Habitat selection and population parameters of Sympetrum infuscatum (Selys) during sexually mature stages in a cool temperate zone of Japan (Anisoptera: Libellulidae)

2004·1.1 MB·English

by  Watanabe, M

Odonatologica33(2): 169-179 June 1,2004
Habitat selectionandpopulationparameters
ofSympetruminfuscatum (Selys)
duringsexually maturestages inacool
temperatezone of Japan
(Anisoptera: Libellulidae)
M.Watanabe¹,H.Matsuoka²andM.Taguch³i
'Institute ofBiologicalSciences,UniversityofTsukuba,Tsukuba,Ibaraki305-8572,Japan
e-mail:[email protected]
2DepartmentofBiology,FacultyofEducation,MieUniversity,Tsu514-8507,Japan
’Yaei-HigashiHighSchool,Sagamihara,Kanagawa229-0029,Japan
ReceivedSeptember1,2003/RevisedandAcceptedOctober30, 2003
Themark-and-recapturemethodwasusedtostudythepopulationparametersofsexu-
allymatureadultS.infuscatuminaforest-paddyfieldcomplexin thecooltemperatezone
ofJapan.Afteremergence,theymoved intotheforestgaps,and theyremained andfedex-
clusivelyintheforestgapsthroughouttheirlives. Mature63capturedoutnumberedmature
59oneachsamplingdayinthepaddyfields,but notinthegaps.However, theestimated
dailynumbersinboth habitatsand/orthewholesurveyarearoughlyindicated a1:1sexra-
tio,Theestimateddailysurvivalratesand dailyimmigrationsshowedthat 99were aptto
stayintheforests,while33sometimesdispersed,though99inthepaddyfieldstendedto
havealong-rangeflight.Inthemorning,someofthepairedcouplesflewtothepaddyfields
intandemandovipositedonthewing;thesewereso-calledflyers.Therestremainedinthe
foreststheentireday;these weredesignatedasperchers.Theforestgapswerethusimpor-
tantforthelifecycleofthisspecies bothasfeedingandroostingsites.
INTRODUCTION
Inmostdragonflyspecies, dailymovementinsearchoffood,mates,androosting sites
isafundamentalprocess thatinfluencesthedynamics of spatially dividedpopulations
(e.g., JONSEN& TAYLOR,2000).Although maidenflightoftentakes anindividual
farfromits siteofemergence(CORBET, 1999),the daily movements ofadultsarise
fromdiurnalbehaviourinresponseto differencesinthepotentialroosting, matingand
ovipositing sitesofdifferenthabitats.
FormanySympetrum species in Japan,ricepaddy fieldsarethe mainhabitatforthe
170 M.Watanabe,H.Matsuoka& M.Taguchi
larvalstages. Afteremergence,teneralsgenerally embarkonamaidenflight,leaving
thepaddy fields. Anumberofstudieshaverevealedthatthe sexually immatureperi-
odofadults is usually the principal period ofodonatedispersal (e.g., MICHIELS&
DHONDT,1991),andmanyspeciesflyaconsiderabledistanceawayfromwater(COR-
BET, 1999),implyingthatsexually mature adultsreturn andtendtostay bythewater.
AmongSympetrum species, S. frequens andS.pedemontanum representtwoextremes
ofmaidenflightbehaviour.IndividualsofS.frequensmovefarfromthepaddyfieldsto
themountains,remain inthemountainsover thehotsummer season,andreturn tothe
paddy fieldsinautumn (TANAKA, 1985;TSUBUKI,1987;MIYAKAWA, 1989).S.
pedemontanum, on theotherhand,exhibitsashortmaidenflight,ifany,and remains
almostexclusively inthepaddyfields(TAGUCHI&WATANABE,1985).OtherSym-
petrum species, including S. infuscatum, display maidenflightbehavioursintermedi-
ate betweenthetwo. Generally, they enterintoforests orwoods nearthepaddy fields,
andstaytherethroughout theirimmaturestages.Aftersexually maturation,manySym-
petrumspecies returntothepaddy fieldsforreproduction, whileS. infuscatum remains
mainly intheforestgaps.
