ECOLOGICAL AND SENSORY ASPECTS OF PREY CAPTURE BY THE WHIRLIGIG BEETLE DINEUTES DISCOLOR
DepartmentofZoology, University ofWisconsin, Madison, Wisconsin
—Whirligigbeetlespreyon a varietyof small soft-bodied invertebrates thatenter theirsurface film habitatfrom above orbelow.Theyalso act as scavengers on deadfloating invertebrates. Deficits in prey capture abilitywere produced byexperimental deprivation of visual,tactile,orsurfacevibrationcues.Visualdeprivationanddeprivationofsurfacevibration cuesproducedstatisticallyequivalentdeficitsinwhirligigpredatory behavior.
Whentactileand surface vibration cues were both unavailable to the beetles, the deficits in their predatory behaviorwere more pronounced. These sensory systemsaresimilar to thoseofHemipteran predatorsthatliveinthe surfacefilm.
The waya predator locates its prey is determined by the predator’s en- vironment, its
andthenature ofits prey.
Forwhirligig beetles,allthreeofthesefactorsareunusual. Thesebeetleslive
onthe surface film ofbodies offresh water,
andgyrinid sensory structures are highly specialized forthis environment. Their separate pairs of
above and belowthe water (Fig. 1) possess different spectral sensitivities suited to theirrespectivesurroundings (Bennett, 1967;
and Goodman,1964; Pappas, 1974).
TheJohnston’s organsoftheir antennae, runningbetween pedicel
andflagellum (Fig. 2), are modifiedinto extremely sensitive surface vibration detectors (Eggers, 1926; Wilde, 1941)
whichcan sense vibrations with an amplitude as small as a few microns (Rudolph, 1967).Gyrinidspotentiallyhaveavailableto
thempreythatenter the surface film
fromunderwater or that live in the surface film itself.
examinedthe predatory activities ofthe river dwelling species Dineutesdiscolor,workingin boththe field
andthe laboratoryto determine the natural diet ofthe beetles
and whatsensory systems were involved in theirprey capture behavior. I investigated forthe use ofvisual,tactile,
surface vibration cues in gyrinid predation, all three of
whichare used by surface film dwelling water striders or
backswimmers(Murphey, 1971a, 1971b, 1973;
Murphey andMendenhall, 1973). Prey animals in thesurface
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Fig. 1. Frontandsideviews ofthehead regionof Dineutesdiscolor, showingtheseparate dorsalandventral pairsofcompoundeyes.
whichconstitutean unusualstimulus availableto
anypredator in contactwith the water’s surface.
MATERIALS AND METHODS
ontheWisconsin River near
Arena(Iowa Co.). Field observations were carried out with binoculars; for laboratory studybeetles were
removed fromthe river
for identification during the
and1977, all but
twowere Dineutesdiscolor.Thisgroup compositiondiffers
found in a lake habitat
Beetlesobserved feeding inthe laboratory belonged to
which underwent nomanipulation oftheir sensory structures.
wastemporarily visually de- prived,
Westinghouse DXC 500-W Pho-toflood
froma distance of30
wascarriedoutin aspecial reflector-lined
aquariumfreshly filled with cool water, so that the beetles were protected
fromelevated temperatures (they always
wasobserved feeding under a
dimred light for 9 min.
The combinationofbright-light adaptation, a short observation period,
andthe insensitivity ofthe gyrinid eye to red light (Bennett, 1967; Carthy
producedthe visual deprivation.
wasdeprived ofsurface vibration cues
byhavingtheir antennal flagellae
removed. TheJohnston’s organ stretched between pedicel
andflagellum (Fig. 2)
twoparts todetect surface vibrations(Eggers, 1926;Wilde, 1941).
wasdeprived ofsensory cues
Fig. 2. The antennaof Dineutesdiscolor. Labelledparts are flagellum(F), pedicel (P)and socketedflotation hairsofthe pedicel(FH).
andflagellae. Inspection with a scanning electron microscope re- vealed that the
havethe typical individually socketed structure of mechanoreceptors,
andso beetles with both pedicels
removedwere deprived oftactile cues
fromthesehairs as well as surfacevibration cues. (Unfortunatelyit
wasimpossibleto eliminate the hairs
andleave the surfacevibration detectorsintact,asthese hairs supply the
buoyancythat keeps the antennae afloat,
and removing themcausesunpredictable changes oftheheightat
whichtheantennal ped- icelsare floating.)
Beetles were observed in the laboratory one at a time, feeding
onlive flightless Drosophila in a 122 x 122 x 30
cmtank that rested
oninflated inner tubes to insulate it
fromextraneous environmental vibrations.
Ap-proximately 150prey captureswere recordedforeach ofthefour treatment groups,withtentotwentybeetlescomprising eachgroup.Threeor fourdays offooddeprivation beforeobservation periods
wasadequateto ensurehun- grybut otherwise healthy beetles.
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Table 1. Objectsclose circledby D. discolorundernatural conditions.
Objectsclose-circled Consumed Notconsumed
Live invertebrateslessthan 3
Live mosquitolarvae 3 88
mmlong 1 1
Deadunidentifiedinvertebrates 5 12
Unidentifiedobjects 46 1,103
andfield, D. discolorperformsa stereotypic predatory sequence I call a close circle.
Thebeetle approaches with its
headdirected toward its prey.
headvery close to the prey the beetle circles
Theraptorial forelegs ofthe whirligig, normally held in grooves underits body, reach out towardthe prey duringthis circle. In the
field, one, occasionally two, or rarely more, close circles were
performedabout prey animals before they were captured
and consumed. Forthe lab- oratory dataI
numberofclosecirclesrequiredforpreycapture as a
howthe various sensory deprivations affected predatory
Atotal of 1,322 close circles ofobjects in the surface film
myfieldobservations (Table 1). Identificationofobjects beingclose circled
dueto theirsmallsize,asthecategories in the table attest. Prey were only scored as “live” if I
saw them move
independently before beingclose circled
All ofD. discolor'sprey were soft-bodiedinvertebrates.
