresidae) Tapinillus(Oxyopidae),and
,
inate against members of other colonies. One Theridion (Theridiidae) (Buskirk 1981; D’An-
spider experimentally introduced into a foreign drea 1987; Aviles pers. obs.). These similarities,
nest was initially approached by other spiders, which include cooperative web building and
probablyinresponsetothevibrationsproduced, maintenance, cooperative prey capture, com-
butwas soontreatedasamemberofthecolony. munal feeding, communal brood care, tolerance
After its introduction, the spider resumed the tomembersofothercolonies,andalackofcastes,
activity (adding cribellate silk) it had been per- are particularly striking given the phylogeneti-
forming when removed from its native nest. A cally diverse set of species in which they have
month later, the spider was still in the colony, evolved.
and, aside from the experimental mark, was in- Onefeaturecommontothisdiversesetofspe-
distinguishable from other spiders in the nest. cies,whichmightbetoalargeextentresponsible
Courtship and mating.—I observed courtship forthese similarities is an irregular type ofweb,
and mating in a colony that contained 106 fe- presentinallbutthesocialthomisid(Main 1988;
males, 4 males, and 1 egg sac. During the 100 Evans&Main 1993),butabsentinothercolonial
min ofobservations, threeofthe malesattempt- but non-cooperative species such as Metabus
192 THEJOURNALOFARACHNOLOGY
gravidus or Philoponella republicana (Buskirk thepreyinalldirectionsduringitstransportand
1981; D’Andrea 1987). An irregular web is the nests are considerably larger than the prey
thought to constitute a preadaptation for coop- beingtransported, sothattheiroverallefficiency
erative behavior in spiders because it allows is not critically affected by damage to a portion
communal habitation and the simultaneous in- ofthe web caused by dragging a prey item.
volvement ofmore than one individual in web The instances of group transport that I ob-
constructionandrepairaswellasinpreycapture served in A. binotata required a degree ofcoor-
(Buskirk 1981). Cooperative web building, be- dination among the individuals involved that
cause it allows the construction of a relatively can only be explained if some sort ofcommu-
large area or volume ofentanglingweb, leads to nication was taking place among them. Prey
thecapture ofrelativelylargeprey items, which, transport in other species usually involves a
in turn, require the concurrence of several in- number ofindividuals pulling in the same gen-
dividualsfortheirsubjugation. Largepreyitems eral direction or some form of, apparently un-
can then be shared by several individuals in a coordinated, relay activity (e.g., Ward & Enders
nest, leading to communal feeding, a trait com- 1985). In the genus Agelena the prey is either
mon again to the non-territorial permanently- carried by a single individual (Krafft 1971) or
social species studied (Buskirk 1981; see also eatenonthesiteofitscapturewhenitistoolarge
Main 1988; Rypstra & Tirey 1989), but absent for individual transport (Darchen 1967; Krafft
in the colonial orb weavers. A communal nest 1971).InAchaearaneadisparata(Darchen 1967)
also facilitates communal care ofthe brood be- some individualspull while othersaidincutting
cause it renders discrimination among spider- thethreadsthathinderthemovementoftheitem,
lingsintermixedinacommonspaceimpractical. suggesting, in this case, some degree ofcoordi-
A. binotataistypicalamongthenon-territorial nation. It must be noted that group transport is
permanentlysocialspidersinhavinganirregular an extraordinary task to be performed by an in-
typeofwebandinhavingdevelopedcooperative vertebrate.Evenamongvertebrates,grouptrans-
preycapture,communalfeeding, andcommunal port is only known among humans, dolphins,
brood care. As in these other species (Buskirk whales, and some canids (Moffett in press).
