Grab, shake, bite, gouge, puncture, split, punch: It's enough to put anyone off sex.
In a brilliant experiment some years ago, Jonathan Waage of Brown University discovered the Rosetta stone to this strange mating behavior. Waage studied the jewelwing damselfly Calopteryx maculata. First he examined the sperm-storage organ of females after a couple of matings to determine whether sperm from a second mating was added to sperm from the first. He was surprised to find that the amount of sperm hadn't changed. Then he dissected pairs in the midst of copulation and studied their sex organs under an electron microscope. The experiment revealed that a male dragonfly uses his penis not just to transfer sperm to the female, but also to remove sperm left in her storage organ from previous matings. When he curls into that wheel position and begins his energetic genital thrusting, he's actually using his rigid, spoonlike, and sometimes spiky, penis to scrape out rival sperm before he deposits his own.
Such a ploy is necessary, Corbet says, because of female choice and sperm competition. A female nearly always mates with more than one male; it's in her interest to "upgrade" her fertilizations if she can, thereby exercising choice over the paternity of her offspring. Males want their sperm alone to prevail, so they have evolved strategies for purging other sperm and for discouraging mates from copulating with rivals. In this game of sexual chess, the last sperm into the female's storage organ wins by fertilizing her eggs.