Do coloured-winged damselflies and dragonflies have flight kinematics different from those with clear wings?

2013·0.44 MB·

by  Dagmar Hilfert-Rüppell

InternationalJournalofOdonatology,2013
Vol.16,No.2,119–134,http://dx.doi.org/10.1080/13887890.2013.763332
Do coloured-winged damselflies and dragonflies have flight
kinematics different from those with clear wings?
DagmarHilfert-Rüppella*,andGeorgRüppellb
aInstitutfürFachdidaktikderNaturwissenschaften,TechnischeUniversität,Braunschweig,Germany;bAn
derWasserfurche32,D-38162Cremlingen,Germany
(Received13August2012;finalversionreceived11December2012)
Theflightsofmaleodonatesencounteringconspecificsattheirreproductionsiteswereinvestigatedby
meansofslow-motionfilms.Werecordedlargeandgenerallyconsistentdifferencesbetweenspecieswith
clearwings(SCLW)andspecieswithcolouredwings(SCOW).SCLWmostlyfoughthavingphysical
contactandmovedtheirwingswithoutpausesinwingbeats(hereafterdesignatedwingpauses),attacking
theothermales.Duringencounters,SCOWmalesshowedpausesofallwingsoroftheforeorthehind
wingsonly.ThewingbeatfrequenciesofSCOWthereforeshowedmuchgreatervariationthaninSCLW.
InZygopteraSCOW,parallelflappingofbothwingpairswasfrequent.Thetwoinvestigatedspeciesof
Calopterygidaeshowedseveralspecialflightpatternswhenencounteringothermales.MaleAnisoptera
withcolouredwingsalsousedwingpauses,andoftendisplayedtheirwingpatternsbyglidingandbanking
totheothermaleorbyflyinginanuprightposture.Thus,wefoundthatmostodonatemaleswithcoloured
wings,inthepresenceofrivals,exhibitedspecialflightstyles,implyingsignallingfunctions.Weinterpret
wingpausesasanadaptivecharacteristicthatallowsrivalmalestoevaluatethequalityoftheiropponent
byassessmentofthecolouredwings.Sexualselectionissuggestedasapossiblecausefortheevolution
oftheseflights.
Keywords: dragonfly flight; coloured wings; kinematics; wing beat frequencies; flight velocities;
signalling;sexualselection
Introduction
Untilnow,thekinematicsofodonateflighthavebeenanalysedmainlytostudyitsaerodynamics
(Usherwood&Lehmann,2008;Wakeling&Ellington,1997;Wang,2008;Wang&Russell,2007).
PatternsofwingmovementduringflightdifferingeneralbetweenthesubordersAnisopteraand
Zygopterainwaysthatareclearlycorrelatedwiththeirdifferentmorphologicalfeatures.Within
eachsuborder,however,differencesamongtheflightkinematicsofthespeciesaremoresubtle,
owingtosimilarmorphologyofbodiesandwings(Rüppell,1989).Basedonphylogeneticanalyses
thatincludedbothrecentandfossiltaxa(Bybeeetal.,2008;Davisetal.,2011),itseemsplausible
thatthethoraxandwingmorphologyofEpiophlebiidaeandLestidaeapproximatetheancestral
condition withinAnisoptera and Zygoptera, respectively. Our previous studies of Epiophlebia
superstesandLestesviridisexemplifythesegroups.Bothhaveclearwingsandshowveryuniform
flight,i.e.mostlyacounterstrokingmodeinwhichtheforeandhindwingsalwaysmoveinopposite
directions(Rüppell,1989;Rüppell&Hilfert,1993;Rüppell&Hilfert-Rüppell,2009b).
*Correspondingauthor.Email:[email protected]
©2013WorldwideDragonflyAssociation
120 D.Hilfert-RüppellandG.Rüppell
CounterstrokinghasalsobeendescribedforspecialmanoeuvressuchashoveringinAnisoptera.
