Egg capsule architecture and siting in a leaf-curling sac spider, Clubiona riparia (Araneae: Clubionidae)

2011·5.8 MB·English

by  Robert B Suter

2011.TheJournalofArachnology39:76-83
Egg capsule architecture and siting in a leaf-curling sac spider, Clubiona riparia (Araneae: Clubionidae)
RobertB.Suter; DepartmentofBiology,VassarCollege, 124RaymondAvenue, Poughkeepsie, NewYork 12604USA.
E-mail: [email protected]
Patricia R. Miller: Department ofBiology, Northwest Mississippi CommunityCollege, Senatobia, Mississippi 38668
USA
Gail E. Stratton: Department ofBiology, UniversityofMississippi, University, Mississippi 38677 USA
Abstract. Femalesoftheleaf-curlingsacspider Clubionaripariabuildthree-sidedcapsules,inwhichtheyencloseboth
themselvesandtheireggs.Acapsuleisusuallyconstructedbybendingasinglebladeofgrassorotherleaftwice,eachtime
causingafoldthatisperpendiculartothelongaxisoftheblade,andjoiningtheedgeswithsilk.Whenconstructedwith
monocotleafblades,theresultingcapsuleisroughlytriangularincrosssectionand2-Atimesaslongasitiswide.We
sampledoccupiedcapsulesfroma0.16-hectaremarshincentralOntario,Canada.Althoughwefoundcapsulesbuiltwith
theleavesofcattails{Typlialalifolia),iris{Irisversicolor),agrass(Calarnagrostissp.),andanunidentifiedwillowshrub
{Salixsp.),forthecurrentanalysisweconcentratedonthemonocotsbecauseoftheirstructuralsimilarity.Capsulesbuilt
oncattails(2.13±0.14ml)weremorevoluminousthanthoseoniris(1.63 ±0.14ml),andcapsulesmadeofgrassblades
(0.67 ± 0.08ml)werethesmallest.Nearly70%ofthetotalvariationincapsulevolumewasassociatedwithdifferences
betweentheplantspecies.Onlyamongcapsulesbuiltoncattailswasthereasignificantpositiverelationshipbetweenpre-
ovipositionspidermassandcapsulevolume;itaccountedforabout37%ofthevariabilityincapsulevolume.Onwillow
leaves,spidersalwaysconstructedcapsuleswiththelowersurfaceoftheleaftotheinsideofthecapsule;andoncattail
blades, spiders always made their bends in a clockwise direction. We discuss the implications ofour findings for an
understandingofthechoicesthesespidersmakejustpriortooviposition.
Keywords: Reproductiveecology,parentalcare,ovipositionsitechoice,clutchmass
Animal architecture has been extensively studied (von largely because of the elegant and simple capsule that the
Frisch 1974;Collias&Collias 1976;Jonesetal. 1997; Hansell femaleconstructsasashelterforherselfandhereggs(Fig. 1:
2005;Gould&Gould2007),withparticularattentionpaidto Comstock 1948; Edwards 1958; Dondale & Redner 1982;
thestructuresbuiltbybirds(e.g., Hansell2000),socialinsects Paquin & Duperre 2003). These retreats are constructed by
(e.g., Jones& Oldroyd 2007), and web-buildingspiders (e.g., bendinga leaf(often ofamonocot) twice, therebyforminga
KHearsbteornstei1n9642;009)B.laAckmloendggesp&idersE,liwaseobnbu2i0l0d7i;ngHisaronmleyron&e cithsasmebaemrstwhiatthissirlko,ugwhiltyhttrhieaengguglsaarnidncfreomsaslesecitnisoind,ea(nCdomssetaloicnkg i
of several architectural modes and at least two of these, 1948). The capsule takes time and energy to construct and
burrow excavation and the construction of aerial shelters ultimately bears all of the spider’s lifetime reproductive
madewith non-silk “decorations” or by leafcurling, involve output, assumingthevalidity ofComstock’sassertionthatit I
the use ofenvironmental (as opposed to secreted) materials. serves “as a nursery for the spiderlings and a coffin for the
Unlikewebs,whichalwaysserveforagingfunctions(Eberhard parent”(Comstock 1948:581).Inthatcontext,thecapsulecan
c1o99m0m;unFiocealtiixon19c9h6a)nnaenlsd(fWrietqtue&ntRlyovdnoeurbl1e982a;sFoinetlriaxsp1e9c9i6f)i,c btheevgireawveiddafsemtaheleco—nswuhmamtatpliaonntotfoausseeraiesssoufbscthroaitcee;shmoawdehibgyh i
burrows and aerial retreats are usually defensive, serving to on the plant to build; how large t—o make the capsule; how
protect against predators and parasitoids, excessive thermal tightly to seal its edges with silk all interconnected and
load, desiccation, and otherthreats to thespiders’ well being presumablyall undertheinfluenceofnaturalselection.
