Web decoration of Micrathena sexpinosa (Araneae: Araneidae): a frame-web-choice experiment with stingless bees

2011·4.1 MB·English

by  Dumas Gálvez

2011.TheJournaloFArachiiology39:128-132
Web decoration ofMicrathena sexpinosa (Araneae: Araneidae): a frame-web-choice experiment with
stingless bees
i
DumasGalvez': SmithsonianTropical Research Institute, RoosveltAvenue, TupperBuilding-401, Balboa, Ancon,
Panama, Republicade Panama. E-mail: [email protected] j
Abstract. Thefunctionofsilkwebdecorationsinorbweavingspidershasbeendebatedfordecades.Themostaccepted
hypothesized functionsarethat web decorations 1) providecamouflageagainst predators, 2)arean advertisement for
vertebratestoavoidwebdamage,or3)increasetheattractionofpreytotheweb.Moststudieshavefocusedononlyafew
genera,Argiopebeingthemostcommon.Inthisstudy,Ievaluatedthepreyattractionhypothesisofsilkdecorationsfora
speciesofapoorlystudiedgenusinthistopic,MicrathenasexpinosaHahn1822.Iusedaweb-choiceexperimentinwhichI
presentedemptyorweb-bearingframesattheendofatunneltostinglessbees(Tetragoniscaangustula).Thisframe-choice
experimentconsistedofthefollowingcomparisons:decoratedwebvs.emptyframe,decoratedwebvs.undecoratedweb,
andundecoratedwebvs.emptyframe.Webswithdecorationinterceptedsignificantlymorebeesthanemptyframesand
undecoratedwebs.Therefore,thedecorationsofMicrathenasexpinosamightplayaroleinincreasingforagingsuccess.
Keywords: Decorated,foraging,stabilimenta,undecorated
Adiversenumberoforbweavingspidersdistributedinboth prey than undecorated webs (e.g., Watanabe 1999; Herber-
tsrtoapbiiclailmenatnad,ttoemthpeeirrawteebszo(nSecsharafdfd&siClokddwienbgtdoenco1r9a9t7i)o.nsT,heiorr 2st0e0i5n;2B0r0u0;ceB&rucHeerebtearls.t2e0i0n12;0C0r5a;iCgheetnagl.&20T01s;oL2i00e7t)a,l.b2u0t0s4;omLei ^!,
function is unknown, and at least six functions have been researchersfoundnoevidenceinfavorofthehypothesis(e.g.,
suggested for these structures (Herberstein et al. 2000; Bruce Blackledge&Wenzel 1999;Hoeseetal.2006;Jaffeetal.2006;
2(e0.0g6.),.Eb1)erThhaeryd m20a0y3),ca2m)oullurleagperetyhetosptihdeerweagbai(nes.gt.,prLeidaettoarls. 2B0u0s8h).eOtnael.sh2o00r8t;comEibnegrhoafrdthi2s00h8y;potGhaewsriysszisewthsaktipr&eyMcootutlad ,
2004), 3)work asadvertisement tovertebratessoas toavoid apparently detect and avoid the web by the presence ofthe
webdamage(e.g., Eberhard2006),4)stabilizetheweb(Bruce decoration(e.g., Blackledge&Wenzel 1999). i|
2006), 5) produce shade for thermoregulation of the spider Usingstinglessbees, Itestedtheprey-attractionhypothesis
(Humphreys 1992), or 6) collect water from the dew for the for the less well-studied Micrathena sexspinosa Hahn 1822. if
spider’s consumption (Walter et al. 2008). The fact that web Micrathena is a Neotropical genus that constructs web :
vdiescuoarlatfiuonncstiaorneo(nSlchyafrofufn&diCnoddiduirnngaltosnpec1i9e9s7)s.trHoongwleyvesurg,geostthsear dheacsortaetsitoendsa(nHyerhbyeprosttehiensisetraelg.ar2d00i0n)g.tNheevefrutnhcetlieosns,onfothoensee :
f
possibilities are not necessarily mutually exclusive, although decorations in any ofthe species. In contrast to the model
evidencesupportingtwoormorefunctionsatthesametimefor genusArgiopewithitspolymorphismofdesigns(Herberstein
anyspeciesislacking(butseeWatanabe 1999,2000). 2000) that perhaps correlate to several functions (Bruce & .