S. infuscatum is aso-called“flyer”,i.e., themature adultsofthisspecies flyoverthe
paddy fields atrelatively high velocity andlayeggs intandemonthewing. Likemi-
gratoryspecies such as S.frequens, thereseemtobenophysical limitationsinpaddy
fieldsforadultbehaviour.Themaidenflightis usually long, andtheindividualsprob-
ably disperse duringtheiradultstages. Accordingly, itisdifficulttoclarify thepopula-
tionparametersofaflyerspecies, bothbecause ofthe difficultyof initially capturing
individualsinthe paddy fields, andbecauseofthe difficultyofrecapture(CORBET,
1999).Therefore, littleattentionhasbeenpaid tothepopulation ecology ofsuch spe-
cies,though thepartiallifehistoryandoccasionalobservationshavebeenreported(e.g.,
ISHIDA, 1969).Using themark-and-recapture method,WATANABE& TAGUCHI
(1988)clarifiedthepopulationstructuresofseven Sympetrumspecies inautumnpaddy
fields,although noS. infuscatumadultsinthemature stageswererecaptured. However,
ourinitialobservationsinthesurveyareaindicatedthataconsiderablenumberofmales
andfemalesinboththesexually immatureandmature stagesspendthedayperching in
theforestgaps:theseindividualsare thusknownas “perchers”. Thus, dragonfliesvary
intheirabilitiesor propensity tomove overthedifferenttypesof habitat,which might
havebroader-scaleconsequencesforpopulation dynamics.
Theaimofthepresentstudy wastoobtainquantitativeinformationontheadultpopu-
lationparametersofS.infuscatum. Morethan20researcherswereemployedtoestimate
the mature adultpopulation size and dailymovements using themark-and-recapture
technique. Reproductive behaviourwas alsoobserved during theexperiments, which
contributestowardsourknowledge oftheS.infuscatum mating system.
SPECIES ANDSTUDYAREA
PopulationsofS.infuscatumwerestudiedinanisolatedplainoftheKamishiroregioninShirouma,Japan
in2000-2001.Theplainliesinacooltemperatezoneatanelevation of750m(Fig.1).Theplainiscomposed
PopulationparametersinSympetruminfuscatum 171
Fig.1.Mapofthestudyarea,Kamishiro,Shirouma,NaganoPrefecture. —Lines(A-D)representthesurvey
linesoftheS.infuscatumpopulationforthemark-recapturemethod.Shadedareasshowpaddyfields,andvil-
lagesareindicatedbytheaggregationofsmallblack squaresthatrepresenthouses andbuildings.
172 M.Watanabe, H.Matsuoka&M.Taguchi
ofricepaddyfields(ca2km2)andvillagessurroundedbyhillsormountains withconiferous(cedar)forests.
Ricereapinginthepaddyfields iscarriedoutannuallyinearly October.
S.infuscatumisanon-territorialdragonfly.Sexuallymatureadultswerefoundintheplaininthemorning
fromlateAugusttomid-September.Oncepairs wereformedinthesurroundingsofthepaddyfields,which
arelocatedinthecentre oftheplain,theymayhaveflownimmediatelytothepaddyfields,ormayhavecome
from agreaterdistance.Inthematingofthisspecies, themaleretainsholdofthefemaleandthepairfliesover
thepaddyfieldsintandemandlayseggsonthewingabovethericeplants.Lonefemalesdonotoviposit.After
oviposition,thepairseparates andthedragonfliesreturntothegapsinthecedarforests,whereaconsiderable
numberofadultS.infuscatumperchthroughoutthedaytoforageorescapefrombirds andotherpredators,
but thereislittlecompetitionforperchingsitesamongthem. Nomatureadultsshowmatingbehaviour.
METHODS
WestudiedtheS.infuscatumpopulationusingthemark-and-recapturemethodintheplainandtheforest
gaps.Althoughthesurveyareawaslarge,thehabitat isroughlyisolated.Wepatrolledalongthepass(total7
km)ingapsandpaddyfieldsoveranareaonadayinlateAugust,whenmostoftheadultshadmatured.
