The objective of my study is to understand the coexisting mechanism of two closely related damselfly species, Mnais costalis and Mnais pruinosa. In the central part of Japan, they coexist. Both species inhabit in forest streams of low mountains, and males show wing color polymorphism linked to alternative mating strategies in which larger orange-winged males are territorial and smaller clear-winged males are non-territorial sneaks. Males prefer to hold territories including oviposition substrates and forest sunlit though they avoid totally sunny sites.

First, I examined the habitat preference of each species in relation to solar exposure at territorial sites in sympatric and allopatric areas. In allopatric area, I found that both species prefer shady site to hold territories, while in sympatric area, M. costalis males preferred sunny site and M. pruinosa preferred shady site. I also found a difference in the body size of males. In sympatric area, M. costalis males were larger than M. pruinosa males on average.

These difference of two Mnais species suggest together a possibility of the difference in thermal ecology in two species. So, I estimate minimum body temperature for flight in two species. As a result, I found that M. pruinosa could fly at lower body temperature than M. costalis.