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Bull.Br.arachnol.Soc.(1997)10(8),295–298 295 The effect of starvation on web geometry in an for a number of days when well-fed. Witt and collabor- orb-weaving spider atorsalsostudiedtheeffectofstarvationontheamount of silk used for a web, and on the size of the web. They Fritz Vollrath1,2 and Ferenc Samu1,3 found:(i)thatthetotalamountofsilkavailabledeclined 1DepartmentofZoology, a few days after the onset of starvation but (ii) that the SouthParksRoad, area covered by the web began to decrease only after Oxford,OX13PS about a week of starvation. These observations led to the conclusion that these spiders compensate for a 2DepartmentofZoology, decreaseinpreybymaintainingalargewebareaevenif UniversitetB135, DK8000AarhusC,Denmark* silk protein (because of malnutrition) is becoming a scarcecommodity.Subsequentstudieshaveinvestigated 3PlantProtectionInstitute, theeffectsofpreyavailabilityinthefieldandhavefound POBox102, significantnumericalresponsesinlifehistoryparameters H1525Budapest,Hungary (Spiller, 1992) as well as functional responses in prey capturebehaviour(Leborgneetal.,1991;Pasquetetal., Summary 1994). Recently Sherman (1994) addressed the issue of variability of web parameters and found that in the Starving juvenile Araneus diadematus (Araneae, Aranei- dae) cross spiders led to an increase in the frequency of spider Larinioides cornutus (Clerck) ‘‘hungry spiders web-buildingactivityaswellastoahighervariabilityinweb invest more effort into foraging, while sated spiders geometry(meshandcapturearea).Anincreaseinfoodled re-allocate energy from continued foraging to egg- toadecreaseinweb-buildingfrequencyaswellassomeweb production’’. Sherman’s observations encouraged us to fine-tuning (by increasing capture area). Our laboratory reportacontrolledexperimentwhichbearsonthisissue. data obtained under controlled conditions support the interpretationoffieldstudiesbyotherswhichsuggestedthat spiders may be capable of responding to changes in prey Material and methods availabilitybyadjustingthearchitectureoftheirwebs. We studied the web-building frequency and web architecture of the orb-weaver Araneus diadematus Introduction Clerck in response to different feeding regimes in our laboratory under highly controlled conditions. The Web-building spiders are sit-and-wait predators with animals were raised and kept individually under our high costs of moving from one web site to another. standardlaboratoryconditions(Samu&Vollrath,1992) These costs are due partly to high predation risks when in 30(cid:1)30(cid:1)5cm frames; 16:8 L:D, 25(cid:2)C, 50% RH, travelling (Vollrath, 1985), partly to a lack of infor- watered daily by spraying. Immature and subadult mationabouttheeconomicvalueofthepresentwebsite female spiders of previously identical feeding history as opposed to the marginal value of the environment were allocated to one of three feeding treatments: (A) (Janetos, 1986) and partly to the risks associated with starvation, no food at all; (B) control, 1 fruit fly daily finding a better site mostly by trial and error (Enders, (wild type Drosophila melanogaster); and (C) satiation, 1977).Notwithstandingthesehighcosts,webspidersdo 10 fruit flies daily. Before the trials, spiders in A and B modifytheirresidencytimeatagivenweb-siteaccording weregiven1flydailyforaweek;spidersintreatmentC to local prey density by staying longer when food is were starved for a week. In each treatment 4 spiders abundant and moving more or less promptly when it built good webs (i.e. typical orbs) at a frequency suf- is not (Janetos, 1982, 1986; Olive, 1982; Vollrath & ficiently high for statistical analysis. Webs were photo- Houston, 1986). However, apart from responding to graphed, digitised (to an accuracy of 0.05mm) and preydensitybyadjustingsitetenacity,webspiderscould analysed by calculating