Adultcensuseswerecarriedoutalong4survey lines simultaneouslyinthe surveyareaonmostlyclear
daysfor5 days and11 daysinlate Augustof2000 and2001,respectively,duringwhichtime theS.infus-
catumpopulationwasconsidered tobestable andreproductiveactivitywashighinthepaddyfields.Ittook
about4hours tointensivelypatroleachsurvey line.AsshowninFigure1,surveylinesA(ca900m)andB
(ca900m)followed anupslopingpaththatincludedmany gapsinthecedarforests.ThesurveylineC(ca
3000m)waslocated in thevillagesand thelineD(ca2000 m)wasin thepaddyfields.Plants inthesurvey
lineswereoccasionallytrimmedbyresidents andfarmers.Consequendy,vegetationalongthelineswasless
than30cmtall, andtheapproximateperchingheightofS.infuscatuminthegapswasusuallylessthan1m.
However,inthepaddyfields,theheightoftheovipositionalflightin tandemwasabout 1.5m,or 10-20cm
abovethericeplants,while thatofmovementflightintandemwasmorethan2m.
Alladultsdetectedwerecapturedwithanetandmapped.Eachwasgivenanindividualnumberthatwas
markedwith ablackcolourfelt-tippedpenontheundersurface ofthelefthindwing.Theywerereleased im-
mediatelyatthesamecapturingsite afterrecordingthedate,sitename,sex, andage.Themarkingwascon-
sideredtohave onlyminoreffectsontheirflightactivities(andtheprobabilityofpredation)becausemost
adultsbegantoflynormallysoonaftermarking.Adultswoundedduringmarkingweretreatedasdeadin-
dividualsin the calculations. Althoughthe age ofsexuallymatureS.infuscatumwasestimated according
tofourage classes,mainlybybody colourandwingcondition,asinthecaseoftheotherdragonflyspecies
(e.g.,McVEY, 1985; WATANABE&HIGASHI, 1989),inlateAugustthemajorityofadultswereinthe
earlymaturestages.There werenooldmatureadults.
Inmost dragonflypopulations,therecapture probabilitiesofthetwosexeswere different(e.g.,WATA-
NABE&HIGASHI, 1989).The sexesofS.infuscatumwereprocessed separatelythroughJolly’sanalysis
(JOLLY, 1965).
RESULTS
Mostmalescaptured weremature individuals, judgingfrom thedarkred colourof
theabdomen,andmostfemaleswerealsomature basedontheirbrownredcolouration.
Adultswerefoundthroughouttheentirehabitat,butmalesweresometimesconcentrated
onthemarginsofforestsoronahedgeinthevillageinthelateafternoonandtheevening,
bothofwhichweresunlitareas. Observationsduringthemark-and-recapturesampling
(M. Watanabe & H. Matsuoka, unpublished) showedthatbothsexes perched onthe
tipsoftwigs or grassbladesinforestgaps,and madeintermittentfeeding flights,after
PopulationparametersinSympetruminfuscatum 173
whichtheyusually returnedtothe Table I
sameperch.Inaddition,therewere Total number ofadultsmarkedinforest gaps andinpaddy
fewinteractionsamongthem,i.e., fields duringthecourseofthe survey(5 daysfor 2000and
11daysfor2001)
therewas nocourtshipbehaviour,
nomatingbehaviour,noterritorial 2000 2001
behaviour,and so on.Forestgaps Habitats males females males females
provide good sources of food for
adults.Thetotalnumberofadults Forestgaps 1,449 1,565 4,201 3,791
Paddyfields 1,863 603 4,046 1,417
capturedingapsindicatedthatthe
sex ratiowasnear unity inboth2000and2001,though the numberof sampling days
was differentineach year(Tab. I). Ontheotherhand,mostadultsinthepaddy fields
flewcontinuously withoutlanding; allfemalescaptured wereintandem,buttherewere
aconsiderablenumberofsinglemales.Consequently, thenumberofmalescapturedin
thepaddyfieldswasabout3timesthatofthefemalesinboth2000and2001. Sincefew
singlemaleswerecaptured intheearly morning,whenthereweremanycouples found
flyingintandemoverthepaddy fields,andsincethenumberofsinglemalesincreased
withtime,the single malesinthepaddy fields appeared tobethosemalesremaining
aftertheseparationoftandemcouples. Singlefemalesappeared toreturn quickly tothe
gapsafteroviposition.