Studies have mostly tested putative visual functions Herberstein 2005), M. sexspinosa consistently produce the ;
(Herberstein et al. 2000; Bruce 2006). Evidence in favor of same decoration (D. Galvezpers. obs.), a line ofsilk on the 1
the two most popular hypotheses (1 and 2) is contradictory. topofthehuboftheweb(Fig. 1). ^
Severalstudiessuggestthatdecorationscandetertheattackof I used a trial tunnel in thefieldcombinedwith decoration
a predator or camoullage the spider (e.g., Blackledge & removal totest thepreferenceofstinglessbeesforwebswith I
Wenzel2001; Eberhard2003; Lietal.2003;Chouetal.2005; decorations. Inmydesign,preynestinginawoodenboxhad I
Gonzaga & Vasconcellos-Neto 2005), but other researchers to fly out of the tunnel and choose an exit in which the '
did not find evidence in favor ofan anti-predator function different web treatments were placed (Galvez 2009). An
(Herberstein2000;Seah&Li2001;Bruceetal.2001;Li&Lim advantage of this approach is that it mimics natural visual ;
2005; Eberhard 2006; Jaffe et al. 2006; Cheng & Tso 2007). conditionsbetterthanlaboratoryexperiments(Bruce2006).I '
Oneofthecriticismsagainstthishypothesisisthatdecorations predictedthatifwebdecorationsfunctiontoattractprey,then
can attract predators to the web as well (e.g., Bruce et al. decorated webs would intercept more bees than the undeco-
2001). ratedwebsoremptyframes.
In contrast, the prey-attraction function suggests that METHODS
decorations could resemble UV gaps in vegetation, eliciting —
escape behavior in fiying insects, or they could imitate food Site&species. IcarriedouttheseexperimentsatLaSelva .
resourcesthatreflectUV,luringprey(Craig&Bernard 1990). BiologicalStationinHeredia,CostaRica(10°26'N,83°59'W),
Many researchers found that decorated webs intercept more a 1550-ha reserve in the Atlantic lowlands with an annual :
averagerainfallof4000mm^(Sanfordetal. 1994).Micrathena .
o'fCuLraruesnatnanded,reBssi:opDhoerpea,rtUmNeInLt-SoforEgceo,loCgHy-a1n0d05EvoLlauutsiaonnn,e,UniSvweirtszietry- streoxpsipcisnotshaaticsonastrsumcatlslitosrwbe-bweianvithnegmisdpisdteorfodcecnusrervienggetiantiotnh,e
land. wovenonaverticalplaneorslightlyinclined(10-20°,Nenwtig
|
128
GALVEZ—WEBDECORATIONOFMICRATHENA SEXPINOSA 129
—
Figure 2. Trial tunnel in which the Tetragonisca angustula
stingless bees were exposed to the different web treatments of
— Micrathena sexspinosa. The walls and roofof the tunnel are not
Figure 1. Araneid Micrathena sexspinosa on its web eating a shown inordertorevealtheinterior. Bothexitsofthetunnelwere
stinglessbee.Thespiderrestsatthecenteroftheholeintheweb;the opened(AandB);thereforebeescouldflyoutofthetunnelfromthe
decoration is built next to it. The arrow indicates part of the nest(N)byeitherexit(arrows).Seetextfordetailsabouttheframes
decoration.Scalebar= 1 cm. bearing the webs. This figure depicts the comparison between a
decorated(Aright)andanundecoratedweb(Aleft).
1985),withacentralholethroughwhichthespidercanmove
easilyfromonesidetotheother(Nentwigetal. 1993).Nextto tunnel. Three variations of the choice experiment were
this hole, the spider usually builds a linear decoration like performed: “decorated web vs. empty frame” (n = 8 pairs,
otherMicrathenaspecies(Herbersteinetal.2000). Iidentified 86bees)“decoratedwebvs.undecoratedweb”(n=9pairs,96
thespidersusingLevi(1985). — bees),and“undecoratedwebvs.emptyframe”{n=7pairs,72
Experimental apparatus and treatments. Without being bees). I kept thespiders on the webs and used individualsof
systematic, I collected samples of M. sexspinosa and their similar sizes with the intention of comparing the two web
websdailyfromthefield(aroundbuildingsandgreenhouses) treatments. I controlled theeffect ofwebsize, sincethewebs
bystickingthewebstocardboardframes(18 X 18cm),witha foreachtreatmentalwayscoveredthesameareaintheframe
hole in the middle (324 cm^). The side ofthe frame used to (324 cnr). The exit ofthe tunnel bearing the frames was in
bear the web had adhesive tape placed with the sticky side frontofherbaceousvegetation,withadarkgreenmeshplaced
facingtheweb.Thistapewasfixedtotheframebywrappingit onemeterfromitinordertoincreasethecontrastbetweenthe
to the corners of the frame with adhesive tape. I removed websandthebackground(Bruceetal. 2005).
