Internal Muscle in the Eye of an Insect

Abstract

THE muscles associated with the vertebrate eye are well known, comprising those responsible for eye movements and the muscles within the eye controlling the iris and lens. The compound eyes of insects form part of the head capsule, and movements of the eye as a whole seem to demand that the whole head be moved as in the peering movements of young locusts¹. There is only one little-known reference to a muscle within the compound eye itself, in the blowfly Calliphora². Lowne called it a “ciliary muscle”, but only noted its outer attachment. Fig. 1a shows that the muscle is attached externally to the medial border of the membrane at the base of the receptor layer, and internally to the tentorium (the internal supporting structure of the insect head). It is supplied by a fine branch of the antennal nerve. We have found comparable muscles in other Diptera, for example, syrphids, tipulids and culicids.