Diverse organisms, from bacteria to mammals, show rhythms with a period of approximately 24 hours, known as circadian rhythms, in their physiological process and behaviors. Social insects are within the insecta, which is the most prosperous group of animals, and have developed remarkable sociality such that they have long been paid much attention in the field of physiology and ecology. This article reviews circadian rhythms in social insects, mainly focusing on honeybees. First, I introduce the studies of social synchronization of circadian rhythms and plasticity in circadian rhythms mainly from the aspect of behavior. In the latter, behavioral circadian rhythms exhibit or diminish within an individual in the context-depend ent manner in a social insect colony. Second, the molecular and anatomical features of the circadian clock in insects are briefly summarized, and then the recent progresses in those of social insects are reviewed. Taken together, I provide over view of circadian rhythms in behavior and the underlying mechanism observed in social insects.

Creatures may exhibit spontaneous self-motions in independ ent on environmental stimuli. Spontaneous self-motions gen erate noises that might disturb the accuracy of environmental responsive behaviors or directional movements, and thus functions of the spontaneous self-motions in animal behaviors have not been studied. Recently, several reports showed, how ever, that spontaneous self-motions specify the efficiency of animal behaviors, suggesting that spontaneous self-motion is a platform that serves for adaptive outcomes of animal behav iors. The freshwater planarian Dugesia japonica belongs to an evolutionarily early group possessing paired eyes with a simple architecture and displays robust photo-response eva sive behavior through neural wiring between the eyes and a simple-structured but stunning brain. More recently, it was demonstrated that spontaneous wigwag self-motion of the head of planarians breaks the symmetry of the eyes’ inputs during photo-orientation behavior. Notably, the angle and fre quency of the spontaneous wigwag self-motion were opti mized and strongly correlated with the angle of the binocular field for ensuring accurate photo-orientation behavior. Furthermore, the angle and frequency of the spontaneous wigwag self-motion were optimized for other adaptive behav iors, such as hiding in a concave space on a stone, even in the absence of environmental cues. Collectively, these insights suggest that spontaneous self-motion ensures the movement in the precise direction for enabling the simple, but efficient and robust outcomes of multiple behaviors.

Animals receive light from their environments using photosensitive proteins, opsins, for many purposes. Since opsins are diversified in some aspects including absorption and biochemical properties, it is essential to investigate properties of opsins for understanding characteristics and mechanisms of opsin-based photoreceptions. We previously investigated properties and functions of visual and non-visual opsins from a jumping spider. Our previous studies suggest that the absorption spectrum of a jumping spider opsin expressed in the retina is involved in receiving defocused images, which could be used for depth perception. The mechanism for spectral tuning of this opsin could involve a hydrogen-bonding network around the chromophore retinal. In this review, we focus on our previous findings about such physiological relevance and spectral tuning mechanism of the absorption spectrum of the opsin in a jumping spider.