Experimental analysis of the effect of genital morphology on insemination success in the ground beetle Carabus insulicola (Coleoptera Carabidae)

Abstract

The hypothesis that the elaborated genitalia of male insects serve to improve insemination success were tested using the ground beetle Carabus insulicola. To enhance variation in genital size, the genital hooks of experimental males were cut, and these males were then mated with virgin females. Logistic regression showed that the length of the male genital hook affected insemination success. Males with a shorter genital hook tended not to deposit spermatophores at the proper site, and failed to transfer sperm into the spermatheca. Therefore, the male genital hook serves to increase insemination success by depositing a spermatophore at the site where sperm are likely to be transferred. The duration of copulation and post-copulatory guarding may also be explained by these determinants. Stepwise regressions indicated that the occurrence of ejaculation, and the location of the spermatophore determined the duration of copulation and post-copulatory guarding, respectively.