Vision In Arthropods Using Hexagonal Shaped Ommatidium Presentation

This Ppt Shows How The Eyes Of A Bee Works And The Small Fragments Like Ommatidium Helps In Clear Vision

1. VISION IN ARTHROPODA:-

2. What Is Vision? ANS:- VISION IS THE ABITILITY TO DETECT THE LIGHT PATTERNS FROM THE ENVIRONMENT AND INTERPRET IT INTO IMAGES. IT VARIES FROM ONE ORGANISM TO ANOTHER LET IT BE EUGLENA OR A SPIDER.

3. As we all know to see or detect something we need a proper optical system and surprisingly 96% of living species has developed an optical system that can discriminate the direction of light to within a few degrees. So The BASIC LIGHT PROCESSING UNIT OF AN OPTICAL SYSTEM IS A PHOTORECEPTOR forming an organ i.e EYE. Arthropods may possess 2 types of eyes based on their structure and complexity. ARTHROPODS EYES COMPOUND EYES SIMPLE EYES DORSAL OCELLI STEMMATA

4. DORSAL OCELLI:- → These are the light sensitive organs found on dorsal or frontal surface of the head coexisting with compound eyes and contains only a SINGLE large aperture LENS. → It consists of:- 1. Lens element (Cornea) 2. Layer of photoreceptor cells. → Cornea is a biconvex transparent cuticle covering these photoreceptor cells which transmit nerve impulses to optic nerve and then to brain.

5. → Ocelli cannot form an image and is unable to recognize objects but actually can sense wide range of wavelengths and shows quick response to changes in light sensity AND THEREFORE THIS IS ONE OF THE REASONS YOU PULL YOUR HAIR OFF WHILE CATCHING A FLY!! LATERAL OCELLI OR STEMMATA :- → They are the only eyes in larvae of holometabolous and certain adult insects. It always occurs laterally. Structurally they are similar to Dorsal Ocelli but often have crystalline cone under the cornea, fewer sensroy rod and are innervated differently from that of the Dorsal Ocelli. → These are also sensitive to light and capable of detecting movement.

6. COMPOUND EYES :-  Number of Arthropods possess compound eyes which are made up of a large no. Of long , cylindrical repeating Photoreceptor units called OMMATIDIA connected to the optic nerve.  Eye is covered with transparent chitinous covering of cuticle forming a Cornea which is divided in many units and each unit is called ommatidium.  Because a compound eye is made up of a collection of ommatidia, each with its own lens, light will enter each ommatidium instead of using a single entrance point.  Each Ommatidium is independent of other ommatidium which means one ommatidium forms a single image and these single image or inputs are combined at the end and form the Whole Image .  These are arranged radially and separated by dark pigmented cells.

7. EACH HEXAGONAL UNIT IS CALLED FACET OR OMMATIDIUM.

8. STRUCTURE OF AN OMMATIDIUM:-  It consists of numerous cells arranged in central axis and many optical parts which are as follows:- A) Cornea :- Outermost layer covered by the transparent cuticle functions as a Biconvex Lens and is known as Cornea. The external surface of cornea is divided into many hexagonal units called a corneal facet . Cornea sheds during each moulting and again gets secreted by underlying corneagen cells . B)Corneagen Cells :- Below the biconvex lens pair of modified epidermal cells are present which secrets fresh cornea when needed and known as corneagen cells. C)Crystalline Cone :- It functions as second lens and focuses light directly into the opening of photoreceptor unit. It is surrounded by 4-6 elongated cone cells or Vitrellae which provides nourishment . Here The Dioptrical Region Ends .

9. D) RHABDOM :- It’s a elongated cylindrical photoreceptor unit which receives light and converts those photon’s energy into electrical energy and then forms image . E) Retinal Cells :- These are a group of 7 elongated cells surrounding the photoreceptor unit i.e The Rhabdome and provide it nutrition and protection . F) Basement Membrane :- The retinal cells rests upon this membrane or internal boundary of ommatidia which is extended into an axon . A bundle of 7-8 axons leaving each ommatidium is connected to the neurons of optic ganglion which is then connected to brain through optic nerve . Rhabdome and retinular cells collectively form receptor region of eye. G) Chromatophores:- Pigment cells like iris and retinal pigment are responsible for separating of one ommatidium to another . These Pigments can shrink or expand to increase or decrease the intensity of light entering the eye. According to this Two Types Of Images Are Form which can be seen later in these slides.

10. As We Studied each ommatidium is capable of producing an independent image of a small part of the object and actually not the entire object. So all these individual images are combined in the brain forming a complete image of the object that is made of small or mosaic dots and i.e why it’s called as MOSAIC VISION .

11. WORKING OF COMPOUND EYE:- Compound Eyes Of Arthropods Can Be Of Two Types on the basis of intensity of light :- A) APPOSITION IMAGE :- The compound Eyes form Apposition image in the bright light while pigment cells in dioptrical and sensory regions SPREAD and completely separate the Ommatidium from each other and prevent light rays passing from one ommatidium to another.  As a result only those rays which fall perpendicularly on the cornea passing through rhabdome form the point of an image. Rays falling obliquely on cornea are absorbed by chromatophores and do not produce any image. In Mosaic Vision if each dot is clearly separated from another it is called mosaic or Apposition Image.  Sharpness of image depends on the no. Of ommatidia present and their isolation from one another.  Apposition images are formed at short distances only and i.e why Arthropods are short sighted .

12. B) SUPERPOSITION IMAGE:-  This kind of image is formed in dim light in Nocturnal Arthropods .  In Weak light the pigment cells shrink to allow more light rays into the eye. Now the ommatidium are not separated by each other allowing the rays to fall in each other’s rhabdom and forming point of image.  Each ommatidium responds to the light rays which had entered through different corneal facets resulting in overlapping of the adjacent patches of images formed by different ommatidium.  This is called superposition image because overlapping images are formed in the brain and the images not sharp but hazy or blurry.

13. ADVANTAGES OF COMPOUND EYES:- Due to the presence of convex corneal surface it results in a wide visual field of more than 300° . It can detect the even movements very easily and responding to it accordingly. They have high Flicker Fusion Frequency. DISADVANTAGES OF COMPOUND EYES:- Image formed here is very crude and unrefined. Ex:- An Insect watches a row of wooden logs which are closely spaced but the image here forms as a continuous horizontal Wooden bar. These eyes can’t form good images in distant vision. They have poor resolving power because of numerous small sized lenses. Therefore, they can’t form detailed picture as human eye does.

14. WE HOPE EVERYONE LIKED OUR PRESENTATION. THANK YOU AND HAVE A NICE DAY MADE BY:- AKANSHA, BIJAYEENI AND PADMATULA GUIDED BY :- SANGEETA MA’AM