Comp. Biochem. Physiol., 1997 118A(4):1353-1357. Cerebral Extraocular Photoreceptors in Beetles FATIMA FELISBERTI 1 , DORA FIX VENTURA 2 AND HORST HERTEL 3 1 Psychology Department, University Park, Nottingham, NG7 2RD, U.K. 2 Instituto de Psicologia, Univ. São Paulo, Av. Prof. Mello Moraes, 1721, CP: 66.261, 05.508 São Paulo, Brazil 3. Bundesanstalt für Materialforschung und -prüfung; Unter den Eichen, 87; 12205 Berlin, Germany Abstract: Cell clusters containing rhabdomeric structures and screening pigments are located in the optic lobes of pupae and adult beetles of Tenebrio molitor and Hylotrupes bajulus . Due to their resemblance with the compound eyes, the cell clusters were called cerebral extraocular photoreceptors (CEPs) and can readily be detected macroscopically as black spots spread between the lamina and medulla. CEPs of T. molitor responded to bright light with a transient depolarisation followed by a sustained plateau. In contrast, CEPs of H. bajulus responded to bright light either with small depolarisations or did not respond to the range of light intensities presented. CEP cells labelled with Lucifer yellow had axons projecting toward the medullar region. The maximum spectral sensitivity of CEPs was broadly tuned to the green region (λ max at 524 nm). The role of CEPs in insects is still a matter of wide speculation, due to the lack of consistent systematic behavioural studies about insects carrying photoreactive CEPs. Here we suggest that the scattered distribution of CEPs in the optic lobes could allow a temporal pooling of information about the surrounding illumination, thereby entraining some rhythmic photobehavioural responses. Keywords: Beetles - Extraocular photoreceptors - Insecta - Optic lobe - Photoreception - Spectral sensitivity - Tenebrio molitor - Hylotrupes bajulus INTRODUCTION Extraocular photoreceptors have been found either in the peripheral or central nervous system of different arthropods. In insects, extraocular photoreceptors are generally deprived of image-forming structures. Nonetheless, they might inform the animal of the presence and intensity of light and measure its duration (31). Hence extraocular photoreception has been associated with rhythmic photobehavioural responses such as eclosion (28), oviposition (20), stridulation (27), and locomotion (25). The optic lobes of the cerebral ganglion were shown to be associated photoperiodism (1,13), but direct links with specific cerebral extraocular photoreceptors (CEPs) in adult holometabolous insects have yet to be shown. CEPs derive from larval eyes (stemmata), which migrate into the optic lobes during metamorphosis keeping an intact rhabdomeric area and screening pigments. Morphological descriptions of such CEPs in insects belonging to Mecoptera (2), Coleoptera (22), Hymenoptera (5), Diptera (23), Trichoptera (9), and Lepidoptera (11) have been published. In addition, a second type of extraocular photoreceptor was described in the optic lobes of the beetles Pachymorpha sexguttata and Zophobas morio, which was characterised by the lack of screening pigments in its cytoplasm (7). However, apart from a few morphological descriptions and a short mention of CEP photoreactivity in Thermophilum sexmaculatum (18), the CEP photosensitivity and the CEP role in adult beetles is not known. Here we show the response of CEPs of Tenebrio molitor and Hylotrupes bajulus exposed to bright light flashes and their spectral sensitivity. A possible role for CEPs in adult beetles is discussed. MATERIAL AND METHODS The mealworm beetle Tenebrio molitor (Coleoptera; Tenebrionidae) and the house borer beetle Hylotrupes bajulus (Coleoptera; Cerambycidae) were kept in a day/night cycle of 14/10 hours, at rather constant humidity (70- 80%) and temperature at 24-28° C. CEP response to light was recorded in pupae (final instar) and adults of T. molitor and adults of H. bajulus. The body and head were immobilised with bee wax. The compound eyes were covered with bee wax followed by a layer of carbon pigment dissolved in shellac. In the