Question

The image formed in the eyes of cockroach is [RPMT 1999]
A) Apposition
B) Superposition
C) Both (a) and (b)
D) None of these

Answer 1

Answer:

A) Apposition

The image formed in the eyes of the cockroach is Apposition type.

Explanation:

• The cockroaches possess a pair of large, sessile, compound eyes. The working principle of the compound eye is very complicated.  

• Although the compound eye is deficient in focusing ability and clarity of image, such an eye is unsurpassed for picking up motion and for peripheral vision.

• It functions as a very efficient organ for photoreception.  Each ommatidium is capable of producing an isolated image of a small part of the object seen.

• Therefore, in cockroaches and other arthropods possessing compound eyes, the image of the object viewed consists of several pieces.

• The vision affected is said to be the mosaic vision because of its similarity to mosaic artwork.

• The nature of the composite image formed varies according to the different intensities of light i.e apposition image and superposition image.

• An apposition type image is one where the sharpness depends on the number of ommatidia involved and the degree of separation from one another.

• In bright light (during daytime), the pigment cells spread in such a way that they completely isolate optically the adjacent ommatidia.

• No light can pass through the chamber walls from one visual unit to the other. In such conditions, the rays of light, that strike the cornea obliquely, are absorbed by the pigment cells without producing a visual effect.

• Only those rays of light which fall perpendicularly upon the cornea, can travel through the ommatidium and reach the rhabdom to form a point of the image.

• Thus the complete image formed is a mosaic of several components placed in juxtaposition in which even the slightest movement is readily detected.

• In other words, each ommatidium responds to a fragment of the total field and then these fragmentary images are fitted together into a single general picture.

• The optic nerve carries impulses to the brain, where they get interpreted & registered as an upright mental image.

• In the case of cockroaches, the pigment sheaths of ommatidia are not retractable (non-contractile).

• Despite being nocturnal, only apposition-type image is formed in cockroaches.

Thus the correct and most appropriate answer is option A) Apposition.

#SPJ6