The male of Ischnura asiatica has a pair of horn at the tip of his genitalia. Because the spermatheca is added to bursa copulatrix with narrow duct, however, the horn is shorter than the duct. Thus, the spermatheca is inaccessible and males could not displace spermathecal sperm directly using the horn. Nevertheless, sperm displacement during copulation occurs in the spermatheca. We have found males stimulate mechano-receptive sensilla which communicate the presence of an egg to the muscles surrounding sperm storage organs for fertilization to induce sperm ejection. The large genitalia are more likely to stimulate sensilla and to displace more spermathecal sperm. From the viewpoint of females, contrarily, there is the possibility that females lose genetic variety of the offspring when much sperm are displaced. Therefore, counter adaptation might be occurred in the population including large males. In order to test this hypothesis, width of genitalia and the number of sensilla were measured in several local populations, each of which had males of different body size. The number of sensilla was decreased with width of genitalia. Because the decrease of the number of sensilla might make difficult to displace spermathecal sperm for males, the counter adaptation that the number of sensilla is decreased might be evolved, when male body size increased.