An unusual type of corneal nipple in the earwig, forficula sp., with a possible anti-reflection role
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Micron and Microscopica Ada, Vol. 23. No. 3, pp. 337—339, 1992. 0739—6260/92 $5.00 +0.00 Printed in Great Britain. 1992 Pergamon Press Ltd SHORT COMMUNICATION AN UNUSUAL TYPE OF CORNEAL NIPPLE IN THE EARWIG, FORFICULA SP., WITH A POSSIBLE ANTI-REFLECTION ROLE SUDIP DEY and BEGONIA DKHAR Electron Microscope Laboratory, Regional Sophisticated Instrumention Centre, North-Eastern Hill University, Shillong 793 003, India (Received 5 September 1991; revised 16 January 1992) Abstract—An unusual type of corneal nipple pattern, with features different from the usual lower height (Group I) and greater height (Group II and III) nipples, has been detected in the corneal surface of the earwig, Forficula sp. However, the nipples show some similarities with each of the two above types possessing certain characters. The height of the nipples, their shape and arrangement in the facet suggest that the nipples may act as an anti-reflection device, and thus increase the intensity of transmitted light, thereby aiding scotopic vision of the insect. INTRODUCTION The compound eye of insects consists of a structural and functional unit, known as an ommatidium. Each ommatidium has a cornea! lens, a crystalline cone and the rhabdom or photoreceptor. The cornea! lens is a modified cuticle which is cast off during each act of ecdysis. In addition to its normal function as a refractive structure, the lens-cuticle performs extensive and varied optical functions, such as an interference filter, selective absorption of light, enhanced reflection at selective wavelengths and as an anti-reflection device (Bernhard et a!., 1970; Miller, 1979; Miller et a!., 1966; Dey, 1988). As an adaptation to the light environment, the lens-cuticle of certain insects shows ultrastructural specialization, such as the presence of conical cuticular protuberances, termed the corneal nipple. The reduced reflection observed in the cornea of some nocturnal moths was attributed to these protuberances and it is argued that scotopic vision may be aided by the presence of these structures by increasing the intensity of transmitted light (Bernhard et a!., 1970). However, the greater height corneal nipples (more than 50 nm high) which can act as an anti-reflection device have been reported only in insects belonging to advanced orders. Studies on structural specialization in relation to photic adaptation in the lower groups of insects have not appeared in the literature. Scanning electron microscopy, due to its large depth of field and high resolving power, has been used to reveal the surface ultrastructural features of many ocular tissues and could provide much information, thereby explaining many of the less understood problems associated with vision. The present study was therefore aimed at determining whether there is a relationship between the ultrastructural feature of a single facet of the earwig (Dermaptera) and its scotopic vision. 337
Transcript of An unusual type of corneal nipple in the earwig, forficula sp., with a possible anti-reflection role
Micron and Microscopica Ada, Vol. 23. No. 3, pp. 337—339, 1992. 0739—6260/92$5.00 +0.00Printed in Great Britain. 1992PergamonPressLtd
SHORTCOMMUNICATION
AN UNUSUAL TYPE OF CORNEAL NIPPLE IN THEEARWIG, FORFICULA SP.,WITH A POSSIBLE
ANTI-REFLECTION ROLE
SUDIP DEY andBEGONIA DKHAR
ElectronMicroscopeLaboratory,RegionalSophisticatedInstrumentionCentre,North-EasternHill University,Shillong 793 003, India
(Received5 September1991; revised 16 January 1992)
Abstract—Anunusualtypeof cornealnipple pattern,with featuresdifferent from the usuallower height (GroupI) andgreaterheight (GroupII andIII) nipples,hasbeendetectedin thecornealsurfaceof theearwig,Forficula sp. However,thenipplesshowsomesimilaritieswitheachof thetwo abovetypespossessingcertaincharacters.The heightofthenipples,theirshapeandarrangementin thefacetsuggestthatthenipplesmayactasananti-reflectiondevice,andthusincreasetheintensityof transmittedlight, therebyaiding scotopicvision oftheinsect.
INTRODUCTIONThecompoundeyeof insectsconsistsof astructuralandfunctionalunit, known as
an ommatidium. Eachommatidiumhas a cornea!lens, a crystalline coneand therhabdomor photoreceptor.The cornea!lens is a modified cuticle which is castoffduring eachact of ecdysis.In additionto its normal functionas a refractivestructure,the lens-cuticleperformsextensiveandvariedopticalfunctions,suchasaninterferencefilter, selectiveabsorptionof light, enhancedreflectionat selectivewavelengthsandasan anti-reflectiondevice (Bernhardet a!., 1970;Miller, 1979;Miller et a!., 1966;Dey,1988). As an adaptationto the light environment,the lens-cuticleof certain insectsshows ultrastructural specialization,such as the presenceof conical cuticularprotuberances,termedthe cornealnipple. The reducedreflection observedin thecorneaof somenocturnalmothswasattributedtotheseprotuberancesandit is arguedthatscotopicvision maybe aidedby thepresenceof thesestructuresby increasingtheintensity of transmittedlight (Bernhardet a!., 1970). However, the greaterheightcornealnipples(morethan50 nmhigh) whichcanactasananti-reflectiondevicehavebeenreported only in insectsbelongingto advancedorders.Studieson structuralspecializationin relationto photic adaptationin the lowergroupsof insectshavenotappearedin the literature.
Scanningelectronmicroscopy,due to its largedepth of field and high resolvingpower,has beenused to reveal the surfaceultrastructuralfeaturesof many oculartissuesand could provide much information, therebyexplaining many of the lessunderstoodproblemsassociatedwith vision.
