COMPILED AND CIRCULATED BY DR. POULAMI ......2020/04/01 · Arachnida and possesses only simple eyes. It has a pair of large median indirect eyes and three pairs of lateral direct

VISION IN INSECTS

COMPILED AND CIRCULATED BY DR. POULAMI ADHIKARY MUKHERJEE, ASSISTANT PROFESSOR, DEPARTMENT OF ZOOLOGY, NARAJOLE RAJ COLLEGE

BY

DR. POULAMI ADHIKARY MUKHERJEEASSISTANT PROFESSOR

DEPARTMENT OF ZOOLOGY NARAJOLE RAJ COLLEGE

ZOOLOGY: SEM- II, PAPER- C3T: NON-CHORDATES II, UNIT 3: ARTHROPODA



VISION IN ARTHROPODA

➢ Arthropods possess simple as wellas compound eyes; the latter evolvedin Arthropods and are found in noother group of animals.

➢ Insects that possess both types ofeyes are considered to be the mostsuccessful animals on earth.



VISION IN CRUSTACEA (PRAWN) – THE COMPOUND EYE

➢ Crustacea includes prawns, crabs, lobsters,shrimps, barnacles, water fleas etc., whichpossess a pair of compound eyes for vision.

➢ Prawn possesses a pair of large stalkedhemispherical eyes on the anterior side ofcephalothorax below the rostrum.

➢ Each eye is composed of a large number ofindependent visual unitscalled ommatidia which are connected to theoptic nerve.



➢ An ommatidium is divisible into theouter dioptrical region for receiving andfocusing light rays and the inner sensoryregion for perceiving light and sending thenerve impulse to the brain, which analyses theimpulses as image of the object.

➢ The cuticle on the surface is modified ascornea over the ommatidia and gives the eyenecessary protection and also allows the lightrays to enter the eye.

➢ Below the cornea, a pair of corneagencells secretes fresh cornea in case of wear andtear.



➢ A lens-like crystalline cone is locatedbeneath the corneagen cells and serves tofocus light rays inwards.

➢ The crystalline cone is surrounded byfour cone cells or Vitrellae that serve toprovide nourishment to the cone.

➢ Next layer is of sensory cellscalled Rhabdomes which are elongatedand transversely striated and are sensoryin function.



➢ Seven retinalcells that surround therhabdome and encircle it provide itnutrition and protection.

➢ Chromatophores are pigment cells whichare responsible for separating oneommatidium from the other so that theyremain as independent units.

➢ They are located around the cone cells andretinal cells and can shrink or expand toincrease or decrease the intensity of lightentering the eye.







THE MOSAIC VISION➢ The compound eye is incapable of

giving distant vision and sharpvision but is efficient in picking upmotion and in providing 360° view,as it is large globular and mountedon a movable stalk.

➢ Each ommatidium is capable ofproducing an independent image ofa small part of the object seen andnot the entire object.



➢ All these small images are combined in thebrain to form a complete image of the objectthat is made of small dots or mosaic of dotsand hence it is called mosaic vision.

➢ The range of the compound eye is not morethan a foot and hence no single ommatidiumcan perceive the entire object.

➢ Movement of the objects can be detectedmuch more efficiently by the compound eyebecause as the object passes in front of theeye, the ommatidia switch on and offaccording to their location in relation to theobject .



➢ This characteristic of the compound eye helps theanimal in detecting the movement of the predatorsand escape before the latter can strike.

➢ Another characteristic feature of the compoundeye is its high flicker fusion rate, which means itcan perceive action as successive independentframes of images and not as a continuous motion.

➢ The flicker fusion rate of the compound eye isabout 50 frames per second as compared to 12-15frames per second of human eye.

➢ By perceiving motion the compound eye helpsarthropods to escape from predators.



THE APPOSITION IMAGE➢ This is perceived in bright light, when

pigment cells in the dioptrical andsensory regions spread and completelyseparate the ommatidia from eachother, so that the angle of vision of anommatidium is only 1 degree and lightrays coming directly from the front canonly enter the ommatidium, whereasthe light rays coming at an angle areabsorbed by the pigment before theycan reach rhabdomes.



