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.