Thebenefitofcounterstrokingisthatitreducesbodyoscillationsandsavesaerodynamicpower
(Wang, 2008). But inAnisoptera the main wing beat pattern is phase shifting of the fore and
hindwings,sothatonepairofwingsfollowstheotherwithadelayofabout1/4period.These
interactions between fore and hind wings increase the power output (Usherwood & Lehmann,
2008). Parallel beating, in which both pairs of wings beat nearly simultaneously in the same
direction, occurs in all Odonata for a few wing beats in special manoeuvres such as backward
or upward flight. Males of Zygoptera with coloured wings, such as Calopteryx splendens and
Megaloprepus caerulatus, fly most of the time using this conspicuous parallel wing beating
(Rüppell, 1985, 1989). The benefit is a greater net force, usable for acceleration; the cost is
greaterpowerexpenditure(Wang&Russel,2007).Assumingthattaxawithplesiomorphicflight
apparatus are also likely to exhibit plesiotypic flight kinematics, then in general the flight of
Odonatashowsadevelopmentfromauniformwingbeatpatterntoonewithmuchmorevariation
in derived Odonata. Particularly the Libellulidae or Calopterygidae (Bybee et al., 2008; Carle
etal.,2008;Dumontetal.,2005,2010)showmuchmorevariation(Rüppell,1989).Hencethe
morphologicalprerequisitesofdifferentsystematicgroupsresultinpartfromtheirphylogenetic
development but also from the structural and aerodynamic requirements of their characteristic
flight-dependentbehaviour,whichleadstodifferentflightpatterns.
However,insectwingsmayhavefunctionsinadditiontoflying:theycanbeimportantforther-
moregulation (Heinrich, 1986, 1993; Kingsolver & Koehl, 1985; Outomuro & Ocharan, 2011;
Tsubakietal.,2010)andforcommunication(Corbet,2004;Schultz&Fincke,2009).Whilein
many odonate species the bodies of males are colourful, as seen by the human eye, the wings
areinmostcasesclear.Insomefamilies,suchastheEuphaeidae,Polythoridae,Pseudolestidae,
Thaumatoneuridae,Calopterygidae,Pseudostigmatidae,afewAeshnidae,someChlorogomphi-
daeandmanyLibellulidae,themaleshaveoftenconspicuouslycolouredwings(Silsby,2001),
and the males of many of these species display their wings in courtship behaviour (reviewed
byFincke,1997).Malecalopterygiddamselfliesusuallypossesscolouredwings(Hooperetal.,
1999) that have been interpreted as a signal trait (Córdoba-Aguilar & Cordero Rivera, 2005;
Siva-Jothy, 1999). The colouration of the wings in species of Calopterygidae has been tied to
fitness-relatedcharacterssuchasfatdepositsandsocialstatus(Fitzstephens&Getty,2000;Plais-
tow&Siva-Jothy,1996;Siva-Jothy,1999,2000).Thesizeandqualityoftheseornamentslead
to reproductive advantages by increasing territorial and mating success (Contreras-Garduño et
al.,2006,2008;Grether,1996a,1996b;Hilfert-Rüppell,2004;Rantalaetal.,2000;Siva-Jothy,
1999). These investigations focused on the structural characters of the coloured wings and on
theirbehaviouralandevolutionaryconsequencesbutnotontheflightofthemalesinwhichthe
ornamentsaredisplayed.CompetingmalesofOdonataaremostoftenflyingwhentheymeetat
reproductionsites.Eachspecialflightpatternappearsasaconspicuousopticalstimulustowhich
thecontestantreactsbyitsownflightpattern(Córdoba-Aguilar&CorderoRivera,2005).Until
now these flights have only been studied in Calopterygidae and Pseudostigmatidae, which fly
relativeslowly(Pajunen,1966;Rüppell,1985;Rüppell&Fincke,1989).Thesespeciesshowa
specialflightbyflappingtheirwingsinparallel(i.e.inphase).FurthermoreC.splendensexhibits
avarietyofdifferentflightpatterns:includingelaboratemanoeuvres,highvelocityflight,wing
pauses,andglidingwithhalf-spreadwings(Rüppell&Hilfert-Rüppell,2009b).MalesofMega-
loprepus caerulatus do not exhibit as many different flight styles. Mostly they hover in front
of another male, displaying their wings beating simultaneously in parallel (Rüppell & Fincke,
1989).Thiskindofflightoccursonlyinthepresenceofanadversary,andsoveryprobablyhasa
signallingfunction(e.g.Schultz&Fincke,2009).
