title: Ultrastructure of the extraocular photoreceptor in the genitalia of a butterfly, Papilio xuthus.
abstract: This paper describes the ultrastructure of a sensory neuron found in the extraocular photoreceptive site on the butterfly genitalia. Our previous studies have shown that there are two pairs of the photoreceptive sites in a butterfly (four per individual). Each photoreceptive site is recognizable by a transparent area in the pigmented cuticle of the genitalia. From the nerve that extends from the transparent cuticle to the last abdominal ganglion, a sustained train of single unit spikes can be recorded in response to a light flash. The single unit spikes disappear when the transparent cuticle is covered, thus indicating that a single photoreceptor exists close to it. Here, we examined complete serial semithin sections of plastic-embedded photoreceptive sites of both male and female, and observed an ovoid structure close to the transparent cuticle. The structure contained the cell body of a sensory neuron whose axon was in the nerve branch from which the photoresponse had been recorded. Further electron microscopy revealed that the cell body extended a few distal processes that protrude tubular membrane from the tip, forming a structure resembling the phaosome (from Greek; phaos = light, some = body) which was first described in the earthworm dermal photoreceptors. The sensory neuron was also found in a surgically isolated nerve-photoreceptor preparation that responded to the light. We therefore propose that the phaosome-containing sensory neuron is the butterfly genital photoreceptor.
authors.affiliation: Department of Biology, Yokohama City University, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Miyako
authors.forename: K
authors.initials: K
authors.lastname: Arikawa
authors.forename: E
authors.initials: E
authors.lastname: Eguchi
mesh_terms: D000818:Animals; D002080:Butterflies / Q000033:anatomy & histology*; D005260:Female; D005835:Genitalia / Q000648:ultrastructure*; D008297:Male; D008854:Microscopy, Electron; D010786:Photoreceptor Cells / Q000648:ultrastructure*