Thermalefectofdarkcocooncapsinpaperwaspnests IbarakiChristianUniversityLibrary (Hymenoptera,Vespidae,Polistes):Anadaptationtocoldclimates 茨城キリスト教大学紀要第 50 号自然科学 p.235~244 235 Thermalefectofdarkcocooncapsinpaperwaspnests (Hymenoptera,Vespidae,Polistes): Anadaptationtocoldclimates SatoshiHozumi Summary Thepaperwasp,Polistesripariusthatdwelsincoldregionsexhibitsabehavioral habitdarkeningofthesurfaceofitscocooncaps.inthisstudy,iinvestigatedthe thermalefectofthedarkcocooncapsbyusingmodelnestsunderoutdoorandindoor conditions.thermoregulationbythefoundresseswasalsoobservedinthefield.the surfacecolorofthecocooncapsinthefieldnestswasbrownishand28% oflightness. Modelnestswithcocooncapswerepreparedandthecocoonswerepaintedindiferent gradesoflightness.when thetemperatureofmodelnestswasmeasured in the outdoorcondition,thenestswith darkercapsexhibitedhighertemperaturesthan thosewiththelightercaps.thediferenceintemperatureamongthemodelswas higherunderwindyconditionsandlowerundercalm conditions.theassociationof highertemperaturewithdarkercapswasconfirmedbymeasurementsintheindoor condition,andstrongwindwasobservedtodecreasethehighertemperatureinduced bythethermalefectofthedarkcap.finaly,iconcludedthatthedarkeningofthe cocooncapsisrelatedtothermoregulationinthecocooncelsandthatitisanadaptive measureincoldregions. Keywords:Paperwasp,lightnessofcocooncaps,nestthermoregulation,adaptation tocoldclimates Introduction Ininsects,thecolorofitsbodysurfaceisecologicalyimportant,anditplaysan importantroleinfunctionssuchascourtship,mimicry,andthermoregulation.the bodysurfacecolorhastwothermalfeatures,absorptionofthermalenergyfrom the sunandemissionofheattoavoidoverheatingofthebody.insmalinsects,suchas flies,beesandwasps,thebodytemperatureiseasilyinfluencedbychangesinthe environment,andthebodysurfacecolorissometimesdetermineddependingontheir habitats(cloudsley-thompson,1999).inhotclimateareas,suchastropicsanddesert, thebodysurfacecolorofsmalinsectsispaleorlighttopreventoverheatingofthe body(cloudsley-thompson,1999),whileincoldregionssuchasthearcticzone,their bodyisalmostblacktoretainmoreheat(danks,1983).on theotherhand,some
236 SatoshiHozumi nocturnalbeetleswithalargebodymassfoundinthetropicsarealmostblackandthis alowsthem toemitheateficientlyduringtheflight(heinrich,1995). However,theroleofsurfacecolorin thermoregulation ofthenestsofthese animalsisnotwelunderstood.thenestsofsomepaperwasps(polistes)havebeen reportedtoexhibitathermoregulatorysurfacedarkening(hozumi& Yamane,2001a). Thenestsofpaperwaspsarecomposedofplantmaterialsandaproteinousoral secretion.theoralsecretioncontainssubstancessuchasglueisawaterrepelent,and turnsgrayorbrownishwhendried.thewaspsoftensmeartheoralsecretiononthe surfaceoftheirneststomaintainthenestsagainstrain(kudô,etal.,1998);this secretionconfersthedarkcolorofnests.suchdarkeningofthenestswouldcontribute to eficientabsorption ofsunlightand consequentincrease in its temperature (Heinrich,1993;Hozumi& Yamane,2001a).Ontheotherhand,thecolorofthenests ofthetropicalpaperwaspssuch aspolybia and Apoica ispaleorlightyelow (Richards& Richards,1951),probablyforreducingtheabsorptionofthesunlightin thehottropicalclimate(yamane,personalcommunication). Ithasbeen suggestedthatthecocoon