(Morse 1985, 1988; Konigswaldetal. 1990; Lubinetal. 1991, We report here on C. riparia\ use of the leaves ofthree
1993; Ward& Lubin 1993). monocots (cattail, Typha latifolia, iris. Iris versicolor, and a
Aerial shelters or retreats are particularly interesting grass, Calarnagrostissp.), and to someextenton theiruseof '
because, relativeto retreatsconstructedat thesoil surfaceor the leaves of a dicot (an unidentified willow, SciHx sp.), in
underrocksorlogs,theydisplaytheinterplaybetweenadded constructing enclosed capsules suitable for egg development '
exposuretowind,insolation,andvisuallyorientingpredators andprotection—. Ouremphasishereis oncapsulevolumeand
and parasitoids on the one hand, and on the other hand itscorrelates subsequentpaperswillcovertheenergeticsof
reduced exposure to ground-foraging predators, high soil- capsuleconstructionandthepossibilitythatthegravidspiders
surfacetemperatures,somepotentialpreyitemsand,possibly, showpreferencesamongtheavailableplantspecies.
prospective mates (Henschel et al. 1992; Ward & Henschel METHODS
1992; Ward & Lubin 1993; Konigswald et al. 1990; Morse —
1985, 1988, 2007). Field site and sampling. The study site was an elongated
The leaf-curling sac spider, Clubiona riparia L. Koch 1866 marsh,0.16hainarea,onasmallislandlocatedat45°27'33.1"
(Araneae: Clubionidae), is known among arachnologists N, 80°25'52.7"W, about 2.7 km off the northeast shore of k
76
SUTERETAL.—ARCHITECTUREOFSACSPIDEREGGCAPSULES 77
grasses, sedges, or rushes. Wemarkedeachcapsulesitewith
flagging tape and did not return to it until we had searched
the entire marsh. Then, as we collected each capsule, we
recordedtheplantspeciesand thecapsule’sheightabovethe
watersurface. —
Measurementsandanalyses. In the laboratory, wephoto-
graphedeachcapsuleand usedacalipertomeasureitslinear
dimensionstothenearest0.1 mm.Forcapsulesconstructedon
monocot blades, these were: leaftip to capsule, width ofthe
leafat the first bend, width at the second bend, and capsule
length (Fig. 1). We also noted whether the capsule was
constructed using a pair of clockwise bends or a pair of
counterclockwise bends (Fig. 1) and whether the bends were
madeinsuchawaythatthetopsurfaceoftheleafformedthe
insideor,conversely,thatitformedtheexternalsurfaceofthe
chamber (we did not score this attribute for cattail or iris
blades because we could not differentiate the two surfaces).
Finally, weopenedeach capsule and weighed the spiderand
theclutchofeggs,eachtothenearestmg. Inafewcases, the
spiderhadnotyetlaiditseggs,soforthesewerecordedgravid
femalemassasthecombinedmassofeggclutchandspider(in
our analyses, we considered gravid female mass as being
equivalenttothesumofeggclutchmassandspidermasswhen
thelatterweremeasuredseparately).
