M. Sasaki 1,3 , N. Monden 1 M. Mitsuhata 2 and M. Ono 1

1 Laboratory of Entomology, Faculty of Agriculture, Tamagawa University, Machida, Tokyo, 194-8610 Japan. 2 Agro-Ecology Dept., Tomen Corporation, 2-14-27, Akasaka, Minato-ku, Tokyo, 107-8677, Japan 3

Bumble bees are well-known to be adapted to the northern climate. We showed their dual photoreceptive pathways and masking of the circadian rhythmicity which enable the arctic, long-day life of the bees.1) Individual worker, queen and male of Bombus terrestris were confined in infrared LED actograph from right after emergence, and the locomotor activity was recorded. Under LD 12:12, worker and male became active shortly before light-on and ceased their activity after light-off. The strong activity continued throughout the photophase and no peak was detected. 2) Under changing LD, active phase (?? ) was constant ca 14 h, irrespective of daylength. In long day with more than 14 h photophase, activity started immediately after light-on. The rhythm was entrained by dusk signal. 3) Under constant darkness (DD), clear free-running rhythm with the average period (?? ) was 23.5 h in workers. Under LL, however, endless activity was observed as far as illumination continued. This masking effect on the rhythmicity occurred only under strong illumination, i.e., more than ca 700 lx by white fluorescent bulb. 4) Surgical removal of compound eyes and/or ocelli showed that the photoreceptor for driving circadian oscillator is extraretinal and most probably in the brain. On the other hand, the compound eye-ectomized bees revealed no more masking, thus the photoreceptor for the masking (direct drive of the activity by illumination) should be compoun

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