TheflightperformanceofmaleAnisopterawithcolouredwingsis,todate,unknownbecause
of difficulties in collecting data from their very fast and erratic flight. Hints from our own
unpublishedobservationsoflibellulidswithcolouredwings(Rhyothemisfuligonosa(G.Rüppell,
Wingcolourationandflightkinematicsinodonates 121
unpublishedobservationonRyothemisfuliginosa,Nakamura,Japan,1993)andNeurothemissp.
(G.Rüppell,unpublishedobservationonNeurothemis spec.,Kerala,India,1989))andthoseof
others(Palpopleuralucia(F.Suhling,unpublishedobservationonPalpopleuralucia,Okawango
near Rundu, Namibia, 2006)) suggest that these males also beat their wings in a specific way.
Newdigitalfilmingtechnology,whichprovidessufficientresolutionandlong-lastingrecordings,
nowmakeitpossibletocollectdatasystematically(Rüppell&Hilfert-Rüppell,2009a).Theaim
ofthisstudyistofindoutwhetherdamselfliesanddragonflieswithclearandwithcolouredwings
flydifferently.Theanalysishadtoberestrictedtocomparableflightmanoeuvressoastoexclude
motivationalandaerodynamicinfluencesasfaraspossible.Wefocusedonmale–maleencoun-
tersatthereproductionsitewhencompetingforterritoriesorfemales.Themotivationforflight
must be very similar for both opponents: eject the other male and obtain access to the limited
resource (i.e. females).Thus the flight parameters of encountering males with clear wings and
withcolouredwingswerecompared.
If coloured wings function as signals to other males, then competing males should exhibit
their wings conspicuously. Our main hypothesis was that males with coloured wings (i.e. used
in communication) move their wings to display the wing ornaments and hence fly differently
to clear-winged males. If this is a general principle it should be true in both Zygoptera and
Anisoptera. Therefore we chose the flight of males of different systematic groups from both
suborders,includingspeciespresumedtoberelativelybasal(plesiotypic)andothersmorederived
(apotypic).Ifinbothgroupswingcolourationprovedtoberelatedtosimilarchangesinflight,
this would be a strong hint of convergent evolution. Sexual selection, rather than systematic
relationshipsoraerodynamicnecessities,probablywouldberesponsibleforthat.
Materialandmethods
Male encounters at typical densities were analysed from high-speed films for 10 species in six
families (Figure 1) to compare their flight parameters (Table 1). For this comparison we chose
speciesfromdifferentsystematicpositions(accordingtoBybeeetal.,2008;Carleetal.,2008;
Dumont et al., 2010; Fleck et al., 2008; Pessacq, 2008) to control for phylogenetic effects on
flightcharacteristics.WithinZygoptera,Lestesviridis(Lestidae)representedamorebasalclade
compared with Coenagrion puella (Coenagrionidae) although both have clear wings (= SCLW
[specieswithclearwings]).Incontrast,Calopteryxsplendens,Neurobasischinensis(Caloptery-
gidae) and Megaloprepus caerulatus (Pseudostigmatidae) all have coloured wings (=SCOW
[specieswithcolouredwings])andrepresentmorederivedgroupsrelativetoLestidae.Similarly,
theclear-wingedAeshnacyanearepresentsamorebasalfamily(Aeshnidae),whereastheclear-
wingedNesciothemisfarinosarepresentsthemorederivedLibellulidae.Theotherstudyspecies,
Libellulaquadrimaculata,TrithemisarteriosaandNeurothemisfluctuans,arealsoLibellulidae,
butallhavecolouredwings.
