Reproductive behaviors related to habitat utilization were studied in males of the damselfly, Mnais nawai, which has two male forms, territorial orange-winged males (nawal) and non-territorial pale-orange-winged males (sahot), at the upper part of a mountain stream where they partially coexist with a related species, Mnais pruinosa, which also has two male forms, territorial orange-winged males (esakit) and non-territorial hyaline-winged males (strigata). These two species showed parapatric distribution; the lower part of the stream was occupied by M. nawai, and the upper part by M. pruinosa. In the present study, cross-matings occurred between both Mnais species, although normal intraspecific matings occurred more frequently than cross-matings. Territorial males of both species copulated with conspecific females that entered their territory and guarded the ovipositing females, probably to avoid sperm displacement resulting from subsequent copulations. Severe competition for oviposition sites by territorial males even occurred between the two species. On the other hand, non-territorial males of both species have alternative mating strategies (including several tactics such as sneaking, takeover and interception). The possible benefits from conflict among territorial males of both species is discussed.

The Japanese endemic Sympetrum frequens is considered as the insular vicariant of S. depressiusculum, widely distributed in the Eurasian Continent. In Korea, morphologically intermediate specimens have been collected, mixed with typical S. depressiusculum. The taxonomical status of these two species is thus questionable. To clarify their status, sequencing of mitochondrial 16S ribosomal RNA (rRNA) was performed on 77 specimens of Sympetrum species collected from Korea and Japan. The pairwise differences between 378 nucleotides of S. frequens and S. depressiusculum, including the intermediate type, revealed only minor differences (< 0.5%, average 0.48%). The neighbor-joining phylogenetic tree indicated that all S. frequens and S. depressiusculum form one clade, suggesting that they pertain to a single species. The tree also suggests that the S. frequens population from Hokkaido is different from all other populations.

Ecological parameters in a population of Mnaispruinosa were investigated in a mountain stream. In the study area, there were two forms of male with regard to wing color, the orange-winged male (esakii) and the hyaline-winged male (strigata), and only one female form with hyaline wings. Emergence of adults began in late April, and the flying season ended in late June. The time after emergence was spent in maturation, and the insects began to mate when they reached maturity. Longevity of adults was 17.6 days for esakii males, 18.4 days for strigata males and 21.9 days for females. There was little difference in emergence time, maturation period, survivorship curve and longevity among the two male forms and female. In other words, the period for reproductive activities was perfectly coincident among them. The factors influencing the synchronization of emergence were discussed.