Morphological variations in external genitalia do not explain the interspecific reproductive isolation in Nasonia species complex (Hymenoptera: Pteromalidae)

External genitalia play a crucial role in the successful copulation of conspecific partners in insects. Due to their structural specificity, external genitalia have often been used for distinguishing closely related species in different insects. However, in the young Pteromalid species complex Nasonia, these structures have not received much attention. Nasonia is a genus of parasitoid wasps, that lays eggs and completes its life cycle in dipteran pupae. All behavioral studies (including this study) conducted on this genus indicate that varied prezygotic isolation operates among these recently diverged ([~]1 million years) species. Whether such prezygotic isolation is caused by the incompatibility in their reproductive organs is not known. Here, we studied the internal male reproductive system of the four species and show that all four species have a similar organisation and thus ruling it out as a potential barrier. Furthermore, to investigate the role of male external genitalia as a prezygotic barrier, we also examined the ultrastructural details, including aedeagal length, aedeagal aperture length, and digital spines. We show that all these parameters and sperm length show variations across the four species. However, these variations do not correlate with the female acceptance of the heterospecific males. Therefore, we conclude that there must be other olfactory or behavioral isolating mechanisms and spatial or temporal factors that maintain the reproductive isolation of these four species.