Prolonged copulation and the internal dynamics of sperm transfer in the water strider Aquarius remigis

Abstract

Copulations lasting much longer than required to effect insemination are common throughout the Insecta, but their adaptive significance remains obscure. We address the hypothesis that prolonged copulations benefit male Aquarius remigis (Hemiptera, Gerridae) by influencing sperm use and storage following insemination. We describe the gynatrial complex of females and document sperm location immediately following 32 naturally terminated and 25 artificially terminated copulations. We also examine sperm storage in 22 females isolated from males for 2–10 days. Our results demonstrate that insemination occurs at the end of copulation, and therefore that prolonged copulation is not a post-insemination mate-guarding tactic, as was previously assumed. Sperm are transferred in a coherent, coiled mass and move rapidly to the spermathecal tube, the primary storage organ. However, a few sperm move directly to the fecundation canal and hence should be capable of immediate fertilization. Prolonged copulation is associated with increased filling of the spermathecal tube, which may indicate some form of copulatory courtship or sperm loading by males during the prolonged pre-insemination phase. We discuss this new interpretation of prolonged copulation in A. remigis in the context of the overall costs and benefits of prolonged copulation for both sexes under the natural conditions of polygynandry.