a number of web characteristics also respond by staying in the same place but adjusting (see Samu & Vollrath, 1992). Since many of these their prey capture strategy and thus reducing metabolic parameters are interrelated, only two were chosen to costs (Vollrath, 1987). This might be achieved by alter- representthechangesinwebs:areacoveredbythesticky ingweb-buildingbehavioursuchasrebuildingfrequency spiral (capture area) and the average distance between and/or construction behaviour responsible for web consecutive spiral turns (mesh size), measured on one architecture. radius in the south (lower) part of the vertical webs. The question whether spiders can and do adjust These are two robust and independent parameters that web-building behaviour was first considered by greatly influence prey capture (Rhisiart & Vollrath, Disjonval (1797) who, confined to the Bastille for a 1994); they were also used by Witt et al. (1968) and considerablesojournattheking’spleasure,observedthe Sherman (1994). We analysed the data with a 3rd order spiders building between the bars of his window. He polynomial regression. observed that web-building frequency and web parameters were affected by the weather as well as by prey. The issue was taken up again by Witt et al. (1968) who Results and discussion foundthatZygiellax-notata(Clerck)ceaseweb-building Our data showed that well-fed spiders reduced web- building frequency whereas starving spiders increased it *Address for correspondence: Prof. Fritz Vollrath, Department of Zoology,UniversitetsparkenB135,DK8000AarhusC,Denmark. (Fig.1).We,unlikeWittetal.forZygiella(1968),found 296 Effectofstarvationonwebgeometry decliningqualityandunabletomoveawaydonotadjust (fine-tune) their web-building algorithm. We failed to findchangesinwebfine-tuninginthetreatmentwhere1 fly was given daily; of our diets this was probably the closest to the natural rate of food intake (Vollrath, 1988). In contrast, the treatment in which 10 flies were given daily had a significant (r2=0.663, p<0.001) effect on web geometry, with spiders increasing web area by approximately 100% and slightly (but insignificantly) decreasing mesh size soon after the onset of the treatment.Itcouldbearguedthatthisbehaviourwouldserve Fig. 1: Web-building frequency of spiders in the three feeding treat- to exploit better a fortuitous increase in a resource. ments (number of days web built/total number of treatment days(+S.E.)).Thetreatmentstarvation(0flies,n=4)lasted40 Our results of the ‘‘starvation to rich diet’’ contradict days,standard(1flydaily,n=6)lasted16daysandsatiation the observations of Witt et al. (1968) who found that (10fliesdaily,n=4)lasted8days.Thedifferencesbetweenthe Zygiella x-notata under similar (but obviously not iden- three treatments were tested with paired t-tests and were tical) laboratory conditions of a sudden food increase significant between the diets starvation and 1 fly (p=0.052, retainedtheoriginalsizeoftheirwebs.Sherman’s(1994) t=3.13),aswellas10flies(p=0.01,t=5.19),butnotbetween1 and10flies(p=0.53,t=0.71). ‘‘normal to rich diet’’ spiders were adults in the wild which could and did lay eggs, and thus had a reproduc- thatoverfedAraneussuspendweb-buildingnotimmedi- tivecomponentthatourspiders(beingjuveniles)lacked. ately but after about 8 days; and at that time they were Accordingly our experiments could be seen as only ready to moult and had to stop web-building. Our providing additional data to his, not testing them. spiders as opposed to Witt’s, however, were all im- However, ours give his observations on Larinioides matureaswellasadifferentspecies,andmighthavehad cornutus an extra dimension. Our observations also a feeding strategy different from Zygiella. To study the observed changes in web geometry in more detail, we plottedwebparametersagainsttime(Fig.2).Incontrast to Sherman’s, our spiders had been raised