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Distribution and abundance of sympatric Mnais costalis and M. pruinosa damselflies were studied in a low mountain stream in Shiga, Japan, from 2008 through 2012. The reproductive seasons of the two species overlapped almost entirely: both species emerged in early May and disappeared in late June each year. Males of both species hold territories within the same stretch of the river; however, M. costalis was more abundant on the lower stream, while M. pruinosa was more abundant on the upper stream. Canopy openness varied at territorial sites. Results of these observations suggest that habitat segregation of the two species is due to different preference for light/shade conditions. Morisita's R δ index suggests that interspecific exclusion (or avoidance) contributes to the habitat segregation of the two species.

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In many cases where two closely related species coexist, ecological interaction or reproductive interference drive species to diversify in their body size and/or other signal traits, often concurrently with microhabitat segregation. However, it is usually unclear how character diversification is associated with microhabitat segregation. We performed laboratory experiments using males of two damselfly species (Mnais costalis and Mnais pruinosa) collected from a syntopic site in Shiga Prefecture, Japan. We analyzed the effects of body temperature and body size on three indices of flight performance: wing-beat frequency and flight speed as measures of thrust production, and minimum body temperature for flight (MBTF). The results showed that the MBTF was correlated with body size: the smaller species (M. pruinosa) flew better than the larger species (M. costalis) in a cool environment. The initial flight speed was positively correlated with body temperature, but negatively correlated with body size. The wing-beat frequency was also positively correlated with body temperature, but negatively correlated with body size. The combined effects of body size and body temperature on wing-beat frequency meant that overall, there was no significant difference in initial flight speed. We suggest that the effect that body size and temperature have on flight performance explains the previously documented microhabitat segregation occurring between these two species, with the larger M. costalis preferring sunny environments and M. pruinosa preferring shady environments.