{98}[Illustration: FIG. 53.—Two ommatidia from the eye of _Colymbetes fuscus_, × 160. (After Exner.) _a_, Cornea; _b_, crystalline cone; _c_, rhabdom; _d_, fenestrate membrane with nerve structures below it; _e_, iris-pigment; _f_, retina-pigment.] The compound, or facetted, eyes are the most remarkable of all the structures of the Insect, and in the higher and more active forms, such as the Dragon-flies and hovering Diptera, attain a complexity and delicacy of organisation that elicit the highest admiration from every one who studies them. They are totally different in structure and very distinct in function from the eyes of Vertebrata, and are seated on very large special lobes of the brain (see Fig. 65), which indeed are so large and so complex in structure that Insects may be described as possessing special ocular brains brought into relation with the lights, shades, and movements of the external world by a remarkably complex optical apparatus. This instrumental part of the eye is called the dioptric part in contradistinction from the percipient portion, and consists of an outer corneal lens (_a_, Fig. 53), whose exposed surface forms one of the facets of the eye; under the lens is placed the crystalline cone (_b_), this latter being borne on a rod-like object (_c_), called the rhabdom. There are two layers of pigment, the outer (_e_), called the iris-pigment, the inner (_f_), the retinal-pigment; underneath, or rather we should say more central than, the rhabdoms is the fenestrate membrane (_d_), beyond which there is an extremely complex mass of nerve-fibres; nerves also penetrate the fenestrate membrane, and their distal extremities are connected with the delicate sheaths by one of which each rhabdom is surrounded, the combination of sheath and nerves forming a retinula. Each set of the parts above the fenestrate membrane constitutes an ommatidium, and there may be many of these ommatidia in an eye; indeed, it is said that the eye of a small beetle, _Mordella_, contains as many as 25,000 ommatidia. As a rule the larvae of Insects with a complete metamorphosis bear only simple eyes. In the young of Dragon-flies, as well as of some other Insects having a less perfect metamorphosis, the compound eyes exist in the early stages, but they {99}have then an obscure appearance, and are probably functionally imperfect.