Although we previously filmed Calopteryx splendens (1982–1986) and Lestes viridis (1985)
we re-filmed these species (2008–2012) in order to obtain enough data for comparisons. For
the cluster analysis we also looked at films of flights of C. splendens when prey-catching. For
comparison we looked at male–male encounters in own slow-motion films ofAnax imperator,
Brachythemisleucosticta,Calopteryxhaemorrhoidalis,Mecistogasterornata,Mnaisnawaiiand
Sympetrumpedemontanum.Otherslow-motionfilmsofMegaloprepuscaerulatusfrom1985and
1986werere-analysedwithrespecttoadditionalvariables.Inthisspeciesweincludedonlyflights
of large males, because the wing beat frequencies in general are related to body size (Rüppell,
1989).Maleswerealwaysfilmedundersunnyconditionswithlittleornowind,attemperatures
inhotclimatesbetween26and32◦C(France,Namibia,Panama,Thailand,)andbetween22and
28◦CinGermany.Until2004,wefilmedusinga16mmfilmslow-motioncameraLOCAM(model
122 D.Hilfert-RüppellandG.Rüppell
Figure 1. Thespeciesinvestigated:Anisoptera(top–lefttoright):Aeshnacyanea,Nesciothemisfarinosa,Libellula
quadrimaculata,Trithemisarteriosa,Neurothemisfluctuans.Zygoptera(bottom–lefttoright):Lestesviridis,Coenagrion
puella,Calopteryxsplendens,Neurobasischinensis,Megaloprepuscaerulatus.Thecoloursofwingsare:L.quadrimac-
ulata–wingbasesamberandalsolargeblackishtrianglesatthebaseofthehindwings;yellowishstripesattheleading
edges;T.arteriosa–redwingbasesandredveins;N.fluctuans–darkbrown;C.splendens–darkbluebands;N.chinensis
–metallicgreenhindwings;M.caerulatus–allwingshavedarkbluespotsdistaltoawhitebandinmales.(Figuresby
J.Craves,R.Lewington,B.A.Orr,B.Strnadova,andW.&M.Tarboton).
51,USA);thereafterweusedadigitalcamera(CasioProEXF1,Japan).Thefilmingfrequencies
were250–600f/s.Inallcasesthecamerawaspre-focusedtotheflightareaofthedragonfliesand
movedparalleltothemovementsofthedragonflies,withoutlookingsimultaneouslythroughthe
viewfinderandonlybrieflyatthedigitalcamerascreen.
TocharacterizethespeciesstudiedweusedthebodylengthmeasurementsreportedbyFincke
(1992),TarbotonandTarboton(2002),Dijkstra(2006),Suhling&Martens(2007)andGünther
(personal communication, 2012).We measured males of Neurothemis fluctuans in the field by
photographing them with a reference scale on graph paper.We measured the body lengths and
wingareasusingthesoftwareImageJ(NationalInstitutesofHealth,Bethesda,Maryland,USA).
Toanalysethe16mmfilmsweusedasingleframeprojector(NACImageTechnology,Japan,
model DF16B) and the QuickTime programme (Version 7.55.90.70) for the digital images.To
obtain the wing beat frequencies we counted the frames of the film, if possible separated into
downstroke, upstroke and wing beat pauses. To reduce the possibility of errors we sometimes
usedthemeansof2–4measures.Toobtainthewingbeatfrequencytheexposureratewasdivided
by the numbers of frames per wing beat. For the 16mm films we used time marks on the film
at an interval of 1/100s; for the digital images we checked the written exposure time using a
stopwatch. Flight velocities were measured by marking the shift of an individual’s image on a
movablefoilplacedonthecomputerscreen.Whenthecamerawasmoved,prominentstructures
inthebackgroundweredrawnonthefoilandwerecoveredbythesestructuresonthefilmwhen
measuringthedragonfly’sbodydisplacement.Wingbeatangles(i.e.theanglethroughwhichthe
Table 1. SpeciesandlocationsofOdonatainvestigated;bodylengths(inM.caerulatus,winglengths);totalnumbersofflightsandwingbeatsfilmed;whethercoloursoccurineither
wingandifsowhetheronlyinthehindwings(hw)orinbothpairsofwings(fwhw);wingbeatfrequenciesduringmale–maleencounters;anddurationofwingpausesasapercentageof
thewholeflightdurationduringencounters.