decorationsfrom 16outof34websbyburningthesilkwitha I counted the numbers of bees either being intercepted
heated fine-pointed forceps while the spider was still on the (includingbeescaughtbyspiders)orfiyingthroughtheempty
web. In case some damage was done to the web during the frame(hereafterreferred toas“numberofbeesintercepted,”
burningprocess, particularly to the sticky spirals, I used the although the empty frames could not intercept bees). I
forcepstodamageasimilarareaoftheorbonthedecorated switched the relative (left/right) positions ofthe frames each
webtobeusedforcomparison.Icollectedatotalof34spiders time two bees had exited the tunnel or were intercepted in
andusedonlyoneorbfromeachspider. order to avoid any possible bias due to frame position. The
Iplacedthewebsattheendofa300 X 120 X 80cmtunnel frameswereplacedattheexitofthetunnelonlywhennobee
(Fig. 2), open at both exits, modified from Galvez (2009). wasleavingthenestorHyingin thetunnel. Incasesinwhich
Since the frames did not match the area at the end of the threeormorebeesaccumulatedinthewebbecausethespider
tunnel, the remaining spaces werecovered with cardboard. I didnotattackthem,Iremovedtheframesandusedforcepsto
placed a wooden box (40 X 30 X 20 cm) with a nest ofthe remove the bees in order to avoid the possibility that bees
stingless bee Tetragonisca angustula Latreille 1811 at one of caughttherewoulddetermorebeesfromflyingintotheweb.
the ends of the tunnel. Thus the bees could fly out of the Thedamagetothewebsusingthisprocedurewasminimaland
tunnelthrougheithertheendbearingtheframes(AinFig. 2) itwasnottakenintoaccountfortheanalysis.Ididnotremove
ortheendnexttothenest(BinFig. 2);however,beesflewin thebeesiftheywerecaptured bythespiderorwrappedwith
oroutalwaysthroughtheendbearingtheframes(duringthe silk by the spider (1-2 bees per trial). After this, I put the
trials). I placed thenestinthe tunnel with bothexitsopened framesbackattheexittocontinuetheexperiment.1used9-10
for48hbeforethebeginningoftheexperimentsinordertoget bees per pair offrames, which required a new pair ofwebs
thebeesacclimatedtothetunnelandthenewnestlocation. madebyfreshspiders.
I carried out a two-frame choice experiment in which the Itestedforasignificanteffectofwebtypeonthelikelihood
beeswereexposedtotwoframesplacedatthesameendofthe ofbee interception usinga linearmixed model. 1 treated the
s
130 THEJOURNALOFARACHNOLOGY
Table 1.—Statistical summary and preferences forthe two-framechoiceexperimentsset for Micrathenasexpinosa. Abbreviations; dec =
decoratedwebs;undec = undecoratedwebs;empty =emptyframes.
%ofbeesintercepted
Totalnumber
Treatment Z P n ofbees dec empty undec
—
decvs.empty 3.90 <0.001 8 86 65 35
undecvs.empty 0.829 0.407 7 72 —46 54
decvs. undec 2.74 0.006 9 95 60 40
counts of bees intercepted per web type in each trial as times in 15 different genera, suggests the possibility of
proportional data. I evaluated web type (between pair of different functions (Scharff& Coddington 1997; Herberstein
frames) as the main effect and trial as random effect. etal. 2000).