Thenumberofrecaptured adultswastoo smalltoestimatethepopulationparameters
withasmall standard deviation.In 2000,31 malesand 28females wererecaptured,
or0.9%and 1.3%oftotalnumberofadultsreleased,respectively. ByJolly’s method,
therewere 10,000malesinthegaps and25,000malesinthepaddy fields, and33,000
femalesinthegapsand20,000femalesinthepaddy fields.
In2001,thenumberofrecapturesincreased 1,5-foldforboth sexes, probably dueto
TableII
DailypopulationparametersofadultsestimatedusingJolly’smethod(±SD)
Year Sex Parameters Wholearea Forestgaps Paddyfields
2000 S number 36,843±12,328 9,453±7,530 25,174±13,226
survivalrate 0.30±0.22 0.27±0.25 0.41+0.16
numberofimmigrations 26,381±4,916 6,851±7,588 19,402+10,130
9 number 26,559±26,822 33,788±28,520 20,433±17,933
survivalrate 0.66±0.40 0.64±0.42 0.78±0.42
numberofimmigrations 29,270±62,686 16,950+36,204 10,277±18,922
2001 6 number 19,061+4,167 I0.664±3,679 11,606±3,250
survivalrate 0.40+0.12 0.45±0.13 0.40±0.12
numberofimmigrations 45,844±97,257 10,206±11,644 261,402±775,094
9 number 28,091+7,528 17,989±5,057 7,994±3,919
survivalrate 0.66±0.08 0.67±0.09 0.65±0.11
numberofimmigrations 61,03)±159,159 32,115±79,941 29,896+77,199
174 M.Watanabe, H.Matsuoka&M.Taguchi
thelong survey period, andthenthestandarddeviationdecreased.Thedailyestimated
numberofadultsinthewholesurvey area,forest gaps,orpaddy fieldsvariedforboth
sexes,butthevariancewas larger forfemales thanmales.Assuming thatthesex ratio
was 1;1,we calculatedthewholedailypopulation by doublingthenumberofmales,to
70,000in2000and40,000in2001.Inaddition.TableII shows thathalfofthe males
wereingapsindaytime,andthenumberoffemalesthatremainedinthegapswastwice
thenumberoffemalesthatremainedinthepaddyfieldsinbothyears.
Thechange inestimateddailysurvivalrate fluctuatedinbothsexes andhabitatsbe-
tween 0.27and0.78 in2000,andbetween0.40and0.67in2001 (Tab.II). Bothsexes
co-existedinthe samehabitats, and gender seemedto havenoeffecton thesurvival
duringthesurveyperiod. Noavianpredation was observedineithersex. However,the
estimateddailysurvivalrate was low,particularly inmales.Sincethelongevity of S.
infuscatumis usually morethan2 months,this dailysurvivalrate maybeareflection
ofthe dailyemigration ratefromthesurvey arearatherthanactualmortality.SCOTT
(1973)pointed out that Jolly’s “survival”estimates shouldbeclosely correlatedwith
residenceforwildinsectpopulations. Inthepresent study, malesinbothgapsandpad-
dyfieldsweremoreapttoemigratethanfemales.Inaddition,becausethepaddyfields
wereopenhabitatsinwhichpairs flewaboutintandem,theimmigration andemigra-
tionrates ofadultsintheS. infuscatum population shouldbe very high.
Themovementbetweenoramonghabitatswas examinedforrecapturedatain2001
(Tab.III). Formales,61movementswere gap-to-gap,16were gap-to-paddy fields, 11
werepaddy fields-to-paddy fields,and 15werepaddy fields-to-gap.Ontheotherhand,
outof73movements observedinfemales,58weregap-to-gap,indicatingthatfemales
weremoreapttostay ingapsthanmales(G=9.2676,0.05>p>0.01).