capsofsocialwaspsalsoregulatethe temperatureofthebroodcels.thecapsinsulatethecelsfrom theouterenvironment (Spradbery, 1973). In vespine nests, the contiguously arranged cocoon caps automaticalygenerateheatbytheirthermoelectromotiveforcedependingon the thermalenvironment(kirshbolm & Ishay,2000;Ishayetal.,2002).Anothermethodof thermoregulationwithregardtodarkeningissuggestedinsomepaperwasps.inthe nestsofpolistesripariusyamaneetyamanedwelinginthecoldregionsinjapan (Yamane& Yamane,1987),thewaspsdarkennotonlythesurfaceofthenestsbutalso thecocooncaps(yamane& Kawamichi,1975).A similardarkeningbehaviorisalso observedinthenestsofp.biglumisbimaculatus(geofroyinfourcroy),whichdwel inthehighmountainousregionofthealps(lorenzi& Turilazi,1986).Theseauthors believedthatunderacoldclimate,darkeningofcocooncapsisanadaptivebehaviorto increase the celtemperature by absorption directabsorption ofearly morning sunlight;however,theecologicalsignificanceofthedarkcocooncapshasnotbeen investigated. Consideringnestthermoregulation,IcarriedoutthisstudyonPolistescocoon capsandneststoassesstherelationshipbetweenthethermalpropertyofthecocoon capsandtheirsurfacecolor.thedarkness(hereafter,representedaslightness)ofthe cocooncapsinp.ripariusnestswasmeasuredandthethermalefectofthesecapswas investigatedusingmodelnestswith darkenedcocoon capsundertheoutdoorand indoorconditions.here,thethermalstrategiesofthepolisteswaspsdwelingincold regionsareproposed.
Thermalefectofdarkcocooncapsinpaperwaspnests (Hymenoptera,Vespidae,Polistes):Anadaptationtocoldclimates 237 Materialsandmethod Measurementoftheheatabsorptionrateincocooncaps Sincethestudyparameterswerenotweldefined,beforeexaminingthethermal efectofthecocooncaps,iinvestigatedthedetailsofsurfacedarkeningofcocooncaps inthenestsofp.ripariusatthecompletionofthepre-emergenceperiod.thenestsof P.ripariuswerecolectedtogetherwithimmaturewasps(N =8)inearlyJuly1999 from Sapporo(43 03'N,141 20'E),northernJapan.Usingpins,thecocooncapswere removedfrom thenestscarefuly,and57samplesofcocooncapswereobtained,i.e.,a meanofsevencocooncapswasobtainedpernest.thecocooncapswerecutintosmal pieces(3 3mm;Fig.1),andtheirlightness(%)wasdeterminedasfolows.The imagesofthecapswereobtainedwith an imagescanner(epson GT-X800)and storedinamacintoshcomputer.theimageswerestandardizedind65,i.e.,daylight condition,usingcolorsyncinmacos10.3software(appleco.).thecolorimageswere thenconvertedtograyscale.on eachimage,tenpointswererandomlychosenand theirlightness(%)wasdeterminedwith Photoshopver.6.0software(AdobeCo.). Firstly,theaveragelightnessvalueofeachnestwasdetermined(approximatelyseven capspernest).subsequently,theaveragelightnessvalueacrosstheeightnestswas determined.althecocoon capsobserved weredarkened,and themean (N = 8) lightnesswas28%.forcomparison,thelightnessofthecocooncapsinthenests(n = 5)ofP.chinensisantennalisPérez,aconsubgenericspeciesofP.riparius,colectedin June2004from IbarakiPrefecture,centralJapan,wassimilarlydetermined,andit wasfoundtobe87%. Temperaturemeasurementofthemodelnest Forevaluatingthethermalefectsofthecocooncaps,IsimulatedP.ripariusnests Fig.1.CocooncapsofPolistesriparius(left)andP.chinensisantennalis(right).Squaresindicate theareaatwhichlightnesswasmeasured(%).
