Table Of ContentOdonatologica31(2):171-175 JuneI,2002
SHORT COMMUNICATIONS
Egg chorionic ultrastructure
of thedragonflyTrameavirginia (Rambur)
(Anisoptera: Libellulidae)
R.J.Andrew
Departmentof Zoology,Shri ShivajiESA’s ScienceCollege,CongressNagar,
Nagpur-440012(MS),India
ReceivedMay28,2000/RevisedandAcceptedSeptember12,2001
SEM studies reveal thatthe egg chorion ofT. virginiais divided into anouter soft
exochorion andaninner toughendochorion. The exochorionexpands intoajelly-like,
stickycoatin water,whiletheendochorionissmooth,thinandunsculptured.Theapically
situatedmicropylarapparatusisformedofalarge,dome-shaped,sperm-storagechamber
andasmall, flat,micropylarstalk which containsapairofcircularmicropylarorifices.
Themicropylarapparatusisencircledbyanexochorionic collar. Thechorionismodified
inaccordance withtheaquatic(still-water)modeofoviposition exhibitedby thisspecies
whilethe micropylarapparatus isshapedtofitin thefertilization pore ofthe vagina.
INTRODUCTION
Theeggchorionininsects is notasimpleenveloping layeroftheeggbutexhibits
various regional and radial complexities. Scanning electron microscopic (SEM)
observations havefurtherdisclosed that the chorion, notonly protects the growing
embryobutalsoundergoes variousmorphologicalmodificationstoassistinfertilization
andoviposition (HINTON, 1981;MARGARIT1S, 1985).TheeggsofOdonatahave
onlybeenrecently examinedultrastructurally (IVEYet.al„ 1988;TRUEMAN,1991;
ANDREW& TEMBHARE, 1992, 1995,1996;MAY, 1995;SAHLEN, 1995)andis
theleastunderstoodstageofthis group(NORLING&SAHLEN, 1997).Thepresent
investigation was undertaken to study the chorionic ultrastructure in relationto
fertilizationandovipositioninTrameavirginia.
MATERIALAND METHODS
Tramea Virginiaisalargelibellulid dragonfly,foundsexually activenearsmallwatertanks infields
172 R.J. Andrew
and gardensinIndiaduringthepost-monsoon period. Ovipositingfemaleswerecollectedneargarden
tanksatBrahmapuriandNagpur(MaharashtraState,India).Eggdumpingwasinitiatedbypassingathin
needlethroughthe thorax (ANDREW& TEMBHARE, 1995).The live fertilized, unwettedeggs were
collectedonacleanclothandthe eggswereimmediatelypickedup withawetneedleatthebasalregion
andfixedin Benin’s fluid. Inpreparationfor examination underthe stereoscan 250 MKIIICambridge
ScanningElectronmicroscope(SEM),theeggsweredehydratedinalcohol,clearedinacetone, air-dried,
mountedonstubsand goldcoated inaPoloronAutomatic unit(E5200).Some completelywettedeggs
werealsoprocessedand observed undertheSEM.All measurementsgivenbelowaremeansofnotless
thantenreadingsalongwith ± standard error. The female reproductiveorgans consistofapairoflong
ovaries anda compactpost-ovarian genital(spermatheca-bursa-vagina)complex (ANDREW&
TEMBHARE, 1994, 1997).The female post-ovariangenitalcomplexwasprocessed and observed as
described earlier(ANDREW& TEMBHARE. 1994).TheterminologyofTRUEMAN(1991)is usedto
describethevarious regionsoftheegg.
RESULTSAND DISCUSSION
Trameavirginiaovipositsinsmall,still-waterbodieslikecementtanksandornamental
garden ponds. Hieunwettedeggs are lightyellow,ovalandhavelineardimensionof
340±15x250+12pm.Theovalshapefacilitatesquick sinking oftheegg.Further, the
ovaleggs caneasily settleinsmallrecesses onthefloorofthewaterbody (CORBET,
1962). Theegg chorion is dividedinto an outer softexochorion (EX) and an inner
toughendochorion(EN)(Figs 1-5).TheEXinunwettedconditionis thin,soft,smooth,
non-porousandunsculptured. As soonasitcomes incontact withwater,itswellsand
expands into ajelly-like, spongy coat which remainssoft throughout theincubation
period. Thelineardimensionoftheegg atthisstage is 450±20 x260±15 pm.This
change provides asticky surface towhichparticlesadhere thereby camouflaging the
egg;alsothe eggs become firmlyglued to variousobjects on the floorofthewater
body.
