Is the copulation of silver-washed fritillary inverted? Function of the female genital projection (Lepidoptera: Nymphalidae: Argynnini)

Females of the silver-washed fritillary butterfly, Argynnis paphia (Linnaeus), and its relatives are known to have a unique genital projection called the cornucopia. Previous observations showed that the cornucopia partly penetrates the male genitalia during copulation, but its detailed structures and functions are unknown to date. Our observations using light microscopy, confocal laser scanning microscopy, scanning electron microscopy, and microcomputed tomography suggested that penetration by the female cornucopia occurs passively, and it is hooked and pulled by the male towards the male genitalia using sharp teeth on the uncus. This hooking and pulling causes wounds on the dorsal region of the cornucopia. Artificial amputation of a cornucopia confirmed that it is indispensable for spermatophore transfer from males and functions to break off the male phallic spines (cornuti). The broken spines may be used by females to help the digestion of spermatophores in the female’s corpus bursae. Females with larger body sizes had more broken spines in the corpus bursae, indicating that secure holding of a large female during copulation is challenging for males.