In calculating the volume of each of the capsules
constructed with monocot blades, we first applied Heron’s
Formulafortheareaofatriangle(Dunham 1990),assuming
the cross-section ofthe capsule to be an equilateral triangle
with side lengths equal to the average of the two widths
measuredabove. Wethen multiplied thisarea bythecapsule
lengthtogetanestimateofthevolume.Thiswasanestimate
because a) the monocot blades are somewhat tapered, more
— towardtheirtipsthanfurtherdown theleaf; b)neartheends
Figure 1. CapsulesofC. ripariashowingtheirtypicalthree-sided ofthecapsule two sidesofthestructureconverge, givingthe
tsuthsreeusdcettueorxead.imsTptlhieensg,cuiirgscrhualscasarpsaaunrldreoswwistlhalatorewa)rienfcrblouuimdletdthutososiencglbacurliilfotycktuwhsieisnecgobncevoneudnnsttie(orin-n ecrnodsss-osfecaticoanpsaulfea,rtlehsesseiqdueislabtuelrgaelsslhiagphet;ly,angdivci)ngawtaheycfarposmulteh’es
clockwise bends (in this example, cattail or iris). The linear cross-section a shape similar to a Reuleaux triangle (i.e.,
dimensions associated with the grass image are those we used to slightlyconvexoneach side;Weisstein2009).
indicatewhereonamonocotbladethecapsulewasconstructedand The most regular of the capsules constructed of willow
tocalculatethevolumeofthecapsule(seetext). leaves are approximately tetrahedral in shape (Fig. 1), but
many were quite irregular, sometimes more conical or even
GeorgianBay,Ontario,Canada.Thewaterofthemarshwas cylindrical. To measure their volumes, we preserved them in
confluentwiththeopenwatersofGeorgianBay,butsheltered 95% alcohol, then dried them and lightly coated them with
fromanywaveaction.Thesitewasabout 10%open,withthe silicone (Ace® Silicone Lubricant) to render their surfaces
remaindercovered byvegetation. Intermsofplantcoverage, hydrophobic. Finally, we submerged each in a graduated
thedominantplantwasagrass, Calamagrostissp. (monocot, cylinder containing distilled water and measured its volume
Poaceae). At the north end of the marsh was a stand of directly. These volumes are reported below, but in our
cattails, Typha kit{folia L. (monocot, Typhaceae), covering subsequentanalyses weconcentrated on thechambers ofthe
about 16m^,andatvarioussitesinthemarshwereclumpsof threemonocotspecies,bothbecausetheirsimilarshapesmake
iris.Iris versicolor L. (monocot, Iridaceae)and individualsof comparisons among them more meaningful and because we
an unidentified willow shrub, Salix sp. (dicot, Salicaceae). used a very different technique to measure the volumes of
Sedges(Cyperaceae)andrushes(Juncaceae),aswellasatleast willowcapsulesandwerereluctanttotreatthetwotechniques
oneotherspeciesofgrass(Poaceae),werealso present. Each asiftheywerecomparable.
cattail, each iris, and each willow was surrounded by Our two primary analytical tools were one-way ANOVA,
Calamagrostis sp., although across much ofthe area ofthe with plant species as the grouping variable and using Sokal
marsh,eachindividualCalamagrostissp.wassurroundedonly and Rohlf’s (1987) method for determining the relative
byothersofthesamespecies. importance of within vs. between treatment variance; and
Our visual search for the egg capsules of C. riparia was linear regression, with spider or clutch mass as the indepen-
carefulbutnotstructured.Wefoundcapsulesoneachofthe dentvariable. Inboth statisticalcontexts, ourinterestwasin
dominant plant species (above), but none on the other elucidatingthesourcesofvariation incapsulevolume.
_
THEJOURNALOFARACHNOLOGY
78 ‘
—
A Table 1. Spiders constructed their capsules without regard to
handednesson iris, grass,andwillow, andwithoutregardtowhich
leafsurfacebecametheexternalsurfaceofthecapsuleon grass. In
contrast,spidersoncattailbuiltonlyclockwisecapsules,andspiders
onwillowalwayslefttheuppersurfaceoftheleafontheoutsideof !
thecapsule. *Topand bottomsurfaceswereanatomicallyindistin-
guishableontheleavesofcattailandiris,renderingthesedistinctions ;'
notapplicable.
i
Proportion Proportionwithtop
clockwise P surfaceoutside* P
— Cattail 21/21 <0.0001 NA
qJ 1 1 , ^ Iris 6/16 0.227 NA
cattail iris grass willow Grass 7/13 0.500 6/13 0.500
Willow 11/18 0.240 18/18 <0.0001
0.02 cm), iris (1.10 ± 0.04 cm), and grass (0.70 ± 0.03 cm)
werehighlysignificantbothinaggregate(F2.49 = 108.4, F<
0.0001)and inpair-wisetests(P< 0.001 foreach).
