CEREBRAL EXTRAOCULAR PHOTORECEPTORS IN ANTS

Clusters of dark pigmented cells, called putative cerebral extraocular photoreceptors (CEPs), are described in the optic lobes of four ant s pecies: Atta sexdens, Camponotus rufipes, Camponotus crassus, and Pseu domyrmex adustus, Electron-micrographs of CEPs show a central rhabdom- like structure formed by tightly interleaved cells containing shieldin g pigments, with axons projecting toward the outer optic chiasm, The p resence of a great number of mitochondria, nucleolus, and multivesicul ar bodies suggests that these clusters might be functional extraocular photoreceptors, Although some branches from GABA-like immunoreactive neurones occur near the CEP region, GABA-immunoreactivity was not dete cted in CEPs. The ant's cephalic cuticle blocks short wavelengths (< 5 00 nm) and the light transmittance is less than 20% for wavelengths be tween 500-700 nm, Ant species with both diurnal and nocturnal habits a nd heightened nocturnal visual sensitivity (A. sexdens and C. rufipes) contain more CEPs than those with diurnal habits and lower visual sen sitivity (C. crassus and P. adustus). The reduction of light reaching this putative extraocular system could act as a signal for a circadian pacemaker and, as a possible consequence, lead to an increase of noct urnal visual sensitivity of the compound eyes and/or nocturnal. locomo tor activity, as already observed in the species studied.