under highly controlledconditionsandwerestudiedunderthesesame conditions which differed only in the treatment (diet). Yet we, like Sherman, observed high individual variability in web geometry (Fig. 3) which renders difficult any analysis of the effects of diet. Our spiders were also (i)controlledastostatewithinaninstar(nottoocloseto eitherprecedingorfollowingmoult),(ii)wereallimma- tures (not concerned with reproduction, like Sherman’s spiders which were adults) and (iii) had been collected assecondinstarsinonesmall,isolatedlocality(suggest- ing genetic similarity). This, in combination with our highly controlled laboratory environment and full security of a spider’s identity (supported by the most accurate measuring possible), suggests strongly that the observed variability is not an artefact but real. It seems largely a matter of internal state, maybe fluctuating semi-randomly. From our reasonably large sample of spiders we further analysed only those (75% per treatment) that built webs at least every other day (with the exceptionofthestarvationtreatment).Here,too,subsequent webs of individual spiders showed large and Fig. 2: Capture area and spiral spacing of Araneus diadematus webs inexplicable fluctuations in web parameters obscuring built during the different feeding treatments. We show 3rd trendsthatmighthaveoccurredduringthecourseofthe orderpolynomialregressioncurvesforeachtreatmentdataset experiment (Fig. 2). containing the subsequent webs of 4 individual spiders per Spidersonthestarvationtreatmentwerefollowedthe treatment:satiation(blackcircles),1flydaily(blacktriangles), starvation (white squares). Correlations were for web area longest. Over 5 weeks of complete starvation was insuf- (upper graph): satiation (upper curve, r2=0.663), 1 fly daily ficient to produce any clear (significant) trends in web (middle curve, r2=0.146) and starvation (lower curve, area or mesh size, since later webs were in size and r2=0.246); and for spiral spacing (lower graph): satiation geometry comparable to earlier ones, even towards the (middle curve, r2=0.073), 1 fly daily (lower curve, r2=0.187) endoftheexperiment(withtheexceptionofthelast2–3 andstarvation(uppercurve,r2=0.113).Notethatspidersfed to satiation (10 flies daily) moulted after about day 9 and days, which statistically were irrelevant). Although thereforeweretruncatedatday8,normallyfedspiders(1fly aware that ‘‘absence of evidence is not evidence of daily)moultedafterabout18daysandweretruncatedatday absence’’wededucethatspidersinanenvironmentwith 16,starvingspidersdidnotmoult.Treatmentbeganatday0. F.Vollrath&F.Samu 297 indicatedthatthewebgeometryofatypicalorbweaver with his conclusions about the difficulties this poses for can be rather variable, although this variability may any study of orb-spider web geometry. Indeed, because needconsiderableinvestigationtobeuncoveredfully.In ofourmuchmorecontrolledenvironmentalanddietary this respect our results of the ‘‘normal to starvation’’ conditions, our observations (Fig. 3) strongly underline dietwererevealingbysuggestingthatthisratherdrastic Sherman’s. However, we also note that the level of change in food intake had at first no measurable effect. individual variability on subsequent days seemed higher Only very much later, in the last few days of the instarvingthanincontrolorover-fedspiders.Compar- treatment, did it show a discernible effect on capture ing the day to day variability in web area and mesh size area without any visible effect on web mesh. Even the wefoundthatthewebsofindividualspidersontherich effectoncaptureareawasnon-significantinthecontext diet showed a clear (and significant) trend but less ofthewholeexperiment,althoughitcouldbemagnified variability in subsequent webs, whereas those on either with selective statistics (i.e. comparing only the first 6 low or starvation diet generally varied