Therefore, I carried out an analysis for each frame choice Multiplefunctionsfordecorationshavealmostnosupport
experiment. I accepted effects as statistically significant for intheliterature,andMicrathenasexspinosa' decorationdoes
P< 0.05,and Icarriedoutallanalysesin R 2.10.0usingthe not seem to be an exception. Eor instance, individuals are
function Imer, specifying the binomial distribution for foundinconfinedspaces(e.g.,shrubs)andthereforeitisvery
proportiondata(R DevelopmentCoreTeam2009). unlikely that the decoration acts as a web advertisement for
RESULTS bEibredrsha(re.dg.,200B6l;acGkalwerdygesze&wsWkeinz&elMot19t9a9;20J0a8f)f.eFeutrthale.rm2o0r0e6,;
Inthistwo-framechoiceexperiment, Icompared“decorat- thedecorationprobablydoesnotworkasamechanicalbarrier
edwebsversusemptyframes”for8pairsofframes(86bees), against predators, because the spider never rests behind the
“undecoratedwebsversusemptyframes”for7pairs(72bees) decorations,abehaviorfoundinArgiopespecies(e.g.,Lietal.
and9pairs(96bees)for“decoratedwebsversusundecorated 2003).Moreover,thesizeandshapeofthedecorationdoesnot
webs.” Decorated webs intercepted significantly more bees provide full cover to the spider. Micrathena sexspinosa
(65%) than the empty frames (35%, Z = 3.90, P < 0.001, generally builds its web in or between the vegetation;
Table 1). Decorated webs intercepted more bees than undec- consequently, one side oftheweb is almost always unreach-
orated webs as well (40%, Z = 2.74, P= 0.006, Table 1). I able to approachingpredators (e.g., spider-huntingwasp). It
found no differences in the number of bees intercepted seemsthatthemainanti-predatorresponseofM.sexspinosais
between undecorated webs and the empty frames (Z = toshuttletotheothersideofthewebthroughthecentralhole
0.829, P = 0.407,Table 1). inthehubordroppingfromtheweb(pers. obs.).
Micrathenasexspinosa'^decorationpatterndoesnotappear
DISCUSSION
to function for thermoregulation of the spider (Humphreys
The prey attraction hypothesis proposes that decorations 1992).Thedecorationdoesnotprovidefullshadeagainstsolar
mayincrease the foragingsuccess ofspiderbyluringpreyto radiation, and the spider does not usually rest behind the
the web. Micrathena sexspinosa spiders on decorated webs decoration(pers.obs.).Amechanicalfunctiononthewebalso
interceptedsignificantlymorebeesthanonemptyframesand seemsunlikely,sinceseveralindividualscanbefoundnearto
spiderson undecorated webs, which isin agreementwith the each other on both decorated and undecorated webs under
hypothesis. The hypothesis has been partially supported similarenvironmental conditions. Ifdecorationswereimpor-
among Argiope species; however, there is almost no support tantforstrengtheningtheweb,thenitisexpectedthatspiders
for other genera of araneids such as AUocyclosa (Eberhard under similar environmental conditions would show similar
2003), Araneiis (Eberhard 2008, but see Bruce et al. 2001), decorating behaviors. However, I could not evaluate if an
Cyclosa (Baba 2003; Chou et al. 2005; Gonzaga & Vascon- increaseofthewebtensionoccursduetothedecoration. For
cellos-Neto2005,butseeTso 1998b)andGasteracantha(Jaffe instance,Octonohasybotides(Bosenberg&Strand 1906)build
etal.2006;Eberhard2006;Gawryszewski&Motta2008).The decorations that lure prey to the web (Watanabe 1999) and
same can be said for the uloborids Philoponella (Eberhard increasewebtension(Watanabe2000),whichallowsthespider
2006) and Zosis (formerly Ulohonis, Bruce et al. 2005; torespond fastertosmall preycaughtintheweb. Therefore,
Eberhard2006). thesetwofunctionsarenotnecessarilymutuallyexclusive,and
Thereisa largevariationofdecorationsatthespeciesand bothincreaseforagingsuccessofthespider.
individuallevelwithinthesegenera(Herbesteinetal.2000);in Luring prey to the web might not depend entirely on the
marked difference, Micrathena only shows a monophormic web decoration but perhaps on the spidercoloration aswell
lineardecoration pattern (Scharff& Coddington 1997). This (e.g.,Argiopespp.,Craig&Ebert 1994;Tsoetal.2002;Cheng
varies from the polymorphism ofdecoration patterns found, & Tso 2007; Bush et al. 2008). The lack of significant
forexample,inthemodelgenusArgiopethatmightberelated differencesbetweenundecoratedwebsandemptyframesdoes
to several functions (e.g., Bruce & Herberstein 2005). The notsupporttheprey-attractionfunctionofbodycolorationas
linearpattern is probably primitive forthe araneids Argiope, suggested for other araneids. However, this study was not
Cyelosaand Gasteraeantha(Herbersteinetal. 2000;Chenget designed toevaluatetheeffect ofspidermorphologyonprey
al. 2010). In contrast, it appeared de novo in Micrathena behavior. In Micrathenagracilis(Walckenaer 1805), Vander-
(Herberstein et al. 2000). Therefore, the function of web hoffetal.(2008)didnotfindanyeffectofspiderpresenceon
decorations in Micrathena might differ from its function in preycapturerate,nordidhefinddifferencesbetweencontrol
othergenera. The lability ofthis trait, evolving at least nine and black-painted spiders. Therefore, bodycoloration ofM.
