Compound Eyes :
There is a pair of compound eyes which are situated on the head on near the sides of the vertex. The compound eyes are black, kidney-shaped and sessile. Each eye is covered by a transparent cornea which has 2000 hexagonal corneal facets.
Each corneal facet represents an ommatidium. Hence, there are 2000 ommatidia in each compound eye. The ommatidium is a structural & functional unit of the compount eye.
Structure of ommatidium :
The corneal facet is like a biconvex lens. This cornea! lens is secreted by two comeagen cells which are situated just below the facet. There are 4 cone cells below the comeagen cells which are also called as vitrelle. The cone cells secrete a crystalline cone which is found in the centre of the cone cells. There are seven retinal cells which ae situated around a rhabdome just below the cone cells. The rhabdome is secreted by the retinal cells. Each compound eye has 14,000 retinal cells which are situated on a basement membrane. Each retinal cell gives out a nerve fibre, hence, all the 14,000 nerve fibres unite to form an optic nerve.
The cornea, the comeagen cells & the cone cells together form dioptric part. Similarly, the rhabdome & the retinal cells together form receptor part.
The ommatidium remains optically isolated with the help of pigmented sheaths. The dioptric part is covered with retinal pigmented sheath. The pigmented sheaths of the cockroach are non-contractile. Hence, there is permanant optical isolation of the ommatidia. In other insects, the pigmented sheath are contractile & they contract in the dim-light. Hence, the optical isolation is lost during dim-light.
Working of the compound eye :
Normally, two types of images are formed in insects viz.
- Apposition image :
It is formed during moderate to bright light. It is also called as mosaic image.
The pigmented sheaths expand to make die ommatidium optically isolated in the bright light. The image of one part of the object is formed in one ommatidium. Hence, it is a distinct image. In cockroach, only mosaic vision is formed.
- Superposition image:
It is formed in dim-light & in the night. There is contraction of the pigmented sheet. Hence the optical isolation is lost. It is an indistinct image. The image of one part of the object is formed in many adjacent ommatidia
