Cantharidin world on islands: Species diversity of canthariphilous arthropods in the Izu–Ogasawara Arc

Cantharidin is a defense chemical produced by only two beetle families, Meloidae and Oedemeridae. This compound also functions as an attractant for certain groups of arthropods (canthariphilous arthropods). Cantharidin‐producing beetles and canthariphilous arthropods build a cantharidin‐mediated community, called cantharidin world. In this study, to clarify the canthariphilous arthropod community formed on small islands, the arthropods were collected by cantharidin‐baited traps and compared among the mainland, peninsula and 11 small islands of the Izu–Ogasawara Arc, Japan. Control traps without cantharidin were also used to collect non‐canthariphilous arthropods. Cantharidin‐producing beetles were distributed across the study area, but the number of species was reduced on islands. We identified 10 species of five families as canthariphilous arthropods in this study area: two species of Anthicidae and Pyrochroidae (Coleoptera), four species of Ceratopogonidae (Diptera), three species of Formicidae (Hymenoptera) and one species of Podoctidae (Opiliones). The number of canthariphilous species was not correlated with island size (area) but was negatively correlated with the distance of island from the mainland. These tendencies were also seen in non‐canthariphilous fauna. The canthariphilous arthropods increased with the number of cantharidin‐producing beetle species, but the non‐canthariphilous fauna did not. Thus, the cantharidin world on islands seems to be affected not only by the geographic characteristics of islands, particularly the degree of isolation from the mainland, but also by the diversity of cantharidin‐producing members within it.