GALVEZ—WEBDECORATIONOFMICRATHENA SEXPINOSA 131
sexspinosa might serve for another function, for instance in Craig,C.L. & K. Ebert. 1994.Colourand pattern in predator-prey
camouflageofthespider(Hoeseetal.2006;Vaclav&Prokop interactions; the bright body colours and patterns ofa tropical
2006). Nevertheless,thebestmethodforevaluatingtheeffect orbspinningspiderattractflower-seekingprey.FunctionalEcology
ocfompthaerinspgidweerbs(ew.gi.t,hcsopliodrearstiaogna)inostnwperbesywiattthroaucttisopnideirss,bya Cra28i0:g06,11.6C-.6SL2i.g0,n.aSl.Gp.olWyomlofr,pJh.iLs.mD.inDatvhies,webM-.dEe.coHraatuibnegrs&pidJe.rL.ArMgaiaosp.e
cspoemcptararlismoenasIudriedmennottsinocflutdheeidnecthoirsatsitoundsy,. sIpnidaedrdsitiaonnd, tthhee aorfgsetnitnagtlaessisbceoersr,eliatstepdriwmiathryrepdreuyc.edEvsoulruvtiivoornsh5i5p:9a8n6d-99th3e.presence
backgroundcan beusedtoevaluatetheirvisibilitytopreyin Eberhard,W.G.2003.Substitutionofsilkstabilimentaforeggsacsby
ordertoconfirmthepreyattractionfunction(e.g.,Bruceetal. Allocyclosahifurca(Araneae:Araneidae)suggeststhatsilkstabili-
2005). mentafunctionascamouflagedevices.Behaviour140:847-868.
ThedecoratingbehaviorofM.sexpinosacouldofferagreat Eberhard,W.G. 2006. StabilimentaofPhiloponellavicina(Araneae:
advantage for resource use; however, further research is Uloboridae)andGasteracanthacancriformis(Araneae:Araneidae):
neededinordertoevaluatewhetheradisadvantageofbuilding evidenceagainstapreyattractantfunction.Biotropica39:216-220.
the decoration exists as in other decorating species (Bruce Eberhard, W.G. 2008. Araneus expletiis (Araneae, Araneidae):
2006,Herbersteinetal. 2000). aJonuortnhaelrosftaAbrialcihmneonltougmyt3h6a:t19d1o-e1s94.not function to attract prey.
ACKNOWLEDGMENTS Galvez, D. 2009. Frame-web-choiceexperimentswith stingless bees
I thank Carlos Garcia-Robledo, Orlando Vargas, Johel Asurpgpioorptestahveignpyrie.y-JaotutrrnaacltioofnAhryapcohtnhoelsoigsyf3o7r:2s4i9lk-25d3e.corations in
Chaves,AriettaFleming-Davies,ClaudiaLizana,andMirjam Gawryszewski, F.M. & P.C. Motta. 2008. The silk tuft web
Knornschild for help duringtheexperiments; Aidan Vey for decorationsoftheorb-weaverGasteracanthacancriformis'. testing
improving the English ofthe manuscript; David W. Roubik the prey attraction and the web advertisement hypotheses.
forhis helpwith the identification ofthe bees; Erin Kurten, Behaviour 145:277-295.
SeanJ.Blamires,AndrewT.Sensenig,andLindenHigginsfor Gonzaga, M.O.&J.Vasconcellos-Neto.2005.Testingthefunctions
theircomments on the manuscript; and finally, I also thank ofdetritusstabilimentainwebsofCyclo.safililineataandCyclosa
theOrganizationforTropical Studiesthatfundedthisstudy. inorretes(Araneae:Araneidae):dotheyattractpreyorreducethe
riskofpredation?Ethology 111:479-491.
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JournalofExperimental Biology212:1832-1839. Manuscriptreceived21August2010, revised12Fehruary2011.