1981),cooperationallowsA, binotataspidersthe Amonginvertebrates,theonlyotherreportedcase
capture of prey items larger than those single ofgrouptransportisrepresentedbytheants(Mof-
individualscouldhandleandfacilitatestheshar- fett in press).
ing ofprey among a larger proportion ofcolony Another feature apparently unique to A. bi-
members than those participating in their cap- notatawhich mayalsoresultfromthespecialar-
ture. The advantages ofcooperation in the care chitecture ofits nests is its nomadic habit that
ofthe brood become specially evident in A. bi- leads to the periodic relocation of its colonies.
notata, wherethe survival oforphanedoffspring The nests of other non-territorial permanently
is only possible because surviving females in- social species, which are usually expanded and
discriminatelycareforalltheyounginanest(see occupiedbymorethanonegenerationofspiders,
also Christenson 1984; D’Andrea 1987). are three-dimensional structures that are prob-
Thewaysinwhichthearchitectureofthenests ably expensive to build and whose prey capture
of A. binotata differs from that of other non- efficiency does not appear to depend on their
territorial permanently social species maybe re- invisibility. The two-dimensional nests ofA. bi-
sponsible for some of the features that appear notata, ontheotherhand, maybelessexpensive
unique to this species. One such feature is the torebuildand, mostimportantly, theyappearto
cooperativetransportofpreyitemsfromthesite criticallydependontheirinvisibilitytotrapprey.
oftheircaptureto theirconsumption site, atrait Suchinvisibilitycanbestbeachievedbyrebuild-
that in this species is developed to a greater ex- ing the nest in a new and debris-free locality.
tentthaninanyothersocialspider.Threeaspects The observationthategglayingisuniversalin
ofthe structure ofthe nests ofA. binotata pose A. binotata (Table 1) illustrates perhaps to an
a special challenge to prey transport in this spe- extreme a feature common to non-territorial,
cies: (1) the sticky nature ofthe web surface; (2) permanently social spiders which critically dif-
therelativelysmallcapturearea;and, (3)thefact ferentiates them from the most highly social in-
thatprey itemsneedtobebroughtovertheedge sects:alackofreproductivecastes(Buskirk 1981;
ofthe leafwith the consequent danger ofacci- Darchen&Delange-Darchen 1986). Eventhough
dentally dropping them out ofthe nest. In other competitionoverresourcesleadingtodifferences
cooperative spiders, webbing usually surrounds in reproductive success are not entirely absent
AVILES SOCIALITY INAEBUTINA BINOTATA SIMON 193
insocialspiders(Riechert 1985; Seibt&Wickler Eberhard, W. 1987. How spiders initiate airborne
1988; Vollrath 1986; Rypstra in press), social lines. J. Arachnol. 15:1-9.
spider colonies critically contain multiple re- Foelix,R.F. 1982. BiologyofSpiders.HarvardUni-
productives ofboth sexes that can mate among versityPress, Cambridge, Massachusetts.
themselves to produce subsequent generations. Krafft, B. 1971, Contribution a la biologie et a la
ethologied'AgelenaconsociataDenis(Araignee so-
This leads to mating within colonies and to the
cialeduGabon). Illpartie. Biol. Gabonica, 7:3-56.
highly subdivided population structure that
Lehtinen, P. 1967. Classification of the Cribellate
characterize non-territorial permanently social spidersand some allied families, with notes on the
species(Lubin&Crozier 1985;Smith 1986;Main evolutionofthesuborderAraneomorpha.Ann.Zool.
&
1988; Roelolfs Riechert 1988; Aviles 1992). Fennici, 4:209.
Incontrast,perpetualinbreedingisrareorabsent Lubin,Y. D. &R. H. Crozier. 1985. Electrophoretic
amongtheeusocialinsectsinwhichnuptialflights evidence for population differentiation in a social
resultinthecrossingofindividualsfromdifferent spiderAchaearaneawau(Theridiidae). Insect. Soc.,
32:297-304.
nests (Wilson 1971). Evidence that A. binotata
Main, B. Y. 1988. The biology ofa social thomisid
has followed the route of other non-territorial
spider. Australian Arachnol., 5:55-73.
permanently social spiders in developing strong
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