Invertebrate Eyes: Variations in Structural Design for Vision

Abstract Invertebrates are the most numerous and diverse among all animal groups, and their eye structures and optical and photoreceptor systems are equally diverse. Image forming compound eyes evolved in arthropods (insects, arachnids, and crustacea) and are generally restricted to them. Compound eyes are structured of eye facets, ommatidia (Figure 12.1). The number of ommatidia varies from only a few in certain species of ants to more than 2,000 in the dragonfly. Each ommatidium is a complete eye, containing two functional systems: the dioptric, or optical, system and the photoreceptor rhabdom system. The optical system is the corneal lens and the crystalline cone that transmit the image on to the rhabdom.