Evidence for lock-and-key mechanisms in the internal genitalia of the Apamea moths (Lepidoptera, Noctuidae)

Abstract. Fifty of the fifty-six species of the genus Apamea known from North America and three Palaearctic species were analysed for lock-and-key characters in their internal genitalia, mainly in the male vesica and the female bursa copulatrix. There were an average of 4.5 such characters per species, structurally corresponding in the two sexes. Anatomically they form a postcopulatory but prezygotic isolation mechanism. In some closely related species, the internal genitalia are very similar, but these species have a precopulatory isolation mechanism in the presence or absence of male abdominal coremata. Closely related species did not have more lock-and-key characters than unrelated species, which is taken to indicate absence of character displacement. The anatomical distribution of the lock-and-key characters was examined and the organs of eight species are illustrated.
The lock-and-key hypothesis has been abandoned by several earlier authors but mainly on consideration of external genitalia. In Apamea the invariable functional correspondence between sexes in the sperm transferring organs, and the overall species-specifity of characters but non-existence of interspecific differences under a precopulatory mechanism indicate that (a) lock-and-keys are functioning and (b) they act as isolation mechanisms. Alternative hypotheses of genitalic evolution are reviewed.