It would seem as if the decussation of the optic nerve-fibrils were a matter of primary importance, as it so generally occurs, but in the young of that most generalized of all pterygote insects, the cockroach (Periplaneta), Hickson states that the optic nerve-fibrils which leave the periopticon pass without decussating to the ommateum, and in the adult there is only a partial decussation. In Nepa there is no decussation, but the anastomosis is complicated by the presence of looped and transverse anastomoses.
Looking at the eye as a whole, Hickson regards all the nerve structure of the eye lying between the crystalline cone-layer and the true optic nerve to be analogous with the retina of other animals. With Ciaccio, Berger, and others, he does not regard the layer composed of the retinulæ and rhabdoms as the equivalent of the retina of vertebrates, etc.
=Origin of the facetted eye.=—The two kinds of eye, the simple and the compound, are supposed to have been derived from a primitive type, resembling the single eye (ommatidium) of the acone eye of Tipula. As stated by Lang, “an increase of the elements of this primitive eye led to the formation of the ocellus; an increase in number of the primitive eyes, and their approximation, led to the formation of the compound facet eye.” This view is suggested, he says, by the groups of closely contiguous single eyes of the myriopods, considered in connection with the compound eye of Scutigera. Grenacher looks upon simple (ocelli) and compound eyes as “sisters,” not derived from one another, but from a common parentage.
Immature insects rarely possess compound eyes; they are only known to occur in the nymphs of Odonata and Ephemeridæ, and in the larvæ and pupa of Corethra.
=Mode of vision by single eyes or ocelli.=—In their simplest condition, the eyes of worms and other of the lower invertebrates, probably only enable those animals to distinguish light from darkness. The ocelli of spiders and of many insects, however, probably enable them, as Lubbock remarks, to see as our eyes do. The simple lens throws on the retina an image, which is perceived by the fine terminations of the optic nerve. The ocelli of different arthropods differ, however, very much in degree of complexity.