Indirect sperm transfer in arthropods: behavioral and evolutionary trends

Arachnids, myriapods, and wingless hexapods exhibit a fascinating diversity of sperm transfer behaviors. Modes of sperm transfer can be categorized by the degree of contact between male and female during transfer, with direct transfer (copulation) involving the greatest contact, paired-indirect transfer an intermediate degree, and dissociated transfer the least. Internal fertilization, spermatophores, and copulation are sometimes assumed to have evolved after invasion of land, but all have evolved many times in the marine habitat. Behaviors associated with indirect sperm transfer include those having close parallels with direct transfer (provision of nuptial gifts) as well as unique phenomena (spermatophore trampling by rival males). The morphology and physiology of indirectly transferred spermatophores have been shaped by environmental factors (e.g. humidity) as well as biological ones (e.g. clutch size of females), and they may provide useful phylogenetic characters. Unanswered questions about indirect transfer include the following: Are females of dissociated species able to choose their partners? What determines size and number of spermatophores? Do speciation rates differ between taxa with direct versus indirect transfer?