Capsules constructed on cattails were substantially higher
above the water(112.4 ± 5.0cm) than werecapsules oniris
(51.1 ± 2.1 cm),grass(59.9 ± 2.1 cm),orwillow(53.8 ± 3.0). '
ANOVA showed these differences to be highly significant
iirFis3,,6sg=ras7s6,.1,an-dP <wi0l.l0o00w1),webruet iinndpiasitri-nwgiusieshtaebsltse, hferiogmhtsoonne ;
another, and heights on cattails were significantly different '
fromtheothers(P< 0.001 ineachcase).
— Capsulesonthefourplantspeciesalsodifferedwithrespect
whiFcighurpelan2t. leAa.veCsawpesrueleusveodluimnesconvsatrriuecdtisoing.nifAicNanOtlVyAdewpaesndaipnpglioend ctoaphsaunldeed(nFiegs.s,1)t,heanddirewcittihonreosfptehcetttwoowbheentdhserustehdettoofpoorrmtthhee
dvsoiiangfrlnfiyieefrdtieocnsatintgthmn.eieftmiBhc.ooandnoMstcl)eyo,atanasmno(dbnwligaaldmlteoohwenwgictdahtrptheshseue,lmmeoavnanlooallcluyopmzateeirdsp-lwawwineistrteehspdmeiAcefifNaeessOru,eVrnacAened,sdupwsaaeilirnsrego- ccbalopotsctukolwmeisse(uTrcafabaplcseeulo1e)fs.,thNaeontldaebalfsypb,iedcesrapsmiedoenrtshewoiolnultoscwiadtetaaliswluaryfbsaucielltefotfonttlhhyee 1
wise tests were all significant. The most voluminous capsules were uppersurfaceoftheleafontheoutsideofthecapsule.Spiders ,
constructedwithcattailbladesandweresolargeinpartbecausethe oniris,grass,andwillowshowednopreferencewithrespectto :
meanbladewidthofcattailswaslarge. handedness,andspidersbuildingongrassappearedtohaveno
preference for one side ofa leafor the other as the outside
RESULTS surfaceofthecapsule.Wecouldnotdistinguishwhichsideof
Among the monocots, capsule volume varied more than acattailoririsbladewastop,sowedidnotscorethisattribute
ten-fold, the smallest beinga grasscapsulewith a volumeof ofcapsulesconstructedonthosetwoplantspecies.
0.29 ml and thelargestbeinga 3.14-mlcapsule madefroma Inlookingbeyondhostspeciesforthesourcesofvariation
cattail leaf. Themeancapsulevolume(± SE)oncattailswas in capsule volume, we regressed capsule volume on spider
2.13 ± 0.14 ml, on iris 1.63 ± 0.14ml, and on grass0.67 ± mass, egg clutch mass, and pre-oviposition spider mass (the |
0.08 ml (Fig. 2A). ANOVA revealed that this variation was sum ofspiderandeggclutch masses). Indoingthis, wewere
significantlyassociatedwithhostplantspeciesiF2j9= 32.04, aware that, because of its constituent components, pre-
P< 0.0001),andTukey’s MultipleComparisonTestshowed oviposition mass would be correlated with spider mass and
thatallthreepair-wisedifferencesbetweenthemeanvolumes with egg clutch mass. We also knew that many studies have
weresignificant(cattailvs.iris,P<0.05;cattailvs.grass,P< foundastrongdirecteffectofspidermassoneggclutchmass
0v.a0r0i1a;tioirnis(Fvisg..g2rAa)ss,waPsa<ttr0i.b0u0t1a)b.leAtboouditff6er9e.n2c%esoafmothnegthootsatl ebtotahl.a2m01o1n)gasnpdecwieisth(iMnarspsehcailelsa(en.dg.,GiKtitllleembarnew1&994F;oNridch1o9l8a5s;
plantspecies. Capsulesconstructed ofwillow, theonlydicot, Brownetal.2003),arelationshipthatwealsosawinourown
were intermediate in volume (1.27 ± 0.11 ml) between those data(Fig. 3;r = 0.221, F,,4„ = 11.32, P= 0.0017).Thuswe
onirisand thoseongrass(Fig. 2A). knew that our several regressions were not independent of
Animportantcomponentofcapsulevolumeinmonocotsis each other. '
thewidthofthebladewhereitbecomesincorporatedintothe In our regression analysis (Fig. 4), capsule volumes (when
capsule,inthiscasemeasuredatthetwobends(Fig. 1).Given pooled across plant species) were significantly influenced
tuhnesursipgrniisfiincgantthadtifbfleraednecewsiditnhscawpesruelealvsoolusimgensifi(caabnotvley),dififteri-s mbyassspi{dPer=ma0s.s0,50e,gg0.c0l2u3t,ch0.m0a12s,s,reasnpdectpirvee-loy)v.ipoTshietiosntrosnpgiedsetr
