T H E Microstructure of T H E Compound Eyes of Insects* by Timothy H. Goldsmith~ and Delbert E. Philpott

The arthropod compound eye differs fundamentally from the vertebrate eye, both morphologically and functionally. In insects it is constructed of a number of units called ommatidia, each consisting of about eight elongate retinula cells arranged around a central axis, the rhabdom (Text-fig. 1). Surrounding the retinula cells is a sleeve of pigment cells which probably serves to isolate, structurally and optically, the different ommatidia. The proximal ends of the ommatidia rest on a basement membrane, below which lies the optic ganglion and through which penetrate nerve fibers and branches of the tracheal system. At the distal end of each ommatidium is a transparent conical structure, the crystalline cone, the consistency of which varies considerably among the different groups of insects. Overlying the crystalline cones and directly in contact with the external world is the cornea, a transparent layer whose surface is sculptured into numerous tiny facets, one for each ommatidium. Incident light penetrates a corneal facet and crystalline cone, undergoing some refraction, and then passes through the length of the rhabdom. I t is now a well established fact that in a number of species the compound eye can analyze plane-polarized light (cf. 8, 1, 26). The means by which this is accomplished, however, is not yet understood. Likewise, the degree to which each ommatidium, though a composite structure, may function as a single physiological unit, and the location and nature of the visual pigment are questions which have not yet been answered. More detailed morphological information is expected to help in understanding these problems.