considerably on with the last 6 days and ignoring the days in between, subsequent days but showed no significant trend over Fig. 2, top). the total period. This suggests that any study aiming to Finally,ourobservationsagreewithSherman’sobser- unravel the spider’s web geometry (and by extension its vations on the high individual variability of spider building behaviour) needs to ensure that housing and web-buildingpatternsonsubsequentdays;wealsoagree diet are well controlled. Summarising our observations we may say that a decrease in prey led to an increase in the frequency of web-buildingactivityaswellasahighlevelofvariability in web geometry (mesh and capture area). An increase in prey, on the other hand, led to a decrease in web- buildingfrequencyandseemstohavebroughtaboutthe behavioural response of web fine-tuning (by increasing capturearea).Sinceithasbeenshownthatweb-building activity and web geometry can be highly interrelated (Eberhard, 1982; Vollrath et al., submit.), any detailed and statistical study of the phenomenon of web fine- tuningmustalsounravelthisinterrelation.Ourdata,on the whole, support the notion that spiders may be capableofrespondingtochangesinpreyavailabilityby adjusting the architecture of their webs, although this is well within the limits of a naturally occurring, and seemingly random, variability of the measured parameters which, in turn, can be linked directly to prey capture efficiency (Eberhard, 1986; Rhisiart & Vollrath, 1994). References DISJONVAL,Q.1797:L’Aranéologie.1–164.J.J.Fuchs,Paris. EBERHARD, W. G. 1982: Behavioral characters for the higher classificationoforb-weavingspiders.Evolution36:1067–1095. EBERHARD, W. G. 1986: Effects of orb-web geometry on prey interception and retention. In W. Shear (ed.), Spiders, webs, behavior and evolution: 70–100. Stanford, Stanford University Press. ENDERS,F.1977:Web-siteselectionbyorb-weaverspiders,particu- larlyArgiopeaurantiaLucas.Anim.Behav.25:694–712. JANETOS,A.C.1982:Foragingtacticsoftwoguildsofweb-spinning spiders.Behav.Ecol.Sociobiol.10:19–27. JANETOS, A. 1986: Web site selection: are we asking the right questions? In W. Shear (ed.), Spiders, webs, behavior and evolution:9–22.Stanford,StanfordUniversityPress. LEBORGNE,R.,PASQUET,A.&SEBRIER,M.-A.1991:Modali- ties of feeding behaviour in an orb-weaving spider Zygiella x-notata (Clerck) (Araneae: Araneidae). Behaviour 117: 206– 219. OLIVE,C.W.1982:Behavioralresponseofasit-and-waitpredatorto Fig.3: VariabilityinwebparametersinwellfedAraneusdiadematus. spatialvariationinforaginggain.Ecology63:912–920. Thespiderswereonthesatiationtreatment(10fliesdaily)and PASQUET,A.,RIDWAN,A.&LEBORGNE,R.1994:Presenceof show in higher resolution and interconnected the ‘‘rich diet’’ potential prey affects web-building in an orb-weaving spider dataalsoplottedinFig.2.Eachlineshowsthedatafromone Zygiellax-notata.Anim.Behav.47:477–480. spider. The bottom graph shows that there is obviously no RHISIART, A. ap & VOLLRATH, F. 1994: Design features of a correlationbetweenspiralspacingandwebarea. spider’sorbweb.Behav.Ecol.5:280–287. 298 Effectofstarvationonwebgeometry SAMU,F.&VOLLRATH,F.1992:Spiderorbwebasbioassayfor VOLLRATH,F.1988:Deducinghabitatqualityfromspidergrowth. pesticidesideeffects.Entomologiaexp.appl.62:117–124. Bull.Br.arachnol.Soc.7:217–219. SHERMAN,P.M.1994:Theorb-web:anenergeticandbehavioural VOLLRATH,F.,DOWNES,M.&KRACKOW,S.submit.Design estimator of a spider’s dynamic foraging and reproductive variables in the orb web of the spider Araneus diadematus. strategies.Anim.Behav.48:19–34. Oikos. SPILLER, D. A. 1992: Numerical response to prey abundancy by VOLLRATH,F.&HOUSTON,A.1986:Previousexperienceandsite Zygiellax-notata(Araneae,Araneidae).J.Arachnol.20:179– tenacity in the orb spider Nephila clavipes. Oecologia 70: 188. 305–308. VOLLRATH, F. 1985: Web spider’s dilemma: risky move or site WITT,P.N.,REED,C.F.