ent, and with the same pattern (Fig. 2B). ANOVA showed relationship was between pre-oviposition spider mass and ‘
thatthedifferencesamongthemeanwidthsofcattail(1.30 ± capsulevolume(Fi40= 6.90;r = 0.15).Whenthedatawere
^
SUTERETAL.—ARCHITECTUREOFSACSPIDER EGGCAPSULES 79
thebottomgraphinFig.4).Wealsonowknow(Table 1)that
these spiders always bend willow leaves to fashion a capsule
thathastheuppersurfaceoftheleaftotheoutside,andthat
whentheybuildonacattail bladetheyalwaysturntheblade
inaclockwisedirection(accordingtotheconventionwe—have
adopted: see Fig. 1). Whatdo these three observations 1)
thespider’sscalingofthevolumeofitscapsuletothespider’s
own mass, 2) the spider’s consistent attention to willow leaf
surfaceproperties,and3)thespider’sproclivityforclockwi—se
handedness when building on cattail but not elsewhere
implyaboutthekindsofpressuresagravid female C. riparia
faces?Weconsiderthesequestionsinord—erbelow.
Scalingcapsulevolumetospidermass. Althougharchitec-
tural featsare not often analyzed in thisway, it isveryclear
thatmanyspidersknowhowtomeasure,andthattheyadjust
0 H ^ ^ ^ thesizesoftheirstructurestofittheirneeds.Asaraneidsgrow,
10 15 20 25 30 35 40 forexample,sodotheirwebs, presumablybothbecausethey
are able to build larger webs and because they have greater
Spidermass (mg) metabolicneeds, and largerwebs intersect larger numbers of
aPttF=riigb0uu.rt0ea0b1l73e).—.toAdbiofufteren2c2es%inofspitdheermvaarsisat(iron=i0n.2c2l1u,tcFh140ma=ss11.w3a2s, 2pin0rc0er3ye)a,s(aeEnbtdehredheasdreirdatmew1ti9e9dr0o)wo.fsSptihimediierlrasrbliuynr,crroebwausserraaoswntiuhnmegbyegrwrooloffwwe(sCpbairdarenerdls
retreatdimensionsasthespidersgrow(Lubinetal. 1991). In
brokendown byplant species, onlyamongcapsules built on that context, the spider size/capsule size relationship in C.
cattails were there significant influences of the independent riparia, and the spider’s implied ability to measure, are not
variables on capsule volume. And again there, the strongest surprising.
relationship was between pre-oviposition spider mass and Moreover, the scaling of capsule volume to spider mass
capsulevolume{Fus = 8.63;r = 0.37). makessensefroma biomechanical perspective. First,capsule
Despite a strong relationship between spider size and volume must be sufficient to enclose both the spider and its
capsule volume, especially among capsules on cattails, we eggs as separate entities (Fig. 6: notjust as the single gravid
found no evidence that larger spiders were predisposed to organismthatconstructedandfirstinhabitedthecapsule)and
buildoncattailsor,conversely, that smallerspiderschose to toallowforthespider’smovementswhilesealingthecapsule
constructcapsulesongrassleaves(Fig. 5). ANOVA revealed fromtheinsideandwhilelayingeggs. Second,ifpredationby
that mean pre-oviposition spider mass did not vary signifi- animalsthatwouldbreachthecapsulebycuttingthroughthe
cantly across the three monocot plant species {F2,47 = 1-05, plantmaterial(Fig. 6:asopposedtotearingthesilkwheretwo
P= 0.358). leafedgesmeet)isimportant,thenlargercapsulesizeisbetter
because,atleastinthemonocots,theleafbladegetsthickeras
DISCUSSION it gets wider. Third, a larger spider’s size means that it can
Many C. ripariaconstructtheircapsulesoncattail, iris, or exert greater forces and, perhaps, can expend more energy
willow, despite the fact that grassblades, on which theycan during capsule construction (R.B. Suter et al. unpublished
alsoconstructcapsules, areclose byand in abundance. This data)thancanasmallerspider,allowingittobendwiderand
suggests that suitable building sites were not a limiting stifferleavesandtherebyenclosemorevolume.