Theyentered the surface film both
from above(gnats, collembolans, flies, etc.)
and below(mosquito larvae).
Althoughsmall pieces ofvegetationwere closecircled, I
never observed the beetles eating
anyplant material. Beetles occasionally
dove belowthe surface, but even
whenthey were surrounded
would haveattacked atthe surface film (mosquitolarvae), Inever
submergedgyrinid attack a prey animal.
In the laboratory, sensory deprivation in visual, surface vibration,
tactile stimuli all resulted in
moreclosecircles being requiredforpreycap- ture. Figure 3
showsthese data displayed as the cumulative probability of prey capture vs. the
numberofclose circles performed,
LU CL CL
3 3 O
Fig.3. Thecumulativeprobabilitiesof prey capturebyfourexperimental groups of Dineutes discolorfedlive flightlessDrosophilainthelab.Thetotalnumberof prey capturedis150 ± 5 foreach ofthegroups ofbeetles.
amenableto analysisusing the
Kolmogorov-Smirnovtest (Siegel, 1956).
Eachofthe four groups ofbeetles includes data for 150
±5 prey captures.
Bothvisually deprived beetles
andthose deprived oftheir flagellae re-
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normalbeetles to captureprey, with signih- cancelevels of 0.025
Thevisually deprived beetles
andthose deprived ofsurfacevibration cues
byflagellarablation were sta- tisticallyindistinguishable
numberofclose circles perpreycapture,
twolines in Figure 3.
Beetlesdeprived of bothsurfacevibration cues
byhaving their antennae
even moreclose circles to capture prey, differing
fromallthree previousgroups at the 0.001 significance level. Pre-
sumablythisdeficit reflectsanadditionalsensory deprivation
duetotheloss ofmechanoreceptive hairs
Whirligig beetles capture
and consumeprey that enters the surface film
from below andabove, therebytakingadvantage ofaquaticinsectsthatneed toreach the surface forair, as wellas flying orterrestrialinvertebrates that
blown downontothewaterorlandedinorderto lay eggs.
ability to clearlydistinguish small invertebrates
severalmetersdistanceundoubtedly skews Table 1 towardslargerpreyitems
morethan isthe actual case. Nonetheless, D. discolor clearly acts as botha predator
and seemsto feed
Virtually all of the live prey of D. discolor listed in Table I are very ephemeral objects
froma whirligig’s viewpoint.
When approached bya predatorinthe surface film,
jump,gnats take off
fromthe water’s surface, etc. Ifa gyrinid is to successfully capture
anyprey, its predatory sequence
mustbe as rapid as possible.
numberofclose circles a gyrinid performs before prey capture, the
doD. discolor live in
movingwater, but they also feed in aggregations
deadinvertebrate not captured immediately is apt to be swept
backswimmers andwater striders are capable ofutilizinga varietyofcuesinlocating
andcapturingprey, including visual, tactile
andsurface vibration cues
(Murphey,1971a, 1971b, 1973;
Murphey andMendenhall, 1973). Potential prey insects inthe surface film generate distinctive vibration spectra (Lang, 1980)
andthe processes of surface vibration orientation are beginning to be unravelled (Lang, 1980;
and Uhlemann,1973; Weise, 1974).
andsurface vibration cuesinprey capture, as indicated
numberofclose circles required in
moregeneral sensebytheirlaboratory treatments, butI
Normalbeetles inFigure 3
and twoclose circles, respectively, that correspond to 77
and129 out of 155 totalprey captured.
The normallaboratoryanimalswerethereforecapturing preywithoneor
Thevisually deprivedbeetleswere exposedonlyto brightlight
and myapparatusensured thattheywerenot subjectedtoelevatedtemperatures,
showdeficits in capturebehavior statisticallyindistinguishable
fromthose ofthe beetles with their antennal flagellae removed. Experimental beetles
whowere op- erated
onwere as vigorous as
anddid not display an in- creased mortality. Ideally I
shamoperations, but there is
whenthe procedure involves an ablation ratherthan an incision.
Congenericwhirligigssuchas Dineuteshorniiare largelyquiescentduring theday
andVogt, 1980).Thisnocturnal feeding
Whetherwhirligigs forage at night or during the day appears to
whichthey live; those that live in a current
mustclimb out ofthe water onto emergent vegetation at night in order to avoid being swept
downstreamwhile unable to use visual cues to maintaintheirpositionrelative to theshore
(Brown andHatch, 1929; Folk- erts
The wavepatterns produced
havebeen well de- scribed by
andthe possibility ofthese
wavesreflecting off prey in asurface film borne echolocation system has
seemeda natural pos-
However,the wavelengths of the
gyrinids are very large
comparedto the portions ofthe natural prey items incontact withthe surface film(siphonsfor
mosquitolarvae, legsfor gnats, etc.)
wavereflection for echolocation
seemsunlikely. It is
bygyrinidsinvolveslocating larger objects in the surface film, like rocks, conspecifics, etc.
amgrateful toProfessorsJeffreyR.Baylis,JackP. Hailman,andStanleyCarlsonwhohave supplied
mewith assistance and advice. I
amalso indebted to Clyde S. Gorsuch, Scott R.
Robinson and Katherine C. Noonan for reading and commenting on various drafts ofthis paper.CheryleHughesproducedthe excellentillustrations.
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NEW YORK ENTOMOLOGICAL SOCIETY
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