Body Number Number Wingbeat Pausesas Placeand
length Coloured of ofwing frequency %oftotal time
Taxon (mm) wings flights beats (Hz;mean±S.D) flighttime filmed
ZYGOPTERA
Lestidae
Lestesviridis 39–48 No 8 69 45±3.7 0 Germany52◦26(cid:3)
N,10◦23(cid:3)E,
1985,2012
Coenagrionidae
Coenagrionpuella 33–35 No 11 183 38±5.2 0 Germany
2009–2011
Calopterygidae
Calopteryxsplendens 45–48 fwhw 6 234 17.3±6.6 25.7 Germany
2008–2011
Neurobasischinensis 56–62 hw 7 74 20.2±6.8 32.4 Thailand8◦37(cid:3)N,
98◦14(cid:3)E,2009 W
Pseudostigmatidae in
g
Megaloprepuscaerulatus 58.6–73.4∗ fwhw 8 58 11.2±4.2 54.7∗∗ Panama9◦9(cid:3)N, c
79◦51(cid:3)E,1986 olo
u
ANISOPTERA ra
Aeshnidae tio
n
Aeshnacyanea 67–76 No 7 160 42.9±5.3 0 Germany a
n
2008–2011 d
fl
Libellulidae ig
Nesciothemisfarinosa 40–45 No 5 198 57.3±9.3 0 Namibia22◦34(cid:3)S, ht
17◦5(cid:3)E,2004 kin
Libellulaquadrimaculata 40–48 fwhw 5 93 36.4±8.6 24.3 Germany2011 em
TNreiuthreomthiesmairsteflruioctsuaans 273.14––3385.4 ffwwhhww 62 11492 4406..38±±1175..64 fw 5456..78hw32.7 TNhaamiliabniad22000049 atics
in
∗Winglength. od
∗∗Duringdescendingflightonly. on
a
te
s
1
2
3
124 D.Hilfert-RüppellandG.Rüppell
wingbeats)werecalculatedonlywhenthemovementofthedragonfliesandthecamera’sfilming
directionwereatarightanglewithrespecttoeachother.
Totestfordifferencesinthewingbeatfrequenciesamongallspeciesweusedunivariateanalyses
of variance. To compare the mean values of the wing beat frequencies between the groups we
usedtheMann–WhitneyU-test,becausemostofthedataarefarfromanormaldistribution.We
hypothesizedthatparticularlyvarianceinwingbeatfrequencybetweenthegroups(SCOWand
SCLW) may be inhomogenous.We used a Levene test to evaluate this independently for both
suborders(ZygopteraandAnisoptera).
Toidentifytaxawithsimilarflightpatternsweusedaclusteranalysis(Euclideandistances).We
chosesixcharactersforthelatter:themean,standarddeviation,andrangeofwingbeatfrequency,
theratioofsummeddurationofwingbeatpausestodurationofflight,andtherangeofvariation
ofthephaserelationshipsoftheforeandhindwingsandofthestrokeangle(bothbasedonranking
thecharacteristicsofthe10species).
Results
Wingbeatfrequencies
Thewingbeatfrequencies(numberofwingbeatspersecond=Hz)duringmale–maleencounters
differedamongspecies(ANOVA,p <0.01).Ingeneral,wingbeatfrequencieswereclearlylower
inZygopterathaninAnisoptera(Figures2and3),andthisappliedparticularlytozygopteranswith
colouredwings,withM.caerulatushavingthelowestfrequencyofall.InAnisopterathefrequency
rangesweremuchgreaterinSCOWthaninSCLW(Figures2and3),withextremevalues(small
black circles) only being present in SCOW. Comparisons of the lowest and highest wing beat
Figure 2. Wingbeatfrequencies(Hz)ofodonatemalesatencounters.Barsinclude50%ofeachdataset,horizontal
linesinthebarsshowthemedian,theverticallineswithsmallterminalcrossbarsshowtherangeexcludingoutliers,and
thesmallfilledcirclesareoutliers.Zygopterawithclearwings:Lv–Lestesviridis,Cp–Coenagrionpuella;Zygoptera
withcolouredwings(verticallyhatchedbars):Cs–Calopteryxsplendens,Nc–Neurobasischinensis,Mc–Megaloprepus
caerulatus;Anisopterawithclearwings:Ac–Aeshnacyanea,Nfa–Nesciothemisfarinosa;Anisopterawithcoloured
wings(verticallyhatchedbars):Lq–Libellulaquadrimaculata,Ta–Trithemisarteriosa,Nf–Neurothemisfluctuans.