resource in this area, but more importantly, it suggests that Iflargerleaf-curlingsacspidersareabletoconstructlarger
predispositionandchoicecouldbeinvolved.Choosingcattail, capsules,andifthereareadvantagestodoingso,whywasthe
forexample,meanshavingtheoptiontomakeasubstantially scalingofcapsulevolumetospidermassonlyobservedwhen
larger capsule than could be constructed on a grass blade thespiders build oncattails? Statistically, this is not a trivial
(Fig.2), and that might well be advantageous for a large dichotomy: on cattails, the relationship is robust, explaining
spider gravid with a large clutch ofeggs. The data on pre- morethan36%ofthevariationincapsulevolume;onirisand
oviposition spider mass contradict that suggestion: the plant grass,therelationshipisinsignificant,andnotjustmarginally
speciesonwhichaspiderconstructedacapsulewasunrelated so(Fig. 4).Theleavesofthegrass, Calamagrostissp.,attheir
tothespider’ssize(Fig. 5).Moreover,atleastforspidersthat widest, where the spiders bend them to make capsules, are
constructed capsules on iris or grass, the size ofthe spider about halfthewidth ofthepartofthebladesofcattailsthat
appearsnot to have influenced thesizeofthecapsule that it thespidersuse(Fig. 2B).Thatmeansthat,wereaspidertotry
made(Fig.4). to make a more voluminous capsule on a blade ofgrass, it
In contrast, we have strong evidence that spider size wouldhavetodosobyelongatingthecapsule;butthatwould
influenced the volume ofcapsules that were constructed on notappreciablyimprovethespidersmaneuverabilityinsidethe
cattails: morethanathirdofthevariationincapsulevolume narrowcapsuleand, becausethespiderwasalreadydoingits
on cattails was attributable to the pre-oviposition masses of constructionatthewidestpartoftheblade,theresultinglong
thespiders,withadoublinginspidersizeresultinginabouta capsule would not be more resistant to the depredations of
20%increaseincapsulevolume(usingtheslopeofthelinein gnawinganimals.
—
80 THEJOURNALOFARACHNOLOGY
Cattail Iris Grass Pooled
Slope 0.227 -0.091 0.114 0.147
r' 0.275 0.030 0.112 0.093
F 5.788 0.376 1.137 4.095
DF' 1,15 1,12 1,9 1,40
P' 0.030 0.551 0.314 0.050
15 20 25 30 35
Massofspider(mg)
Cattail Iris Grass Pooled
Slope 0,030 0.005 0.012 0.033
= 2 0.252 0.006 0.091 0.123
F 5.043 0.075 0.900 5.583
DF• 1,15 1,12 1,9 1,40
P’ 0.040 0.788 0.368 0.023
20 30 40 50
MassofEggs(mg)
Cattail Iris Grass Pooled
Slope 0.101 0.003 0.036 0.065
3 2 0.365 0.001 0.077 0.147
F 8.629 0.009 0.754 6.896
DF 1,15 1,12 1,9 1,40
P 0.010 0.927 0.408 0.012
30 40 50 60 70
Massofspider+eggs(mg)
—
Figure 4. Oncattail(filledcircles;regressionline),capsulevolumevariedsignificantlywithspidermass,eggclutchmassandpre-oviposition
spidermass(thesum ofspidermassandclutch mass).Thoserelationshipswerenotfound withcapsulesbuilton iris(opencircles)orgrass
(triangles). On cattail, the strongest relationship was between capsule volume and pre-oviposition spider mass, where differences in mass
accountedfor36.5%ofthevariationincapsulevolume.