238 SatoshiHozumi bypreparingpapermodelnestsusingthesamematerialandsurfacecolorbecausethe nestsbuiltbywaspsvaryinthesizeandshapeofcombsandinsurfacecolor.the modelshadhexagonalcelswithadiagonaldiameterof5.6mm,whichissimilarto thatofp.ripariusnests.themodelnestwasprepared with whitetissuepaper (thickness<0.1mm)madefrom purewoodenfibers,andthesurfacestructurewas similartothatofthepolistesnests(hozumi& Yamane,2001b).Inthepresentstudy, themodelnesthad61celsthatwere40mm inlength;thenumberandlengthofthe celsofthemodelwascomparabletoanaveragep.ripariusnest(68cels,38mm in length)atthecompletionofthepre-emergenceperiod(hozumietal.,2008).dividing walsimitatingcocooncapswereatachedtosevencelslocatedatthecenterofthe modelnests(fig.2).thewalsweremadeofthewhitetissuepaper,andthesurfaces ofthewalsweredarkened togray;thelightnesswasidenticaltothemeasured lightnessvalueofthecocooncapsofp.riparius(28% lightness;hereafter,pra).in addition,threeothermodelnests modelnestwithwhitecocooncaps(90% lightness; hereafter,wh),blackcaps(10% lightness;hereafter,bk),andwithnocaps(nc) werealsopreparedforcomparison.inthisstudy,iconsideredthatthethermalefect ofthecapsofp.cininsisantennalis(87%)iscomparabletothatofthecapsinthewh model. The temperature ofmodelnests was measured in the outdoor and indoor conditions.foroutdoormeasurements,themodelnestswereplaced1m abovethe ground.ineachmodelnest,acopper-constantanthermocouple(ø =0.32mm)was insertedinthecenterofthecappedcels,i.e.,5mm from thebotom (Fig.2).The temperaturewasmeasuredtothenearest0.1 C,andthedatawerestoredinadata logger(nr1000,keyence).theceltemperature,insolationexpressedasheatflux (W/m2,Medtherm,64-0.2-16/SW-1C 150),and ambient temperature (Ta) were measuredevery1min.thewindvelocitywasmeasuredwithadigitalanemometer (ModelDP-70,Hiyoshi)every10min.Themeasurementswerecarriedoutfrom 0630 Fig.2.Theschemeofamodelnest.Left,cross-section;right,frontsection.
Thermalefectofdarkcocooncapsinpaperwaspnests (Hymenoptera,Vespidae,Polistes):Anadaptationtocoldclimates 239 to1630hoursonseptember13,2004. Indoormeasurementswerecarried outin a dark room maintained at25 C constanttemperature.amodelnestwassetathorizontalcelaxesat1m abovethe floorandwasheatedfrom aboveat900w/m2.initialy,themodelwasheateduntil theceltemperaturestabilized understilairconditions(approximately15 min); subsequently,airwasblowntowardsthemodelsuchthatitcouldenterthecelsfor10 min(fig.2).thewindvelocitywassetateightlevels;0(stilair),1,2,3,4,5,6,and 7m/s.Thetemperaturemeasurementswereperformedfivetimesforeachmodelnest ateachwindvelocity,andthevaluesofthefiveexperimentswereaveraged. Dataanalysis Mann-WhitneyU-testwasemployedtodetectthediferencesbetweenthetop andcentralcelinthefieldnestsofp.riparius.nostatisticaltestssuchasanova wasappliedtothetemperaturedatameasuredwithmodelnestsunderoutdoorand indoorconditionsbecausethetemperaturedatawerenotindependent.however,the thermalefectduetodiferentgradesoflightnessofthecapcolorwasrepeatedly observed,and the values between the outdoor and indoor measurements were inconsistent. Results Thermalefectofthesurfacecoloronthecocooncaps Onthedayofmeasurement,insolation(mean±standarddeviation(SD),511± 378W/m2)gradualyincreasedfrom 0700to1130hours,anditlargelydecreasedfrom 1300to1400duetothecloudyenvironment(Fig.3).Thewindwasstrongfrom morningtonoonandcalm from afternoonuntilthemeasurementwascompleted.the Tarangedfrom 19.6 Cto26.0 C(mean,23.6 C).Theceltemperatureswereafected bythechangesininsolation,andthetemperaturewashigherinthemodelswith darker cocoon caps (Fig. 3). During early morning (0630 0730 hours), the temperaturewasalmostsimilaramongalthemodels,andthenesttemperatures gradualyincreasedwiththeincreaseininsolation.from 1000to1300hours,the temperature diference (Td) among the models gradualy increased. The cel temperatures decreased in a cloudy environment untilthe completion ofthe measurementperiod.the mean temperatureswere 28.2 C (WH),29.3 C (PRA), 30.3 C (BK),andtheTdwas1 C betweenthemodels.thetemperatureofthenc modelwaslowerthanthatofthewh modelfrom 0630to1100hours,whilethe temperaturesofthesenestswerealmostsimilarafter1130hoursunderthecalm wind condition. Figure4showstheindoorTdbetweenthemodelswiththecocooncaps,i.e.,WH, PRAandBK,andthemodelwithnocaps(NC).Thetemperaturetendedtobehigher
240 SatoshiHozumi Fig.3.Temporalchangesofenvironments(upper)andnesttemperature(lower);lightnessofcocooncaps,BK,PRAandWH hadcocooncapswith10%,72% and90% lightness,respectively;nchadnocaps. Fig.4.Temperaturediferencebetweenmodelswithcocooncaps(BK,PRAandWH)andamodel withoutcaps(nc).
