Morphology and function of male genitalia (spermatophores) in Euscorpius italicus (Euscorpiidae, Scorpiones): complex spermatophore structures enable safe sperm transfer.

The structure and function of the spermatophore of Euscorpius italicus are analyzed. We show how the spermatophore gets shaped from two hemispermatophores and for the first time the sperm transfer mechanism is shown in detail, illustrating function and importance of all complex lobe structures of an euscorpiid spermatophore. A detailed description of the interaction of spermatophore and female genitalia is given. The capsular region of the spermatophore bears different lobes: The distal and basal lobes hook into two cavities on the inner side of the female's genital operculum. A so-called "crown-like structure" hooks into a membranous area in the genital atrium. During sperm transfer, these crown-like structures move backwards, in this way widening the female's genital opening. The sperm duct of the spermatophore is coated with numerous spicules on its outer side, which could serve as a sealing mechanism and/or may stimulate the female. Furthermore, we conclude that "safeguarding of sperm transfer" is one driving force for evolution of male genital complexity in scorpions, but also sexual selection by cryptic female choice could partly play a role.