Thatlineofreasoning,whichprovidesatenableexplanation leaves used by spiders at our study site were strongly
for the constrained volumes of capsules on grass blades, asymmetrical, with a relatively smooth, shiny, dark upper
irrespectiveofspidersize,does notservewell forcapsuleson surfacethatwasdevoidofstomata,andamuchmoretextured
iris blades. These blades, though about 15% narrower than and lighter lower surface with vascular tissue in relief and
cattail blades, are ofmuch the same shape and share with manystomata(R.B.Suterunpublisheddata).Thepresenceof
cattail blades theproperty ofbecoming thickerand stifferas gasexchange pores, the stomata, consistently on the interior
onemovesdown theblade fromthetip. Thus, astheydoon faces of the capsule walls suggests that the consequent
cattails, largerspiderscould makemorevoluminouscapsules differences in humidity and possibly respiratory gases are
on iris, but they do not. We do not currently have a way to importanttothespiders.
explainwhythescalingofcapsulevolumetospidermassdoes Desiccation is surely a problem for spiders and theireggs
nothappenon iris. (Gillespie 1987; Hieber 1992; DeVito & Formanowicz2003),
Bendingwillowleavestoputthetopsurfaceoutside. When and probably led to the evolution ofknown behavioral and
aspiderconstructsitscapsuleusingawillowleaf,itdoessoby architectural solutions (Humphreys 1975; Suter et al. 1987;
bendingtheleaftowarditslowerside, resultinginachamber DeVito & Formanowicz 2003). We presume that capsule
that has the lower surface ofthe leafon the inside and the construction by C riparia also serves to reduce desiccation,
uppersurfaceoftheleafontheoutside(Table 1).Thewillow bothofthespiderandofitseggclutch. Partofthatfunction.
SUTERETAL.—ARCHITECTUREOFSACSPIDEREGGCAPSULES
Thishypothesisismildlysupportedbytheobservationthat
£ 80- (thTeabslpeid1e)rsbedcoausneotthfeavsoprecoineseosfideCaolramathgeroostthiesroonfwthhiecghrawses
a 70- foundthespidercapsuleswasamphistomatal,withstomataon
g* 60- bothsurfacesofeachblade(R.B. Suterunpublisheddata),as
isusual inthisgrassgenus(Maetal. 2005).
t 50- Wehavenodirectevidenceconcerningtherelativehumidity
S 40- insidethecapsuleson anyofthehostplants, soatestofour
30- contention that the particular structure of willow capsules
S- functionstoboostinteriorhumiditymustawaitfurtherstudy.
O 20- Three alternative hypotheses about the topside-outside con-
2(nAs io0-j sutnrduecrtsiiodneoofftwhielwliolwlocwaplseuafleissmrueclahtemotroetrheeflefcatcitvetthhaatntthhee
catt,ail iris gra,ss topside:buildingacapsulewiththeundersideinsidea)makes
thecapsulelessconspicuoustovisuallyorientingpredators;b)
Figure 5.—Pre-ovipositionspidermassesdidnotvarysignificantly causes thecapsule to absorb more solarenergy undersunny
dependingonwhichplantleaveswereusedinconstruction(^2,47= conditions,therebyraisinginternaltemperature;andc)keeps
1.05, P = 0.358), an indication that a spider’schoice ofone plant the more photosynthetic layers ofthe leafexposed, thereby
speciesoveranotherwasnotbiasedbythespider’smass. possibly inhibiting abscission and prolonging the life ofthe
theprovision ofshelterfrom the forcedconvection ofwinds leaBfe(nTdaiynlgorc&attWaihlitbellaadwes200c1l)o.ckwise.—Our data on cattail
andfrominsolation,wouldbeprovidedevenifthesidesofthe capsules show a striking and highly significant handedness:
capsulesweremadeofwillowleavesthathadnostomata.But allofthespider-bearingcapsulesoncattailswereconstructed
thepresenceofstomataontheinsidemeansthatwatervapor by bending the blade clockwise (Table 1), whereas on the
lost fromthe plant during normal transpiration would dwell otherthreeplantspeciestherewasnoevidenceofhandedness.
insidethecapsuleuntilitdiffusedoutwardthroughthespaces Asymmetriesofthissort,inwhichananimal’smorphologyor
between the silk-joined edges ofthe leaves, thus keeping the behaviorisinsomewaychiral, havereceivedmuchattention
relativehumidityofthecapsule’sinterioratcloseto 100%. inrecentyears,particularlyasresearchershavedemonstrated
that some chirality at the level ofgross morphology, brain
laterality, and behavior, is a consequence ofchirality at the
molecular and early developmental levels (Levin & Palmer
2007;Okumuraetal.2008;Davisonetal.2009).Inthecurrent
case, we do not know whether the asymmetry resides in the
gravidspiderorinthecattail leaf.
