Abstract
Newly hatched larvae of Bombyx mori displayed positive phototactic response, but the response was rapidly suppressed after the larvae were fed on mulberry leaves or exposed to the odour of the leaves (Fig. 2) . The suppression of phototaxis continued subsequently in the absence of mulberry leaves, but the response was recovered gradually. This suppression was satisfied by placing the larvae in the presence of methanol extracts prepared from mulberry leaves. It was demonstrated by a paper chromatography that the suppressive principle was distinct from attractants (Fig. 5) . From electrophysiological study it was shown that peripheral photoreceptive organ was not involved in the suppression of phototaxis by mulberry leaves (Fig. 3) . Electric shock treatment caused a recovery of phototaxis immediately, and this strongly suggested an involvement of central nervous system in the suppression (Fig. 4) . Adaptive significance of the loss of phototaxis was presumed to be an arrest of the silk-worm larvae on the host plant.
When the larvae were fed on artificial diet which contained no mulberry leaves, their responses were not affected at least by 24 hr-feeding (Table 1) . However, the phototactic response was decreased gradually with larval growth (Fig. 6) . This observation showed that physiological changes caused by the growth affected the phototaxis of the silkworm larvae.