Wingcolourationandflightkinematicsinodonates 125
Figure 3. ComparisonofthewingbeatfrequenciesbetweenZygopteraandAnisopterawithclearandcolouredwings
(graphsymbolsasinFigure2).
frequenciesshowedthatallspecieswithcolouredwings,whenencounteringothermales,hada
verywiderangewithespeciallylowvalues(Figure2).InAnisopteratherelativelysmallspecies,N.
fluctuans,withlargewingornaments,hadnohighermedianwingbeatfrequenciesthanthelarger
A.cyanea(Mann–WhitneyU,p =0.411).ThatofL.quadrimaculata,ofmediansize(Table1)
withonlysmallornaments,bycontrastdifferedfromthefrequenciesofA.cyaneasignificantly
(p =<0.01).
We then compared all SCLW with all SCOW of both suborders. The wing beat frequencies
ofthetwogroupswithineachsuborderdifferedsignificantly(U-test,bothp <0.01)(Figure3).
The homogeneity of variance of the mean wing beat frequencies also differed between SCOW
and in SCLW in both suborders (Levene test, for both comparisons, p <0.01). It was always
lower in SCOW. To discriminate the variations of the wing beat frequencies we calculated the
standarddeviationasapercentageofthemeanbeatfrequency(=coefficientofvariation,CV)
andfoundlargedifferences(Figure4).InSCOWtheCVwas3–4timeshigherthaninSCLW.
However, males of L. quadrimaculata, which possess only small coloured areas on the wings,
had less variable wing beat frequencies, as shown by the relatively low values of the standard
deviation(Table1)andtheCV(Figure4).
Kinematics
Phaserelationshipsbetweenforeandhindwings
At male–male encounters the two pairs of wings were beaten in different patterns: Zygoptera
SCLW(L.viridisandC.puella)showedcounterstrokingoronlyaslightphaseshift,theforewings
126 D.Hilfert-RüppellandG.Rüppell
Figure 4. Coefficientofvariation(=standarddeviationasapercentageofthemeanwingbeatfrequencies)ofthewing
beatfrequenciesduringmale–maleencounters.Lightbars=specieswithclearwings(SCLW),darkbars=specieswith
colouredwings(SCOW),SpeciesabbreviationsasinFigure2).
Figure 5. WingbeatpatternsofencounteringmalesofZygoptera(semi-schematic).Top:Lestesviridiscounterstroking;
middle:Coenagrionpuellacounterstrokingandsometimesphaseshifting;bottom:Calopteryxsplendensparallelstroking
withpauseswiththefoldedwingsinthebackwardposition(horizontalblackbars).Darklinesillustratethedisplacements
offorewings,lightlinesofhindwings.Linesgoingdown,downstroke;linesgoingup,upstroke.X-axis=seconds.
leadingthewingmovement.IncontrasttheSCOW(C.splendensandM.caerulatus)beattheir
foreandhindwingsnearlyparallel(i.e.inphase),thehindwingshavingadelayofonly5–10%
ofthewholewingbeatdurationduringtheupstrokeinC.splendens(Figure5).
InallAnisopterawefoundaphase-shiftedwingbeatingoftheforeandhindwings,thehind
wings leading and the forewings following by about a quarter of a whole wing beat period.
Wingcolourationandflightkinematicsinodonates 127
Figure 6. WingbeatpatternsofencounteringmalesofAnisoptera(semi-schematic).Top:Aeshnacyanea(unbroken
lines,displacementsofhindwings;brokenlines,offorewings);middleTrithemisarteriosa(thicklines,displacementsof
hindwings;thinlines,offorewings);bottom:Neurothemisfluctuans(unbrokenlines,displacementofhindwings;broken
lines,offorewings).Linesgoingdown,downstroke;linesgoingup,upstroke.X-axis=seconds.Horizontalblackbars,
pauseswithwingsspread.Notethesmallwingbeatangles(=smallamplitudes)oftheforewingsinN.fluctuans.
