Short Communication Multimodality of Ocellar Interneurones of the American Cockroach

Many insects possess two or three dorsal ocelli in addition to the paired compound eyes. The dorsal ocellus is characterized by a high convergent ratio of many retinular axons upon several thick second-order neurones in the posterior region of the ocellus. The thick second-order neurones extend towards the brain as an ocellar nerve together with other thin processes, and their central projections have been well demonstrated by cobalt backfills in many insects (reviewed by Goodman, 1981). However, our knowledge about how information is processed in the ocellar system is limited (but see Chappell & Dowling, 1972; Wilson, 1978a,6; Goodman, 1981; Simmons, 1981). In the present study we present some evidence that in the cockroach, Periplaneta americana, the ocellar second-order neurones function as CNS integrative multimodal neurones. The cockroach Periplaneta americana possesses two ocelli, each occurring near the base of the antenna. In the Periplaneta ocellus more than 10000 retinular axons synapse with only three or four thick second-order neurones (10-15 jttm in diameter) which are referred to here as L-neurones (Cooter, 1975; Bernard, 1976; Weber & Renner, 1976; Toh & Sagara, 1984). The L-neurones are hyperpolarized by the ocellar illumination, and respond with a few off-spikes at the cessation of the illumination (Fig. 3A) (Mizunami, Yamashita & Tateda, 1982). In addition to these thick afferent neurones, several thin processes, which respond to various sensory stimuli other than ocellar illumination, are included in the Periplaneta ocellar nerve and are referred to here as small multimodal ocellar interneurones (SM-neurones). The SM-neurones respond with spike discharges to the following stimuli; illumination to compound eyes, movement of antennae including tactile stimuli and air puffs to antennae, air puffs to cerci, vibration to legs, and spontaneous/forced wing beats (Fig. 1). Details of stimulus conditions are given in the figure legends. Recordings from suction electrodes attached to a mid-region of the ocellar nerve revealed spikes originating in more than two SM-neurones (Fig. 1A-E). Whether each SM-neurone responds with spikes to all of the five stimuli used here has