Thermalefectofdarkcocooncapsinpaperwaspnests (Hymenoptera,Vespidae,Polistes):Anadaptationtocoldclimates 241 inthemodelswithdarkercaps,andthediferenceamongthemodelswaslargestata windvelocityof3m/s;thetdwas2.7 C,3.6 Cand4.8 CforWH vs.nc,pravs.nc, and BK vs.nc,respectively.thetd gradualy reduced when thewind velocity increasedfrom 5to7m/s,andtheTdamongthemodelsweresimilar(2.9 C 3.2 C)at 7m/s.OncomparingtheWH andncmodels,itwasfoundthattdwaszerowithstil air,whileitincreasedasthewindvelocityincreased. Discussion From the results oftemperature measurements with the modelnests,two thermalefectsofthecocooncapswereobserved the wind-break efectbythecaps andthe heatabsorb efectduetothedarkeningofthecapsurface.underboth outdoorandindoorconditions,thewind-breakefectofthecocoonwasobservedwhen airwasblowntowardthenest.forexample,whilethetemperaturesinthewh and NCmodelsweresimilarunderthecalm condition,thetemperatureinthewh model washigherthanthenc modelwhenairwasblowntowardthenest.theheat-absorb efectwasobservednotonlywhenthemodelnestsreceivedsunlight(orinfraredlight) butalsowhenthesurfacesofthecapswerenotexposedtosunlight.inaddition,the temperatureincreasedduetothethermalefectofthedarkcapreducedtothewhite cap levelby the externaldisturbances,such aswind.the phenomenon can be explainedasfolows.duetohighheatabsorption,thedarksurfacecanretainmore heatuntilathermalequilibrium isachieved,andtheheatisthenradiatedtoboththe sidesofthecocooncap,resultinginahigherceltemperature.ontheotherhand,the heataroundthecapscanbeeasilyemitedbyanenvironmentaldisturbancedueto thehighemissioncapacityofadark-coloredsurface.thisexplanationissupported from theresultsoftheindoormeasurementthatrevealedthattheceltemperature wasalmostsimilarinthewh,pra,andbk modelsunderthehighwindvelocity(over 5m/s). Incoldclimaticregionswherethenestingperiodisshort,nestthermoregulation playsanimportantroleinacceleratingthecolonialgrowth,becausethedurationof thefoundingstageinfluencesthenumberofreproductivegynesunderlimitednesting periods(wilson,1971;heinrich,1993).inordertogainheatfrom theenvironment, thefoundressesofp.ripariusstrivetoachievenestthermoregulation,asapartofnest building.thenestofp.ripariusisacharacteristicstructurewithextracelsaround thebroodsanddarkenedcocooncaps(yamane,1969,1971;yamane& Kawamichi, 1975);boththesefeaturesincreasethenesttemperature(Hozumi& Yamane,2001a, b;hozumietal.,2008;thisstudy).from thesestudies,itisestimatedthatinapreemergencep.ripariusnest,theceltemperatureatthebroodareacanbeincreasedby 5 Cunderstilair(buildingextracelsincreasestemperatureby4 Canddarkeningof thecocooncapincreasestemperatureby1 C),comparedtothepre-emergencenestsof
