ABSTRACT.To locate the photoreceptor involved in the photoperiodic induction of diapause inBombyx moriL., covering of larval head with black paint or local illumination using chemiluminescent paint was carried out. A silkworm race showing a response of long‐day type during the larval stage was employed. The results demonstrated that the photoreceptor is located in the head but is extraocular. The optical properties of the larval body suggest that during the first and second stadia light is admitted through the translucent clypeus of the head, but during later stadia enters over the entire larval body including the head, and that it reaches the cerebral lobe where a photoreceptor is possibly locate