Question

What is formed by corneagen cells of an ommatidium?
A. Cone cells
B. Crystalline cone
C. Lens
D. None of these

Answer 1

First, we must know what an ommatidium is. An ommatidium comprises a cluster of photoreceptor cells that are surrounded by support cells and pigment cells. The compound eyes of arthropods like for the insects, crustaceans and millipedes are composed of units which are called ommatidia. Corneagen cells are basically the epidermal cells that produce the cornea, and later produce the corneal pigment cells.

Complete answer:
The specific composition of ommatidium is different in different organisms. Each ommatidium is innervated by one axon bundle and provides the brain with one picture element. The brain forms an image from these independent picture elements.
Below the cornea, there is a pair of corneagen cells that secretes fresh cornea in cases of wear and tear. A lens-like crystalline cone is situated beneath the corneagen cells and focuses the light rays inwards. The crystalline cone is surrounded by four cone cells or Vitrellae that serve to provide nourishment to the cone.
Cone cells or also called as vitrellae surrounds the transparent crystalline cone which is located below the corneagen cells and functions as a second lens. The vitrellae secrete the crystalline cone.
An ommatidium possesses a bio-convex lens or cornea that is formed by the cuticle which becomes thickened and transparent, these form the facets. Below the lens the epidermis forms two clear corneagen cells or lenticular cells which secretes the lens.

The diagram below depicts an ommatidium.

Therefore, the correct option is B which is Crystalline cells.

Note:
An ommatidium can be divided into the outer dioptric region that receives and focuses light rays and the inner sensory region for perceiving light and sends the nerve impulse to the brain, that analyses the impulses as an image of the object. Ommatidia are hexagonal in shape in cross-section and approximately ten times longer than wide.