Fig. 51.—One element of the Compound Eye of the Cockroach, × 700. Co. F, corneal facets; Cr, crystalline cones; Rm, nerve-rod (rhabdom); Rl, retinula of protoplasmic fibrils. To the right are transverse sections at various levels. Copied from Grenacher.
Fig. 52.—Diagram of Insect Integument, in section. bm, basement-membrane; hyp, hypodermis, or chitinogenous layer; ct, ct′, chitinous cuticle; s, a seta.
Each facet of the compound eye is the outermost element of a series of parts, some dioptric and some sensory, which forms one of a mass of radiating rods or fibres. The facets are 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 represent 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 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)[111]; 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 segments (retinulæ), corresponding in number with the rhabdomeres. Each retinula possesses at least one nucleus. The retinulæ were found by Leydig to possess a true visual purple. To the hinder ends of the retinulæ are attached the fibres of the optic nerve, which at this point emerges through a “fenestrated membrane.”
Fig. 53.—Section through Eye of Dytiscus-larva, showing the derivation of the parts from modified hypodermic cells. L, lens; Cr, crystalline cones; R, nerve-rods; N. Op. optic nerve. From Grenacher.
In the simple eye the non-faceted cornea and the retinula are readily made out, but the crystalline cones are not developed as such. The morphological key to both structures is found in the integument, of which the whole eye, simple or compound, is a modification. A defined tract of the chitinous cuticle becomes transparent, and either swells into a lens (fig. 53), or becomes regularly divided into facets (fig. [55]), which are merely the elaboration of imperfectly separated polygonal areas, easily recognised in the young cuticle of all parts of the body. Next, the chitinogenous layer is folded inwards, so as to form a cup, and this, by the narrowing of the mouth, is transformed into a flask, and ultimately into a solid two-layered cellular mass (fig. 53). The deep layer undergoes conversion into a retina, its chitinogenous cells developing the nerve-rods as interstitial structures, while the superficial layer, which loses its functional importance in the simple eye, gives rise by a similar process of interstitial growth to the crystalline cones of the compound eye (fig. [55]). The basement-membrane, underlying the chitinogenous cells, is transformed into the fenestrated membrane. The nerve-rods stand upon it, like organ pipes upon the sound-board, while fibrils of the optic nerve and fine tracheæ pass through its perforations. The mother-cells of the crystalline cones and nerve-rods are largely replaced by the interstitial substances they produce, to which they form a sheath; they are often loaded with pigment, and the nuclei of the primitive-cells can only be distinguished after the colouring-matter has been discharged by acids or alkalis.
Dr. Hickson[112] has lately investigated the minute anatomy of the optic tract in various Insects. He finds, in the adult of the higher Insects, three distinct ganglionic swellings, consisting of a network of fine fibrils, surrounded by a sheath of crowded nerve-cells. Between the ganglia the fibres usually decussate. In the Cockroach, and some other of the lower Insects, the outermost ganglion is undeveloped. The fibres connecting the second ganglion with the eye take a straight course in the young Cockroach, but partially decussate in the adult.