The spermathecal duct; the left end corresponds to the beginning of the spermatheca and the right end corresponds to the beginning of the vagina.
It's well known that human sperm have a long way to travel in the female body if they are going to fertilize an egg. But that's nothing compared to the tortuous path taken by the sperm of leaf beetles. Female leaf beetles have a spiral-shaped tube through which the sperm must travel, including turn reversals to make this maze even trickier (the "spermathecal duct"). So, how do these beetles ever get lucky? Well, according to this research, it's all about hardness... of the female! The female tubes are uniformly stiff, whereas the sperm are soft at the tip and gradually stiffen along their length, a combination that maximizes the sperm's speed through the tube. Sorry, ducks - looks like the corkscrew penis doesn't make you that ...