Fromthemark-and-recapturedataobtainedamongthestudysites,theaveragedistance
offlightforadultswascalculatedas showninTableIV.Sincetheadultswerenotalways
recaptured the dayafterbeing released,theindividualdistancesper day calculateddid
notcoincidewiththeactualdistancesofflighttrajectory.Furthermore,thesurveyperiod
wasdifferenteachyear.However,themeandistanceperdaybetweencaptureandrecap-
ture sites throughout
Table HI
Releaseandrecapture siteswithdurationafterreleasein2001 the whole area was
similarinboth years
Number Daysafterrelease Max. for both males and
Sex Direction ofadults (±SD) days females, and males
showed significantly
6 Gaps-*■Gaps 61 2.0+1.4 5.5
higher activities than
Gaps —￿Paddyfields 16 2.5±1.7 5.0
Paddyfields-»Gaps II 4.1±3.2 8.5 femalesin2001.
Paddyfields—> Paddyfields 15 2.0±2.1 6.5 Withinforestgaps,
both sexes showed
5 Gaps—*Gaps 58 2.1+1.7 9.0
low rates of move-
Gaps —+Paddyfields 7 2.0±1.5 4.5
Paddyfields-*Gaps 2 2.3±0.4 2.5 ment activity. The
Paddyfields-*Paddyfields 6 2.1+2.3 6.5 rate of less than 100
PopulationparametersinSympetruminfuscatum 175
TablelV
Distance ofmeandailymovement(m/day, ±SE); — inparentheses():numberofadults
Year movement <J 5
2000 throughoutthesurveyarea 259.3± 70.4 (26) 101.2± 35.0(27) U=292.0 P=0.130
2001 throughoutthesurveyarea 231.2± 40.1 (103) 145.7± 40.1 (73) U=2982.0 P=0.016
within gaps 187.9± 45.5 (62) 95.4± 35.3 (56) U=1395.5 P=0.046
between gapsandpaddyfields 378.6±102.2 (28) 295.9± 87.1 (8) U= 106.0 P=0.837
withinpaddyfields £U) vO1+ 4^ Ö (15) 479.7±211.4 (6) U= 20.0 P=0.055
m/day forfemalesindicatedthatfemales movedonly tothe next gap withinasingle
day,andthatfemalesremainedsignificantlymore stableingapsthanmales.Relatively
fewadultswererecaptured inpaddy fieldsafterbeing releasedingapsandvice versa.
On theotherhand, withinpaddy fields, femaleswere recaptured ata greaterdistance
fromthereleasethanmales.
DISCUSSION
Ourstudy revealedthatS. infuscatum isnotrestricted toawholesurvey areawheth-
er itshows lowflightactivity inforestgapsor high flightactivity inpaddy fields.The
availablemethodsdonotallowustotrace migrationsofgreatdistance.However,some
of ourresults coincidedwith thefragmental informationreported by UEDA(1997),
i.e., thatthis species always fliesoverthepaddyfieldsinautumn. Thehighdensity of
tandemflyinginthepaddyfieldsthroughoutthereproductive seasonsuggeststhatthis
species doesnotexhibitterritoriality,andlittleintraspecificaggression forperchingsites
inforestgapswasobserved(M.Watanabe&H.Matsuoka,unpublished).FewS. infus-
catummaleswere“opportunists” thatflewaboutandsearchedformatesintheforests,
andthustherewerenopairs intheforestgaps.Tandemflyingandcopulation seemedto
occuralongtheborderbetweentheforestsandthepaddyfieldsinearlymorning. Out-
side-pondpairing informationhasbeenreported inS.parvulum(UEDA, 1979)andS.
danae(MICHIELS&DHONDT, 1989).
Morethanten Sympetrum species thatinhabitJapanese paddy fieldshavebeenre-
corded(ISHIDA, 1969).Someofthem(e.g., S. frequens andS. kunckeli) werefound
inthe present survey area. In lateAugust mostof thesewere sexually immatureand
consideredtohavebeenaccidentally capturedinthepaddy fields,andvery fewadults
wereobservedintheforestgaps.Therefore,noodonatespecies coexistswithmatureS.
infuscatum inthesurvey areaduringlateAugust. TherewerefewS.pedemontanum in-
dividualsinthesurveyarea, aspecies inwhichsexualmaturationisreachedtheearliest
amongSympetrumspecies(MICHELS&DHONDT, 1987).SincetheotherSympetrum
speciesappearaftermid-September, thematurationofS.infuscatumistheearliestinthe
surveyarea.WATANABE& TAGUCHI(1988)pointedout thattheSympetrum com-
176 M.Watanabe,H.Matsuoka&M.Taguchi
munityofthepaddy fieldsinearly Octoberconsistsofdiversespeciesand individuals
ofdiverseage,duetodifferentdurationofmaturationafteremergence.
