Cuticular structures on the labium of the larva of Crocothemis servilia (Drury) (Anisoptera: Libellulidae)

5 Pages·1992·0.84 MB·English

by  Gupta, A

Odonalologica21(1): 97-101 March I. 1992
Cuticular structures on the labiumof the larva
ofCrocothemis servilia (Drury)
(Anisoptera: Libellulidae)
A. Gupta¹,S.Dey²,S.Gupta²
1 DepartmentofZoology, St.Edmund’s College,Shillong-793003,India
2 Electron MicroscopeLaboratory, RegionalSophisticatedInstrumentationCentre,
North-Eastern HillUniversity, BijniComplex,Shillong-793003, India
Received September24, 1990/ Revised andAcceptedNovember 12, 1991
The various types ofsensilla occurringon the labium aredescribed with the help
ofscanningelectron microscopy. In addition tobristles andspines, various sensilla
trichoidea are located on the ligulaand the palps. Thepalps also contain sensilla
basiconica, arow ofpapillaeand simplepits. Thepossiblefunctional roles playedby
these types ofsensillum in the life ofC.serviliaare discussed.
INTRODUCTION
Scanning electronmicroscopyofthecuticularstructures foundontheappenda-
ges and body surface of aquatic insects has extended our knowledge ofthe
structure ofanumberoftypesofsensillum(GOUTEAUX, 1977;SLIFER, 1977;
DAHL, 1978; LEE & CRAIG, 1983; KAPOOR, 1985a, 1985b; WOLFE &
ZIMMERMAN,1984).Thepresentstudy describestheexternal sensory receptors
on the labiumofthelarval Crocothemisservilia. Thelabiumis consideredto be
a very important structure inodonate larvae. Itis foldedupon itselfandtucked
beneath the head ventrally. Itis used to capture prey organisms by thrusting it
forward. Therefore, ascanning electron microscopic examinationofthevarious
types ofsensillumpresenton it is likely to provide abasis forunderstanding the
relationship betweenthe morphology ofthe larvaeand their significant role as
predators. Furthermore, as the bristles and other structures on the labium are
extensively usedin differentiatinggeneraandspecies inthelarvalstage,adetailed
knowledge of labialstructures is likely to be important frojn ataxonomic point
of view.
98 A.Gupta, S, Dey &S. Gupta
MATERIALANDMETHODS
Crocothemis servilia inhabits the pondsof Shillongand itsneighbouringareas (lat.25° 34’N;
long.90°52’E)inthenorth-eastern regionofIndia.Larvaewerecollected fromWardlake,aperennial
lentic waterbodyinShillong,fromthemarginalweedbeds.Thelabiumwasfixedin 2.5%glutaralde-
hyde in 0.1 Msodium cacodylatebuffer for 2-4 hours. This was followed by washing in buffer,
post-fixationin 1%osmium tetroxide, dehydrationin gradedconcentrations ofacetoneand drying
(WOOLLEY& VOSSBRINCK, 1977).Itwassecured tobrass stubsbyanelectro-conductive paint,
’Dotite’. This was followed by coatingwith athin layerof goldina Fine Coat Ion SputterJFC
1100. They wereexaminedin aJSM-35 scanningelectron microscopeoperatedat 15 KV.
RESULTS
InOdonata,the labiumis foldeduponitselfataboutits midlength andturned
backward. In C.servilia, thelabiumhas well-developed premental setae,which
are bristlesor sensilla chaetica. Thefront marginoftheprementum is produced
intoatriangular ligula. Thehinged palps arearmed withtwo curvedhooks.The
palpal margin is beset with bristles and other sensilla (Fig. 1). The premental
setae are arranged ina group,somewhat like abristle-field, each setaemerging
from aroughly conical socket. The basalregion ofboth the ligula andthepalps
bear sensilla trichoidea, designated here as type 1, 28-57 pm long, each with a
bulb-shaped socket and arelatively stout basal region tapering sharply towards
the apex (Fig. 2). Similartrichoid sensilla, 20-33 pm long are also strewn on
the central and apical regions ofthe ligula. Interspersed among them are a few
sensilla trichoideaof type la, around 80 pm long. These have similar sockets
to thoseoftype 1,butare more slenderand whip like(Fig.3). Sensillatrichoidea
designated type 2 are arranged atregular intervals along thefrontmargin ofthe
ligula, forming a definitepattern. They are 33-60pm long and emerge from
circular punctures fringed with collar-like structures. Their tips are relatively
blunt (Fig. 3). The proximal outer margin of the palps is beset with a row of