&PEAKALL,D.B.1968:Aspider’sweb: dependentgrowth.Oecologia68:69–72. problemsinregulatorybiology.Springer,NewYork. VOLLRATH,F.1987:Foraging,growthandreproductivesuccess.In W.Nentwig(ed.),Ecophysiologyofspiders:357–370.Springer, Heidelberg. Bull.Br.arachnol.Soc.(1997)10(8),298–302 Carorita limnaea (Araneae: Linyphiidae) and other The site Araneae at Wybunbury Moss, Cheshire—a unique Wybunbury Moss is a National Nature Reserve refuge for two relict species of spider in Britain (NNR) and a Site of Special Scientific Interest which is givenGrade1StatusintheBritishNatureConservation C. Felton and S. Judd Review(Ratcliffe,1977).Itispositionedalmostcentrally ZoologyDepartment, in the recently designated Midlands Meres and Mosses LiverpoolMuseum, WilliamBrownStreet, Ramsar Site and is one of the two best examples of LiverpoolL38EN schwingmoorintheBritishIsles(EnglishNature,1991). ChartleyMossNNR,Staffordshire(Gridref.SK02-28-) and Wybunbury Moss are ‘‘subsidence raised mires’’ Summary andarestructurallyuniqueinBritain.Theyareofgreat The distribution of the endangered, British Red Data scientific importance within a national and a European Bookstatus1,linyphiidspider,Caroritalimnaea(Crosby& context (English Nature, 1991). Bishop) is described at Wybunbury Moss, Cheshire. The Reserve is small (11.43ha), very compact, with C. limnaea was not recorded during a survey of ten different habitats compressed together, and is sur- other schwingmoors in Cheshire, Shropshire, Staffordshire roundedby3.8haofwetpasture,whichisalsomanaged and Clwyd. Both it and Gnaphosa nigerrima L. Koch are relict species, apparently restricted in Britain to by English Nature. It is not contiguous with any areas Wybunbury Moss. One hundred and thirty-three spider of known ecological importance, being isolated in an species are recorded from the site. Eleven species of agricultural and urban landscape. harvestmenandonepseudoscorpionarealsorecorded. Four major plant communities occur in the Reserve—Sphagnum lawn; pine woodland; fen woodland; and mixed birch, oak, and rowan woodland (Poore&Walker,1958;Green&Pearson,1968).These, Introduction together with the surrounding wet grassland were most In 1994 English Nature funded a Liverpool Museum recently mapped in the site management plan (English surveytodeterminethestatusandconservationrequire- Nature, 1991). The post-glacial history of the site was ments of Carorita limnaea, a minute linyphiid spider discussed by Green & Pearson (1977). knowninBritainonlyfromWybunburyMoss,Cheshire Its small size and isolation make the site vulner- (Gridref.SJ697503).Itwasfirstrecordedtherein1962 able, particularly to run-off water containing fertiliser (Duffey & Merrett, 1963) and subsequently by J. R. fromsurroundingagriculturallandanduntilrecently,to Parker in 1970 and C. Fulton in 1982 (English Nature, eutrophic water from adjacent houses. This pollution 1991), and S. Dobson and J. D. Stanney in 1993 (Scott, extends across nearly two-thirds of the site and has led 1993).Thepossiblethreatstoitssurvivalatthissitewere to the deterioration of the surface-floating peat raft and briefly summarised by Merrett (1991). inseveralplacestoitscollapseandalsotothespreadof Little is known about the biology of this species. It is rheophilous mire communities (Rieley & Page, 1990). rare throughout its extensive Holarctic range and is ThisisapotentialthreattoC.limnaeaandotherspiders. recorded from the United States, Belgium, Germany, Recent management at the site has included the Sweden and Finland (Duffey & Merrett, 1963) and reversal of the successional advancement of pine/birch recently,fromanumberofriverbasinsitesinnorth-east woodland on the Sphagnum lawn and the successful Asia (Marusik, Eskov & Kim, 1992), and Canada and instalment of a pumped drainage system to stop house- throughout northern Asia (see Eskov, 1994). hold pollution.