Diurnal activity patterns, particularly onreproduction, inmatureSympetrum adults
inthe paddy fieldshavebeenwell documented (KINOSHITA&OBI, 1931;ARAI,
1983;UEDA,1979).CORBET (1980)statedthatthemaleactivity patternatthewater
isareflectionofthearrivalrate ofreceptive females.Particularly, inthespecies adopt-
ingterritorialtactics,mature malesreturn towatertoestablish territories, whilemature
femalestendtovisitonly tooviposit (VANBUSKIRK, 1987).Thus,theprobability of
recaptureforterritorialmalesis higherthanthatforfemales(CORBET, 1999).Conse-
quently,such diurnalbehaviourseems toleadtoamale-biasedoperational sex ratioin
thepaddyfieldsasreproductive sites (e.g.,WATANABE&TAGUCHI, 1988).Inthis
study, however,morethanhalfofthematureS.infuscatum femalesremainedatforest
gapsevery day.Femalearrivalatthepaddy fieldsdepends uponmaleactivity, since
femaleswerebrought inintandemby amalefromthemargins oftheforests.Thus,the
sex ratiointhepaddy fieldsmustbeunity.
Althoughthe flying5. infuscatum adultswerehardertocapturebecausethey flewat
thecentreofthe paddy fieldsoratmorethan2 mabove therice plants, thesex ratios
basedontheestimateddailynumbersforthetwo yearsofmeasurement appeared tobe
roughly 1:1,with aslight predominance ofmales.Anequal sex ratiointhereproduc-
tivearea was alsoreported for,S.pedemontanum, which inhabitspaddy fieldswithout
any flights away from the fields (TAGUCHI & WATANABE, 1985). In our study,
however,only tandempairsofS. infuscatum cametothepaddy fieldsandstartedovi-
positionwithoutdelay. Thesurplus malescaptured mustbetheindividualsdelayed in
theirreturn toforest gapsafter separating from atandempairfollowing oviposition.
Someofthelonemalesintermittently perched ontheground orthetipofrice blades,
andthesemay havebeenwaiting fortheflightactivitiestoresume. They havetopay
high energycostassociatedwithtandemflightincluding oviposition intandeminthe
paddyfields.TAGUCHI& WATANABE(1995)observedthatmalesmightwithstand
higher thoracictemperatureintandemflightthanfemales.Theotherlonemalesflew
aboutthepaddy fields,asifsearching forany strayreceptive femalesthathadseparated
fromothermalesbeforeoviposition,though wedidnotdetectanydifferenceintheaim
oftheflightbehaviourofthesinglemalesinthepaddy fields.MICHIELS&DHONDT
(1991)reported that,S. danaemalesreadilyrematedon thesame daywheneverpossi-
ble.McMILLAN(1996)separated S. vicinumtandempairsandfoundthatmostfemales
wereimmediately chasedby lonemalesandtakenintotandem.
Thedailypopulation densityofS.infuscatumwashigh(2.25 and 1 adultsper 100m2
ofthepaddyfieldsfor2000and2001,respectively),andthedistributionoftandempair-
ings appeared toberoughly random.Nomutualattractionamongpairs was observed.
Therice paddy fields supported the random distributionofthepotential oviposition
sites, wherethefemaleslaideggsintandemonthewing.Thesameovipositionbehav-
iourwasobservedinS. darwinianum(TAGUCHI&WATANABE, 1995),whichalso
has littlesite preferenceforovipositionwithinthepaddyfields. Ontheotherhand. S.