long (about 580 pm) bristles with heavily sclerotizedbases. A few shorter but
sharp-tipped spines canalsobe seen (Fig. 4). Thefront andinnermargins ofthe
palps containarow ofsensilla trichoideatype2a, having similarsockets tothose
of type 2 found on the ligular margin, but with relatively pointed tips. Their
length gradually diminishesfromabout 100pm inthecentre toabout30 pmon
thesides. Beyond them, almost along theedge, is alinearrow ofpapillae (Fig.
5). At higher magnification, each type2a trichoidsensillum can beobserved to
be associated with a pit about6 pm in diameter. Each papilla has three lips,
diminishing in size from the innerto theouter. On one side ofeach papilla is a
short (3-5 pm) basiconic peg and, onthe other side, a still shorter (1-1.5 pm)
basiconic peg;these are designated as types 1 and la respectively (Fig. 6). Type
1 basiconic pegsarealso strewn overthesurfaceofthe palps withtype 1 sensilla
trichoidea(10-23 pm long) interspersed amongthem(Fig. 7).
Larval labium ofCrocothemisservilia (Dru.) 99
Figs 1-7.Crocothemis servilia,cuticular structuresonlarval labium: (1)Labium,generalview;(2)
Prementalsetae, magnified;(3)Ligulashowingsensilla; (4)Bristles and spinesonthe outer margin
ofthepalp;(5)Sensilla onthe frontmarginofthepalp;(6) ditto,magnified;(7)Surfaceofthepalm
withsensilla basiconica and sensilla trichoidea. [Ig: ligula;pm: prementum; pms: prementalsetae;
pi:palp; br: bristles;s:spines; p: papillae;l.p: lipsofpapillae;t 1: sensilla trichoidae, type 1;t la:
ditto type la; t 2:ditto, type2; t 2a: ditto, type2a;bl: sensilla basiconica,type 1;bla: ditto, type
la.Allthe barsare 100pm except those inFigs. 1and6, whichare 1000pm and 10pmresp.]
DISCUSSION
Scanning electron microscopy, because ofits large depth offield and high
resolvingpowerbecomes indispensablefordetailedmorphologyofinsectsensilla
(SCHMIDT& SMITH, 1987).
Basedon theheterogeneity, complexity andabundanceofsensilla, the labium
appears to be a very important sensory region ofthe head. This is in keeping
100 A.Gupta, S.Dey & S.Gupta
with itsfunctionsofcapturingandsampling theprey.Thebasiconicpegsassocia-
ted withthe lineararrayof papillae on the margins of the palps are likely to be
chemoreceptors, as concludedby AMRINE & LEWIS (1978, 1986) for similar
sensory structures foundin the fleaCediopsylla simplex. Theligular andpalpal
surfaces are clothedin sensilla trichoideaofafew types. These are presumably
all mechanoreceptors. However, the longer, more slender ones (type la) could
also possibly detect the presence ofanother object withoutcoming intoactual
contact with it. This would be possible due to a force on the insect caused by
the increased drag as it passes an object, or vice verse. Thus any object within
10to 100 diametersofthe insect would be readily detected due to a wave of
resistancewhichwouldbereflectedbackagainsttheinsect(WOLFE& ZIMMER-
MAN, 1984). Thereforeit appears that C. servilia, which is known to be an
efficient predator ofother insects and fish spawn, couldbank upon its array of
sensilla tolocate andambushitsprey.Thepremental setae are grouped together
intheformofabristlefield, andprobably functionas proprioceptors in addition
toproviding usefultactileinformationtothenymphs (AMRINE&LEWIS, 1978,
1986; SCHMIDT & SMITH, 1987).Thus they are likely to be instrumental in
providing feed-backinformationaboutthestretchingofthe palps duringpreycap-
ture.
The functional significance ofthe numerous pores on the palps cannot be
ascertainedat present. AMRINE &LEWIS (1978, 1986) suggested that similar
pores found in the flea Cediopsylla simplex are openings to epidermal glands
which secrete anoily substance. However, theirrole inaquatic larval stages can
only be determinedthrough studiesonfinestructure, innervationandphysiology.
Finally, the occurrence, density and arrangement of the labial sensilla are
expected to serve as usefulmorphotaxonomic charactersinspecies differentiation.
ACKNOWLEDGEMENT
We thank Dr D.T. KATHING, Head, RegionalSophisticatedInstrumentationCentre, Shillong,
India,forhis help and encouragement.
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