THE chemical composition of a homogenized compound eye was reported by Langer1, but until now nothing was known about the components of rhabdomeres alone, although the presence of photosensitive substances in rhabdomeres has been anticipated by many investigators from a long time ago. During the course of histochemical studies on the compound eye of a house fly, Musca vicina Macquart, periodic acid–Schiff-positive substances were found to be distributed in rhabdomeres of a dark-adapted eye. The periodic acid-Schiff reaction of rhabdomeres was always positive both in a dark-adapted eye treated with saliva for 1 h in an incubator and in a light-adapted eye (Fig. 1). In addition to this, Bauer's reaction of rhabdomeres was very weak.