However, this pattern was not steady, as the example of A. cyanea shows (Figure 6). In fights
of A. cyanea and L. quadrimaculata, short counter stroking could be seen when hovering or
climbing.Oneortwoparallelwingbeatsofforeandhindwingsoccurredinfightswhenflying
fastbackwards.Neurothemisfluctuans(SCOW),however,variedthesephaserelationshipswidely
(Figure6).
Beatingpausesandwingbeatangleinflight
Pauses of both wings (i.e. interruption of wing beats) occurred frequently in male encounters
inSCOW.PauseswereleastfrequentinL.quadrimaculataandoftenmuchmorefrequentinT.
arteriosaandM.caerulatus.Thevalueforthelatterspecieswastheresultfromadescendingflight.
Inlevelorclimbingflights,nowingpauseswereobservedinM.caerulatus(sevenflights).Alsothe
wingpausesinL.quadrimaculataweremostlyexhibitedinbriefdownwardorforwardflights.The
highvaluesofwingpausesasapercentageofwholeflightduration(55.7%)inTrithemisarteriosa
werearesultofmeasurementsmadeinthemorning,whenterritorieswerefirstestablished.Later
in the day, when the territorial male circled around the intruder at high velocities (7.6; 8.9 and
3.9ms−1),wefoundlowervalues(24.3%,n=88wingbeats).InC.splendenswingbeatpauses
wereshorterwhenmalesflewfast.Thus,infastpursuingflight(max3.75ms−1)thecumulative
percentage of time during pauses was only 17% (n=27 wing beats) while at relatively slow
undulatingflight(around0.6ms−1)itwas56%(n=118wingbeats).
WingpausesofonlyonepairofwingswereseeninC.splendens,N.chinensis,T.arteriosaand
N.fluctuans.InC.splendenstheforewingsstoppedbeatingduringpassagesoflateralthreatening
flight, which often occurred during horizontal zigzag (undulating) flight, in T. arteriosa after
periodsofgliding,andinN.fluctuansduringfrontalencounters.Furthermore,themalesofthe
lastspeciesreducedthebeatinganglesoftheforewingsfromabout70◦ to90◦ to5–10◦ to40◦
128 D.Hilfert-RüppellandG.Rüppell
whenapproachingaperchedconspecificmale,whilethehindwingsbeatnormally.Duringsome
pursuing flights of N. chinensis the hind wings of both males (with a metallic shining surface)
stoppedbeating,beingheldnearlymotionless,withtheleadingmaleflyinginalaterallyundulating
path.When performing a banked turn towards the middle of the flight paths, only the lower of
thehindwingswasstroking,andonlyatverysmallangles.Thiswasalsothecasewhencircling
around each other for some seconds. Only the clear forewings were beating at the maximum
angle.
Duringfrontalencounters,malesofSCOWsometimesreducedthebeatinganglesofbothpairs
ofwingsattheendofthedownstrokeandbeatthemmostlyinthefrontalrangeofnormalwingbeat
angles.InmalesofC.splendensthemaximumwingbeatangleof90–110◦,withwing-clapping
at the end of the upstroke, was reduced to only 40–70◦ without wing-clapping. Megaloprepus
caerulatusinadescendingflightbeatitswingsatanangleof10–30◦ incontrastto100–130◦ in
allotherflights.
Groupingofspecies
Bycombiningwingbeatfrequencieswithfiveotherflightcharacters(seeMaterialandmethods)
weobtainedacleardivisionintotwoclustersrepresentingSCLWandSCOW(Figure7).Theonly
exceptionwasC.splendensflyingtowardpreyandbacktotheperch.Weincludedthosefilmsin
this study to compare “normal” flight with the flight of encountering males.These prey flights
clusteredwithintheSCLW,butthemalesofC.splendens threateningeachotheratencounters
groupedamongalltheothermalesofspecieswithwingornaments.
Figure 7. Similaritiesofflightcharacteristicsatmaleencounters.Allspecieswithcolouredwings(SCOW,vertically
hatchedbar)clustertogether(left),asdidthespecieswithclearwings(SCLW)(right).However,C.splendensflying
towardsprey(Cst)iswithintheSCLW;stillwithinitbutalittlefurtherseparatedisC.splendensflyingawayfromprey
totheperch(Csf).(Clusteranalysisexplains84%ofthevariability).