PopulationparametersinSympetruminfuscatum 177
pedemontanum, S. frequens, S.eroticum, andS. vicinumlayeggs inpuddles beforethe
patchilydistributedwater surfaces inthepaddy fieldsdryupfollowingthericeharvest,
thus,thesespecies musthavestrongpreferences foroviposition siteswithinthepaddy
fields(KINOSHITA &OBI, 1931;MIZUTA, 1978;McMILLAN,2000).
BothmaleandfemaleS. infuscatumindividualsthatremainintheforestgapsgener-
ally use asit-and-waitforagingtacticinorderto catchprey throughout theentireday.
HIGASHI(1973) observedthefeeding behaviourofS.frequens inconiferousforests
wheretherewas anexcess offemales.Since nomatingbehaviourwas observedinthe
hills,thishabitatmaybeafeeding site, aswellas,presumably, aroostingsite.TAGUCHI
& WATANABE (1987) suggested thatfemalesof S. eroticum visitthepaddy fields
only onceevery few daysto mateandoviposit. MICHIELS&DHONDT (1991)also
showedthatinS. danaefemaleseach oviposition boutwas usually followedbyanon
reproductive period ofapproximately 4 days. Therefore, in thisstudy area, thelarge
numberofS.infuscatum femalesintheforestgapsmightdevelopclutchesofeggs cy-
clically, as suggested by FINCKE(1982).
In general, immatureindividuals of most dragonfly species fly away from water
(MOORE, 1953), and itis consideredthatthey disperse mainly by random move-
mentsduringthemaidenflight(e.g., JOHNSON,1960;CORBET,1999).Although the
maidenflightplays arolein dispersal forbothsexes ofmanyodonatespecies, mature
malesalsodispersed inthepresentstudy. Sinceallmatureadultsreturnedtotheforest
gaps,thelattermustbethemajorhabitatofS. infuscatum. Then,two-waymovement
betweentheforestandthepaddy field needstooccur inboth sexes forreproduction,
indicating thatthey havetheability to switchbetweentwo distinctflying habits, i.e.,
percherandflyer habits.
Mostodonatefemalesarepolygamous, although ithas frequently beenreportedthat
theycanrefusetocopulate(CORBET, 1999).As inthecase of S.infuscatum staying
intheforestgaps,thehigh density seems torelease mature malestoattack or harass
single females forthepurposeofmating. However,inour study, alltheadult males
tendedtosimply perch, anddidnotinterferewiththefemales.Theremustbeanaddi-
tionalmechanismwhichprevents thereleaseofunnecessary courtship behaviour,be-
causeperching femalesdidnot avoidmalestrying toperchatthe samesiteinthefor-
est gaps,as they didinthecase ofS. danae(MICHIELS&DHONDT, 1989).Thisis
probably theirlifetimehabitas apercher intheforest gaps.Although furtherdetailed
behaviouralstudy of thisspecies is needed,ourresults make cleartheimportance of
theforestgapsforthe lifehistoryof S. infuscatum. Itisapparentthat dragonflies are
good indicatorsofhabitattypesandoftheecological qualityoftheland-water-inter-
facebecauseoftheircomplex habitatrequirements (seereviewinCORBET,1999).In
the species studied,theeffectofhabitatfragmentation isnot likely toleadtoisolation
effects.Theabandonedforestshavesupplied alarge numberofgapswhichare amajor
habitatforS. infuscatum. Thisfact furtherincreasestheindicatorpotential ofthespe-
ciesintheforest-paddy fieldcomplexecosystemandhas aneffectontheevaluationof
conservationstrategies.
178 M.Watanabe, H.Matsuoka&M.Taguchi
ACKNOWLEDGEMENTS
WewouldliketothankH.TANI,T.MURAKAMI,K.HOSOI,Y.MIMURA,T.HIGASHI, Y.NA-
KANISHI, M.BON’NO, T. IMOTO,N.KATAYAMA,andK.FUTAMURA fortheirassistance inthe
field. Thestudents oftheBiologicalScienceClubofYaei-HigashiHighSchoolandthestudentsoftheDe-
partmentofNaturalScience,FacultyofEducation,MieUniversityalsohelped.This workwassupportedin
partby agranttoMWfromtheUniversityofTsukuba ResearchProjects.
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