Separation possibilities and genital measurement variations in two cryptic species of European pierid butterflies, Leptidea juvernica Williams, 1946 and L. sinapis (Linnaeus, 1758)

Of the three recently separated cryptic butterfly species of the Leptidea sinapis complex, the two species L. sinapis and L. juvernica occur sympatrically and syntopically in central Europe. As the separation of these species requires genital or genetic characters, their correct identification is a fundamental problem to be solved prior to any biological and distributional studies. In the present study a morphometric approach was applied to test for separation possibilities and to examine genital measurement variations based on large population samples of the two species (347 females and 636 males). Butterflies were collected at 456 localities distributed across Poland. Specimens of both sexes could be separated using either the shape or the length of the antrum bursae (females), phallus and saccus (males). Intraspecific seasonal differences in these measurements may affect the discrimination of males only. Genital characters were significantly larger in butterflies of the spring brood than in those of the second brood. In females, antrum bursae length ranges were separated by an interval, allowing for the exact identification of each specimen. In males, no such intervals were found between phallus and saccus length ranges, which slightly overlapped. Discriminant analysis resulted in 100% of males correctly classified. The present study confirmed the validity of interspecific differences in the shape of phallus and saccus for species identification without measurements. The length of these two characters discriminates the two species when separation is limited to specimens from the same generations. As an alternative for discriminant analysis, either phallus length and vinculum width or phallus length and the ratio of the ventral edge of genital capsule length and saccus length (VEL/SL ratio) is recommended for the practical separation of the two species. The differences in the shape and size of the copulatory organs of the two species seem to indicate reproductive isolation due to mechanical incompatibility of their genitals.