A NEW-TYPE OF PUTATIVE NONVISUAL PHOTORECEPTORS IN THE OPTIC LOBE OF BEETLES

A putative photoreceptor organ is described in the carabid beetle, Pac hymorpha sexguttata. The elongated structure, about 20-40 mum wide and more than 300 mum long, is situated within the optic lobe at the fron to-dorsal rim of the lamina. It lies, deep in the head capsule, in fro nt of the compound eyes and beneath window-like thinnings of the cutic le. The organ is composed of two types of cells: (1) clear sheath cell s and (2) well-organized inner receptor cells that appear in a horsesh oe-like or circular array in cross-section. Common histological featur es 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 cel l, lamellar and multivesicular bodies within the trunk, and clusters o f small mitochondria. The organ has no shielding pigment. It is connec ted by thin axons to a circumscribed neuropil that parallels the organ , and thence via a fiber tract to the medulla accessoria, a possible s ite of the circadian pacemaker in insects. Immunoreactivity to anti-pe r(s), an antibody recognizing the Drosophila period (per) protein that plays a central role in the function of the circadian pacemaker in fr uit flies, is demonstratable in thin efferent terminals within the org an, in the associated neuropil and in its fiber connection to the medu lla. A second receptor organ displaying the same fine structure lies n ear the second optic chiasm. This set of putative photoreceptors also occurs in the tenebrionid beetle, Zophobas morio, and its pupa. The po ssible function of these receptor organs is discussed with respect to former chronobiological data and some recently described types of extr aretinal photoreceptors in arthropods.