Page:Jardine Naturalist's library Entomology.djvu/192

186 only two, and in a few instances only a single ocellus has been detected. Their internal structure renders it obvious that they are organs of vision. Müller is of opinion that their refraction must be very great, the convex cornea, the anterior convex surface of the lens, the posterior convex surface of the lens, and, finally, the convex surface of the glassy body itself, each having the power of refracting a ray of light. It is improbable that they are of any use in distant vision, but they are fitted to give a distinct view of objects close at hand, and are doubtless designed to facilitate the insect's operations in the narrow passages, tubular flowers, &c., where they are so often employed. When we look at a composite eye, it is found to present a reticulated appearance, which is occasioned by its being composed of an aggregation of minute hexagonal pieces. Each of these pieces is a distinct lens, and constitutes in itself a complete organ of vision. Their amount has been calculated in a variety of species, and in most cases found to be astonishingly great: 7000 have been counted in the eye of a common fly; 12,544 in a dragon fly; 11,300 in the goat-moth; 17,355 in a butterfly; 25,088 in a species of mordella; 1300 in the convolvulus sphinx; 50 in an ant, and about the same number in Xenos. The eye consists of various layers; first of a hard, transparent, facetted membrane, which may be regarded as