242 SatoshiHozumi P.chinensisantennalis,dwelinginthewarm areasofcentraljapan.ontheother hand,theefortinvestedbyp.ripariusfoundressesiscommendable,sinceactivities involvedinbuildingsuchnestsrequiregreatefortintermsofnestmaterials,suchas producingproteinousoralsecretionandcolectingplantmaterials.whentheefort requiredforconstructingapre-emergencenestwith regardtotheamountoforal secretionandplantmaterialsarecomparedbetweenthetwospecies,p.ripariusdoes twice the amountofwork done by P.chinensisantennalis,and thisresultsin increasingtheweightofthenestbyfourtimes(yamaneetal.,1998).thissimply indicatesthatthedurationofextranidalactivitiesperformedbythefoundressesofp. ripariusisfourtimesthatofthefoundressesofp.chinensis.furthermore,duringthe pupal substage, the foundresses of P. riparius may spent more time for thermoregulatorycoolingbyfanningandwatercolection;p.riparius,10.4% and62.3 times/day,respectively(thisstudy);p.chinensisantennalis,0.5% 1.8% and0 25 times/day,respectively(kasuya,1983;kudô,personalcommunication).theseeforts investedfornestthermoregulationarerepaidintheform ofaccelerateddevelopment oftheimmaturewaspsandtheproductionofmorereproductivegyneswithinashort nestingperiod. ItisconcludedthatthedarkenedcocooncapsofP.ripariusisatypeofthermal adaptationtocooltemperateregions,andthedarkcapspromotethermoregulationin thecocooncelsviatheirthermalefect,i.e.,highabsorptionandemissionability. However,somePolisteswaspsdarkenthecocooncapsalsoinhotregionssuchas tropics(kojima& Kojima,1988).Itisconsideredthatinadditiontothermoregulation, thedarkcolormayactasaprotectionstrategy.inthenestswithagrayorbrownish surfacecolor,thewhitecocoon capsexposed totheoutsideenvironmentmaybe obviousduetothecontrast.itisalsopossiblethatinhotclimates,thiscoloration functionstoreducethechanceofpredationfrom mammalsandbirds,ratherthan thermoregulation,because the wasps are not necessary to achieve high nest temperatureduetoahighambienttemperature. References Cloudsley-Thompson JL (1999) Multiple factors in the evolution of animal coloration. Naturwissenschaften86,123-132. DanksHV(2004)Seasonaladaptationsinarcticinsects.IntegrativeandComparativeBiology44, 95-94. HeinrichB(1993)Thehot-bloodedinsects.HarvardUniversityPress. HeinrichB(1995)Insectthermoregulation.Endeavour19,28-33. HozumiS, YamaneSô(2001a)Efectsofsurfacedarkeningonceltemperatureinthepaper waspnests:measurementsusingpapermodelnests.entomologicalscience4251-256. HozumiS,YamaneSô(2001b)Incubationabilityofthefunctionalenvelopeinpaperwaspnests (Hymenoptera,Vespidae,Polistes):I.Fieldmeasurementsofnesttemperatureusingpaper models.journalofethology19,39-46.
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244 SatoshiHozumi アシナガバチの巣における暗色化した繭の熱的特性 穂積 訓 寒冷地に生息するトガリフタモンアシナガバチ (Polistesriparius) は, 繭の表面を暗色化する特徴がある 本研究では, この繭表面の暗色化が持つ熱特性に着目し, 巣の模型 ( 以下, 模型巣 ) を使って野外および実験室で温度を測定して検証した また, ガリフタモンアシナガバチの創設メスによる温度調節行動についても合わせて調査した ガリフタモンアシナガバチの繭の表面は黒褐色で, 明度は 28% であった 実験で用いる模型巣には, 明度を 4 段階に設定した仕切りを繭としてつけた 模型巣の内部温度を野外でしたとき, 巣内の温度は明度が低い巣ほど上昇した 模型巣の間の温度差は, 風が強いほど小さく, 風が穏やかなときほど高かった 実験室の温度測定において, 無風条件では明度の低い巣ほど温度の上昇は高くなった 一方, 風が強くなると, 繭の明度による温度の変化は小さくなった 最後に, これらの結果から, 繭の暗色化と温度調節の関係, およびガリフタモンアシナガバチの寒冷地の適応について議論する キーワードアシナガバチ, 繭表面の明度, 巣の温度調節, 寒冷な気候への適応