Limited variability of genitalia in the genus Pimpla (Hymenoptera : Ichneumonidae): Inter- or intraspecific causes?

We studied the morphometric variability of genitalia in five species of the genus Pimpla (Hymenoptera, Ichneumonidae). This genus is characterized by a high intraspecific variation in body size, a simple structure of the geni talia and many closely related species. We found that genitalic characters of all studied species vary less than characters related to body size. Howe ver, there exists an overlap in genitalic characters between different spec ies. The pattern of variance and the ecology of the species studied suggest s that low variance of genitalia cannot be explained by interspecific cause s (mechanical isolation) or sperm competition. The most likely explanation for the low variance of genitalia is assuring mechanical fit between male a nd female during copulation. Sexual selection by female choice may be a cau se of the observed pattern of variance as well, if females have active pref erence for males with larger genitalia. We suggest that genitalia of insect s with a large variation in body size vary less than other morphological ch aracters to ensure intraspecific mechanical fit.