A new type of putative non-visual photoreceptors in the optic lobe of beetles

A putative photoreceptor organ is described in the carabid beetle, Pachymorpha sexguttata. The elongated structure, about 20-40 microns wide and more than 300 microns long, is situated within the optic lobe at the fronto-dorsal rim of the lamina. It lies, deep in the head capsule, in front of the compound eyes and beneath window-like thinnings of the cuticle. The organ is composed of two types of cells: (1) clear sheath cells and (2) well-organized inner receptor cells that appear in a horseshoe-like or circular array in cross-section. Common histological features of all inner cells include a distal trunk ending in microvilli that form a rhabdom-like structure, an axon at the proximal end of the cell, lamellar and multivesicular bodies within the trunk, and clusters of small mitochondria. The organ has no shielding pigment. It is connected by thin axons to a circumscribed neuropil that parallels the organ, and thence via a fiber tract to the medulla accessoria, a possible site of the circadian pacemaker in insects. Immunoreactivity to anti-per(s), an antibody recognizing the Drosophila period (per) protein that plays a central role in the function of the circadian pacemaker in fruit flies, is demonstratable in thin efferent terminals within the organ, in the associated neuropil and in its fiber connection to the medulla. A second receptor organ displaying the same fine structure lies near the second optic chiasm. This set of putative photoreceptors also occurs in the tenebrionid beetle, Zophobas morio, and its pupa. The possible function of these receptor organs is discussed with respect to former chronobiological data and some recently described types of extraretinal photoreceptors in arthropods.