Our working hypothesis is that the amphistomatal (Kaul
1974)c—attailleafisasymmetricalwithrespecttohoweasilyit
bends that it is somewhat lessenergetically costly for the
spider to bend it with a clockwise bias than with a
counterclockwisebias,andthatthisdifferenceislargeenough
tomatterintheevolutionarycalculusleadingtoanoptimum
architecture. Support for this hypothesis could come from
measurementsoftheworkrequiredtobendcattailsclockwise
vs. counterclockwise, and that study is underway. To make
good sense, however, that support would have to be paired
with similar measurements of iris blades, because they are
superficially nearly identical to cattail blades but are not
treatedasidenticalbythespiders(Table 1).
Despitethestructuralsimplicityoftheelegantcapsulesbuilt
by C. riparia, our analyses oftheir sizes and their locations
revealed substantial complexity. The gravid spiders that
constructed the capsules did so not only on narrow-bladed
grassleavesandonthebroaderbladesofirisandcattailsbut
alsoonwillowleaves.Giventhisvarietyofconstructionsites,
A.FAignurienha6.b—iteTdwcoaposupleen,eodpecanpesdulbeys,thsehaouwtnhorasp,prsohxoiwmsattehleyfteomaslcealCe.. isstmuarilpslreinssotitnghsaudtrhpaartistieonnngtlhythoafotntcheacpasvtutoallieulsmveowloaufsmethtehvealrraeiregdeastw)i,sdigebnluiytfii(cttahniets
rtihpearvioalwuimtehiotfsetghgemcaaspssulaen.dBp.orAtrnayesmtphteyreclaaptsiuolneshisphoowfitnhgeirdasimzaegteo relationshipbetweenspidersizeandcapsulevolume.Capsules
probablycausedbyapredatorthatgainedaccesstothespiderand found on cattail blades were also unusual in having been
eggsbytearingthroughtheplantmaterialratherthanbyseparating consistently constructed by bending the blades clockwise,
thegrassbladesatasilk-closedseam. while no chiral preference was seen in capsules built on the
82 THEJOURNALOFARACHNOLOGY
otherthreeplantspecies. Finally, thespidersalwaysfoldeda Harmer, A.M.T. & M.E. Herberstein. 2009. Takingit toextremes;
willow leaf so that its stomata-bearing surface faced, and what drives extreme web elongation in Australian ladder web
could perhaps modify or modulate, theenclosed atmosphere spiders (Araneidae: Telaprocerci mamlae)! Animal Behaviour
ocfonTtdhhuiecstciaapnscguclofeuo.nutr rieslatoendlystuadiesb:egminenaisnugr.ingWetheareenercgeutrircesntloyf Hen7ts8hc:eh4re9ml9a,-l5J0.f4Ra..c,torDs. fWoarrdnest&-siYt.e Lseulbeicnt.ion,199w2.ebThceonsitmrpuocrttiaonnceanodf
capsuleconstruction,testingthespidersforpreferencesamong bNeehgaevvioduerserto.fJSotuergnoadlyopfhuTshelrimnaealtuBsio(lAorgayne1a7e::97-E1r0e6s.idae) in the
the available plant species, analyzing the ways in which the Hieber, C.S. 1992. The role of spider cocoons in controlling
microenvironment inside a capsule differs from external desiccation.Oecologia89:442-448.
conditions, and seeking the source(s) of the chirality in Humphreys,W.F. 1975.Theinfluenceofburrowingandthermoreg-
capsuleconstructiononcattails. ulatory behaviouron the waterrelationsofGeolycosagodeffroyi
ACKNOWLEDGMENTS (Araneae: Lycosidae), an Australian wolf spider. Oecologia
21:291-311.
We are grateful for the editorial contributions of Linden Jones,C.G.,J.H.Lawton&M.Shachak. 1997.Positiveandnegative
Higginsandforimprovementssuggestedbythreeanonymous effects of organisms as physical ecosystem engineers. Ecology
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Classof’42 Faculty Research Fund. Jones, J.C. & B.P. Oldroyd. 2007. Nest thermoregulation in social
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