Distribution of sensory hairs on the head horn of the Asian rhinoceros beetle Trypoxylus dichotomus Kono. (Coleoptera, Scarabaeidae)

The prominent head horn on the male Asian Rhinoceros Beetle, Trypoxylus dichotomus Kono (formerly Allomyrina dichotoma), is used in combat with other males for reproductive access to females. These beetles engage in stereotyped combat behavior that takes place on the trunks and branches of trees. The ultimate goal of combat is either to chase off a rival or to wedge one's horn underneath an opponent and flip them off of the substrate. Logically it follows that the horns should have some measure of sensory input yet surprisingly little is known about the distribution of sensory structures on the horns. By analyzing fight videos, we were able to identify the regions of horns that are most likely to contact an opponent during combat. We combined this analysis with an investigation of the ultramorphology of the horn using a scanning electron microscope. Our results demonstrate a correlation between points of contact during combat and an increase in amount of sensilla on the horn of T. dichotomus. We also identified differences in cuticle morphology on the head horn. By combining behavioral analysis with morphological assessment we were able to investigate a potential exaptive function for the elaborate weaponry of T. dichotomus.