Phototactic behaviours are observed from prokaryotes to amphibians and are a basic form of orientation. We showed that the marbled crayfish displays phototaxis in which the behavioural response reversed from negative to positive depending on external light conditions. Animals reared in a 12-L/12-D light cycle showed negative phototaxis during daytime and positive phototaxis during night-time. Animals reared under constant light conditioning showed negative phototaxis during day- and night-time, while animals reared under constant dark conditioning showed positive phototaxis during day- and night-time. Injection of serotonin leads to a reversal of negative to positive phototaxis in both light/dark-reared and light/light-reared animals while injection of dopamine induced reversed negative phototaxis in dark/dark-reared animals. Four hours of dark adaptation were enough for light/dark-reared animals to reverse phototaxis from negative to positive. Injection of a serotonin 5HT receptor antagonist blocked the reverse phototaxis while serotonin 5HT receptor antagonists had no effects. Similarly, dark/dark-reared animals reversed to showing negative phototaxis after 4 h of light adaptation. Injection of a dopamine DA receptor antagonist blocked this reverse phototaxis, while dopamine DA receptor antagonists had no effects. Injection of a cAMP analogue into light/dark-reared animals blocked reverse phototaxis after dark adaptation, while adenylate cyclase inhibitor in dark/dark-reared animals blocked reverse phototaxis after light adaptation. These results strongly suggest that serotonin mediates positive phototaxis owing to decreased cAMP levels, while dopamine-mediated negative phototaxis occurs due to increased cAMP levels. Supporting this, the ratio of serotonin to dopamine in the brain was much higher in dark/dark-reared than light/dark-reared animals.

Silphinae (Coleoptera: Silphidae) is an abundant decomposer that plays important roles in the ecosystem. However, there is little information about the life history of this taxon. We found sperm displacement behavior in carrion beetle Silpha perforata. Copulating males bit the female's antenna strongly and inserted the penis into the partner's genital organ more than once. We found a white substance on the tip of penis during copulation. We examined whether this white substance is a previous male's spermatophore, which was removed from the mating partner. When females were dissected just after mating, the same substance that often presents on the penis of mating males was found in the bursa copulatrix of females, although the bursa copulatrix of virgin females was empty. Male behavior during copulation with females of different mating history was also observed to confirm that the removal of spermatophores was observed only in copulation with females that have the spermatophores of previous males. Consequently, we estimated that S. perforata males removed spermatophores of previous males from mating partners. In addition, we dissected the males frozen during copulation, and inspected the penis morphology. This observation revealed that the apical part of the penis was usually hidden in the basal part of penis, but expanded and appeared during insertion. This apical part had many spines, which play an important role in sperm displacement and sexual conflict in some species. These results indicate that there is the sperm competition in S. perforata. This is the first report on sperm competition in Silphinae.

Overwintering sites have recently been a focus of research into adaptive insect behavior in areas where winter is severe; however, little is known about the overwintering sites of most insects, including common ground or carrion beetles. Here, we reveal the overwintering site of Japanese carrion beetles Eusilpha japonica (Motschulsky) (Coleoptera: Silphidae). These beetles occupy various locations between spring and autumn; however, they are not observed in most of these places in winter. To investigate where they overwinter, a thorough scan was made of 109 sites. Overwintering beetles were found at 21 of 73 sites near to trees, and no beetles were observed at 36 sites that were not close to trees. To evaluate the influence of trees on the abundance of overwintering beetles, we established a quadrat consisting of 518 subquadrats in a suitable habitat with varying distances to the nearest trees. We caught 118 overwintering beetles in the quadrat. More beetles existed in subquadrats closer to trees, and analyses that considered spatial autocorrelation revealed a negative correlation between beetle frequency and distance to the nearest tree.

In several species, males frequently immobilize females during copulation. In some species, female immobilization enables males to copulate with unwilling females, while in others, female immobilization prolongs postcopulatory guarding. Male carrion beetles often bite and pull hard on one of the female's antennae during copulatory mounting. Previous descriptive studies have hypothesized that antenna biting is important for postcopulatory guarding in Silphinae. Here, we observed the mating behavior of Oiceoptoma subrufum, to understand the roles of antenna-biting in the initiation and termination of copulation. We compared the success and duration of intromission and pre- and postcopulatory mounting duration between males that did and did not bite female antennae during copulatory mounting. The success and duration of intromission and precopulatory mounting duration were unaffected by antenna biting. However, antenna-biting males mounted females for longer after intromission compared to non-biting males. These results indicate that antenna biting contributes to postcopulatory guarding behavior, not coercive copulation, in O. subrufum.

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