➢ The image formed in brain is a mosaic of severaldots, each one of which is formed by anommatidium.

➢ Each ommatidium uses only a tiny portion of thetotal field of vision and then in brain these tinyimages are grouped together to form a singleimage of the object.

➢ Since each dot is clearly separated from the other,it is called mosaic or apposition image.

➢ The sharpness of the image depends on thenumber of ommatidia and their isolation from oneanother.



THE SUPERPOSITION IMAGE

➢ This type of vision occurs in dimlight in nocturnal arthropods.

➢ The pigment cells shrink to allowmore light into the eye, so that theommatidia no longer remainoptically isolated from one another,enabling even oblique light rays tostrike one or more ommatidia.



➢This results in overlapping of theadjacent blotches of imagesformed by different ommatidia.

➢This is called superposition imagebecause overlapping images areformed in the brain.

➢This image is not sharp but hazybecause of overlapping images.















VISION IN ARACHNIDA (SCORPION) – THE SIMPLE EYE

➢ Scorpion belongs to the classArachnida and possesses onlysimple eyes.

➢ It has a pair of large median indirecteyes and three pairs of lateral directeyes which function in different waysin different situations.



THE MEDIAN INDIRECT EYES

➢ The median eyes are large convex andcovered with the thick cuticle thatforms cornea or lens.

➢ The hypodermis forms a thick vitreousbody that nourishes the lens.

➢ The sensory rhabdomes pointbackwards towards the reflecting layercalled tapetum.



➢The rhabdomes are surrounded bymany sensory retinal cells whichtransmit nerve impulses to the opticnerve and then to the brain.

➢Median eyes of scorpion are used forvision in the night or in dark placesbecause the dim light entering theeye is reflected by the tapetum tostrike the rhabdomes again to formvision.



THE LATERAL DIRECT EYES

➢ Lateral eyes are small in size, 3 pairsand located on the lateral sides ofprosoma.

➢ This eye is covered externally by abiconvex lens formed from thetransparent cuticle.

➢ The epidermis forms a thinner vitreousbody under the lens.



➢ Inside the eye cup are severalrhabdomes which point directlytowards the source of light as thetapetum is absent in these eyes.

➢Each rhabdome is connected on theposterior end to a sensory retinalcell that is connected to the nerve.

➢The lateral eyes are used to providevision in day time or in bright light.



VISION IN INSECTA (COCKROACH) – SIMPLE AS WELL AS

COMPOUND EYES

➢ Insects possess one pair ofcompound eyes and 1-3 simple eyesor ocelli on top of the head.

➢ In cockroach the ocelli arerudimentary.



THE INSECT COMPOUND EYES

➢ The compound eyes are sessile in the formof convex brownish-black, kidney-shapedstructures on the lateral sides of head.

➢ Each eye contains about 2,000 ommatidia,similar in structure to those alreadydescribed earlier.

➢ The pigments separating ommatidia arenot retractable in the eyes of cockroachsince the animal is nocturnal and spendsdaytime in dark places.



➢ But the eye produces mosaic vision similarto the crustaceans.

➢ Compound eyes are specially adapted toperceive movements of objects.

➢ The insect compound eye is advancedstructure because the number ofommatidia in insect eyes increases givingthe eye sharpness of vision.

➢ Also the distance of vision increases inpredatory insects and fast flying insects.





THE INSECT OCELLI➢ Ocelli are simple eyes, more or less similar to

the simple eyes of arachnids and provide theeye with distant vision.

➢ Ocelli also give nocturnal vision to night flyinginsects, which find their way by aligning themat an angle with the moon or stars.

➢ By possessing both types of eyes, insects enjoyboth types of visions, namely detection ofmovement with compound eyes and distantvision with simple eyes or ocelli.





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COMPILED AND CIRCULATED BY DR. POULAMI ......2020/04/01 · Arachnida and possesses only simple eyes. It has a pair of large median indirect eyes and three pairs of lateral direct