ITS NERVOUS SYSTEM AND SEXSE ORGANS. 



101 



transparent, biconvex, and polygonal, often, but not quite 

 regularly, hexagonal. In many Insects the deep layer of each 

 facet is separable, and forms a concavo-convex layer of different 

 texture from the superficial and biconvex lens. The facets, 

 taken together, are often described as the cornea ; they repre- 

 sent the chitinous cuticle of the integument. The subdivision 

 of the cornea into two layers of slightly different texture 

 suggests an achromatic correction, and it is quite possible, 

 though unproved, that the two sets of prisms have different 

 dispersive powers. Beneath the cornea we find a layer of 

 crystalline cones, each of which rests by its base upon the inner 

 surface of a facet, while its apex is directed inwards towards 



Fig. 52. Diagram of Insect Integument, in section, bm, basement-membrane ; 

 hyp, hypodermis, or chitinogenous layer ; ct, cl', chitinous cuticle ; s, a seta. 



the brain. The crystalline cones are transparent, refractive, 

 and coated with dark pigment ; in the Cockroach they are 

 comparatively short and blunt. Behind each cone is a nerve- 

 rod (rhabdom), which, though outwardly single for the greater 

 part of its length, is found on cross-section to consist of four 

 components (rhabdomeres)* ; these diverge in front, and receive 

 the tip of a cone, which is wedged in between them ; the 

 nerve-rods are densely pigmented. The rhabdom is invested by 

 a protoplasmic sheath, which is imperfectly separated into 



* The number in Insects varies from eight to four, but seven is usual ; four is the 

 usual number in Crustacea. 