TheEN is thin (1.5±0.6pm),smoothand does not undergo any post-oviposition
changesinwater. InBradinopyga geminataandLandonadeplanta, theENiscomposed
oflarge flatplates (IVEYet at., 1988;ANDREW &TEMBHARE, 1996),butsuch
platesare notfoundin T. virginia.
Theapical tipof thechorionismodifiedintoacomplex micropylarapparatus(MA)
(Figs 2, 3). TheMAis lodgedinacircularcollarwhichis formedby afoldoftheEX.
Thecollarhasadiameterof 75±10pmandaheightof 10±1.5pm.Itprobably protects
the MAfrom mechanicalinjury. The MA consistsofalarge dome-shaped, sperm-
storagechamber,40pmindiameter.Asmall, plate-like, rectangular micropylarstalk
arisesfromthecenter ofthestorage chamber.Thestalkis 18pmlong and 6pmwide.
Apairof micropylar orificesarepresentsub-apically ontheflat surfaceofthe stalk.
Each orificehas a diameterof 4-5 pm. Inlibelluliddragonflies, the MA generally
consists ofa convex/concave or hyperboloid stalk and asmall storage chamber
(TRUEMAN, 1991; MAY, 1995).Thedome-shaped chamberand smallflatstalk has
so farbeenrecordedonly from T. Virginia.
Thefertilizationpore(FP) inthevagina of T. Virginia is narrow, spiny, tunnel-like
Eggultrastructure inTrameaVirginia 173
andformedfromamid-dorsaland pairedlateral cuticularplate(Fig. 6). TheFP lies
justabovethefemalegonopore(ANDREW& TEMBHARE, 1994).Theeggs inthe
medianoviduct ofT. virginia are not randomly orientiedas foundin Orthetrum
chrysostigma (MILLER, 1984),but theMA is always directedupwards as the eggs
Figs 1-5.SEMmicrographsofTramea virginia egg: (1)partlywetted egg, showingexpanded sticky
exochorion atthe wetted basal region(arrows)and unwetted smoothregion atthe anteriorend; — (2)
anteriorregionoftheegg,showingmicropylarapparatus encircled by thecollar(arrows); — (3)lateral
viewofthe above, showingthe micropylarorifice (arrow)ontheprojecting micropylarstalk; — (4)
fracturedsectionoftheunwettedegg, showingthin,softexochorion; — (5)fractured sectionofthewetted
egg,showingthin,toughendochorion; —(6)longitudinalsectionofthe vagina,showingthenarrowspiny
fertilizationpore(arrow) [x90].
174 R.J. Andrew
movedowntheoviductintothevaginathroughthefemalegonopore.TheMAtherefore,
comesto liejustbelowthe FP.
InT. virginiathestructure oftheMAoftheeggiscomplementary totheshape ofthe
FP. During fertilization, the stalkoftheMA enters theFP. Spermtrickles down the
spiny FP dueto muscularcontractionof thebursa (SIVA-JOTHY, 1987;MILLER,
1991)andenters theMAthrough theorificesofthestalk. Thestructure oftheMAof
T. Virginiasupportsthetwo-stepfertilizationmechanismasproposed by TRUEMAN
(1991). He postulated that “[the] sperms [is] mechanically scooped into theatrium
duringoviposition but only later penetratfes] the endochorion”. This aids rapid
fertilization,whichisnecessary since thevaginais toosmalltoholdmorethantwoor
threeeggs atatimeand T. Virginia exhibitsa highrate ofoviposition (8-12egg/sec.).
ACKNOWLEDGMENTS
Iamdeeplyindebted toDrD.B.TEMBHAREforconstantencouragement. ThisworkisapartofUGC
minorresearchproject(23-382/99WRO).The helprendered byS.V.RAO(RSIC, NagpurUniversity)
andBABASARODEYinSEM studiesandphotomicrographyisgratefullyacknowledged.
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