VISION IN ARTHROPODADA

CLASS – B.Sc. PART – I(HONS. & SUBSI.) PAPER – I

BY KUNDAN PATEL GUEST FACULTY DEPARTMENT OF ZOOLOGY B.N. COLLEGE, PATNA INTRODUCTION Sensation of vision is one of the most effective strategies to understand the beauty of nature and its creations. In Arthropoda, the of compound has made it curious to understand the way they visualize their environment. Apposition eyes are the most common form of and are presumably the ancestral form of eye. Most have at least one or two types of eye – lateral compound eyes, and smaller median ocelli, which are simple eyes. INTRODUCTION COMPOUND EYES IN ARTHROPODA

 Most arthropods have compound eyes on each side of the head(dichoptic)  Made up of repeating units, the ommatidia, each of which functions as a separate visual receptor  Each ommatidium consists of:- 1. A lens – transparent cuticle form a biconvex corneal lens or facet 2. A transparent crystalline cone 3. Light-sensitive visual cells arranged in a radial pattern like the sections of an orange 4. Pigment cells which separate the ommatidium from its neighbours COMPOUND EYES IN ARTHROPODA

 The pigment cells ensure that only light entering the ommatidium parallel(or almost so) to its long axis reaches the visual cells and triggers impulses. Thus, each ommatidium is pointed as just a single area in space and contributes information about only one small area in the field of view  The is excellent at detecting motion. As an object moves across the visual field, ommatidia are progressively turned on and off. Because of the resulting “flicker effect”, respond far better to moving objects than stationary ones. E.g. – honeybees will visit wind- blown flowers more readily than still ones COMPOUND EYES IN ARTHROPODA COMPOUND EYES IN ARTHROPODA WORKING OF COMPOUND EYES Resolution and sensitivity:-  Arthropods that are apt to be active in dim light(e.g. – crayfish, praying ) concentrate the screening pigments of their ommatidia into the lower ends of the pigment cells. This shift enables light entering a single ommatidium at an angle to pass into adjacent ommatidia and stimulate them also. With ommatidia responding to a single area in the visual field, the image becomes coarser  The shift in pigments make it more sensitive to light than it is in the daytime as more ommatidia can detect a given area of light WORKING OF COMPOUND EYES Colour vision:-  Some insects are able to distinguish colours.  In the honeybee, four of the visual cells in each ommatidium respond best to yellow-green light(544 nm), two respond maximally to blue light(436 nm) and the remaining two respond best to UV light(344 nm) Mosaic vision:-  The compound eye is incapable of distant and sharp vision but is efficient in picking up motion and in providing 360o view, as it is large globular and mounted on a movable stalk  Each ommatidium is capable of producing an independent image of a small part of the object seen and not the entire object. All these small images are combined in the to form a complete image of the object that is made of small dots or mosaic of dots and hence it is called mosaic vision  The compound eye has high flicker fusion rate(about 50 frames per second) which means it can perceive action as successive independent frames of images and not as a continuous motion  has a flicker fusion rate of 12-15 frames per second WORKING OF COMPOUND EYES

The apposition image:-  In bright light, pigment cells in the dioptrical and sensory regions spread and completely separate the ommatidia from each other, so that the angle of vision of an ommatidium is only 1o and light rays coming at an angle are absorbed by the pigment before they can reach rhabdomes  The image is formed in brain is a mosaic of several dots, each one of which is formed by an ommatidium  Each ommatidium uses only a tiny portion of the total field of vision and then in brain these tiny images are grouped together to form a single image 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 the no. of ommatidia and their isolation from one another WORKING OF COMPOUND EYES The superposition image:-  This type of vision occurs in dim light in arthropods  The pigment cells shrink to allow more light into the eye, so that the ommatidia no longer remain optically isolated from each other, enabling even oblique light rays to strike one or more ommatidia. This results in overlapping of the adjacent blotches of images formed in the brain. This image is not sharp but hazy because of overlapping images WORKING OF COMPOUND EYES THE SIPMLE EYE OF ARTHROPODA

The median indirect eyes:-  The median eyes are large convex and covered with the thick cuticle that forms or lens  The hypodermis forms a thick vitreous body that nourishes the lens  The sensory rhabdomes point backwards towards the reflecting layer called tapetum  The rhabdomes are surrounded by many sensory retinal cells which transmit nerve impulses to the optic nerve and then to the brain  Median eyes of scorpion are used for vision in the night or in dark places because the dim light entering the eye is reflected by the tapetum to strike the rhabdomes again to form vision THE SIPMLE EYE OF ARTHROPODA The lateral direct eyes:-  Lateral eyes are small in size, 3 pairs and located on the lateral sides of prosoma. This eye is covered externally by a biconvex lens formed from the transparent cuticle  The epidermis forms a thinner vitreous body under the lens  Inside the eye cup are several rhabdomes which point directly towards the source of light as the tapetum is absent in these eyes  Each rhabdome is connected on the posterior end to a sensory retinal cell that is connected to the nerve  The lateral eyes are used to provide vision in day time or in bright light THE SIPMLE EYE OF ARTHROPODA Ocelli:-  In Latin, ‘ocellus’ means little eye  These are simple eyes that provide the ability of distant vision  These also provide nocturnal vision to night flying insects  There is no crystalline lens  The refractive focusing of light is carried out solely by the curved cornea  They consist of a single lens and several sensory cells THE SIPMLE EYE OF ARTHROPODA CONCLUSION Arthropods are not only most abundant and diverse animals rather they are equipped with many adaptive features that make them advanced compared to other invertebrates. Their eyes and vision are also evidences in this regard. It makes them enable to have a highly sensitive vision even in night and during flight. Mechanism of image formation, although complex, is highly evolved in them. BIBLIOGRAPHY & REFERENCES

1. Invertebrate Zoology – E.L. Jordan & Dr. P.S. Verma 2. Invertebrates – R.L. Kotpal 3. Invertebrate Zoology – R.D. Barnes 4. Invertebrates – Janet Moore THANK YOU