Thepresentstudywasthereforeaimedat determiningwhetherthereisa relationshipbetweenthe ultrastructuralfeatureof a singlefacetof theearwig(Dermaptera)anditsscotopicvision.
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MATERIALS AND METHODSTheeyeswerefixedin 3% glutaraldehydein 0.1 M sodiumcacodylatebufferat4~C
for 4 h. The sampleswere washedwith the buffer, dehydratedthrough increasingconcentrationsof acetone,then critical point dried in a Samdri PVT 3 (Tousimis)critical point dryerusingacetoneas intermediateandcarbondioxide as transitionalfluid. The dry sampleswere then coatedwith gold in a JFC 1100 (Jeol) ion sputtercoater and were observedin a JSM 35CF (Jeol) scanningelectron microscopeoperatingat an acceleratingvoltageof 15 kY.
RESULTS AND DISCUSSIONThescanningelectronmicrographof the cornealsurfaceof Forficula sp.,shown in
Fig. la, revealedcertaininterestingfeatures.Comparisonof the surfacepatternof asinglefacetwith that of the housefly,Muscadomestica(Fig. ib), andnocturnalmoth,Attacusatlas (Fig. lc), showedthat the cornealprotuberancesof Forficula sp. havea
(ci) —, .~.
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(b) (c)
Fig. I. Scanningelectronmicrographof a ~ingIefacet showingthecornealntpple patternofForficula sp., as comparedto that of Musa domesticaand Attacus atlas. (a) Forficula sp.
(bar= I Mm); (b) Musca domestica(bar— I Mm): (c) Attacusat!a.s (bar=0.1 Mm).
CornealNipple in the Earwig 339
numberof similaritiesas well asdifferencesfrom thoseof thenocturnalmothandthehousefly. As farasthe height (distancebetweenbaseandtip) is concerned,the nipplesat mostof thefacetpartsaremoreor lesssimilarto thoseof thenocturnalmoth,buttheperiodicity (centreto centredistance)is not constant.Further,in somepartsof thefacet,thenipplesareflat andaresimilarto thoseof thehousefly;whereasin someparts,thenipplesappeartoassumeaconicalshapewith considerableheightwith referencetothe cuticularsurface(70—140nm) andare moreor lesssimilar to thatof the nocturnalmoth (180 nm).
Thecornealnipplesare classifiedon the basisof their height asGroupI (50 nmorless), GroupII (between50 nm and 200 nm) and GroupIII (200nm or greater)protuberances(Bernhardeta!., 1965).AlthoughGroupI protuberancesweredetectedin thefacetsurfaceof manyinsectspeciesbelongingto differentorders,theirfunctioniscompletelyunknown.GroupII andIII nipples,on the otherhand,play adefinite anti-reflection role due to their conical shape,regularperiodicity and height. However,reportson theoccurrenceof thesegreaterheightnipplesin insectsotherthannocturnalLepidopterais very rare (Dey, 1988).Thepioneeringwork of Bernhardet a!. (1970)showedthatnippleswith greaterheighttend to occurin insectsof anadvancedorder,andlowerheight nipplesaremostlyfoundin insectsrepresentinglessadvancedordersindicatingthe absenceof any anti-reflectiondeviceassociatedwith them.Ourpresentobservationson the cornealsurfaceof theearwig,however,showan exception.In themajorportionof thefacet,it canbeconsideredthat thecorneawith a refractiveindexn = 1.5 is coveredwith athin film (cornealnipple)of refractiveindexn2 andthicknessd.The effectiveindex of refractionof the nipple increasesgraduallyfrom the refractiveindex of air at the tip to that of the corneaat thebase.It is well known that if n2 is agradual transition betweenn1 and n3 with a thicknessd=A0/2, a broad bandanti-reflectioneffect is achieved(Miller, 1979).Sincein the presentcase,mostof thenipples fulfil this physical condition, the same anti-reflection effect is expected.However, it may not be as efficient as in nocturnalmoths,as it appearsthat theperiodicityof thenipplesis notregular.Nevertheless,thenipplesare likely to help thescotopicvisionof the insectby increasingtheintensityof transmittedlight. Thepresentstudyfurther suggeststhat anti-reflectiondevicesin the facetsurfaceneednot be acharacteristicfeatureofhighergroupsalone,andmaybeexpectedin lowergroups,alsoasan adaptationto scotopicvision.
Acknowledgements—Theauthorsare thankful to Prof. D. T. Khathing,Head, R.S.I.C. for constantencouragement,to Mr. A. Shadapfor photography,alsoto Mrs. RajammaJohnsfor typingthemanuscript.
REFERENCESBernhard,C.G.,Miller, W. H.andMøller, A. R., 1965. Theinsectcornealnipplearray.ActaPhysiol.Scand.,
63, Suppl.243: 1—79.Bernhard,C. G., Gemne, G. and Salström, J., 1970. Comparativeultrastructureof corneal surface
topographyin insectswith aspectson phylogenesisandfunction.Z. Vergi. Physiol.,67: 1—25.Dey,5. 1988.Scanningelectronmicroscopicdetectionofcornealanti-reflectioncoatingin thegrasshopper,
Epacromiadorsalis andits physiologicalsignificance. Vision Res.,28: 975—977.Miller, W. H., Møllar, A. R. andBernhard,C. G., 1966. In:FunctionalOrganizationoftheCompoundEye,
Bernhard,C. G. (ed.),pp. 21—23. PergamonPress,Oxford.Miller, W. H., 1979. Ocularoptical filtering. In: Handbookof SensoryPhysiology,Autrum, H. (ed.), Vol.
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