PROSTITUTION is supposedly the oldest profession, which means that sex itself must be one of the oldest behaviors, a conclusion underscored by the recent discovery of two bugs fossilized as they coupled 165 million years ago. Simply being ancient doesn’t make a sex act interesting, but this one was noteworthy in part because behavior is generally so ephemeral, leaving none of the preserved remains we rely upon to tell us about how early forms of life ate, or ran, or saw the world.

The fossil also showed in breathtaking detail the positioning of the pair in flagrante delicto. While the finer points of the orientation of this particular duo may have been affected by the pressure of the sediment that caused their preservation, the finding underscores the unlikely seeming importance of sexual habits in evolution. Scientists care about the intricacies of how insects mate not out of a desire to create a bestial Kama Sutra, but for two reasons.

The first is that we can use such aspects of sex as mating position and the shape of genitalia to reconstruct evolutionary history and see how species are related to one another. Evolution is all about reproduction. What happens at the business end of an animal is essential to whether eggs are fertilized and genes passed on, and nowhere is the variation in sex organs more breathtaking than in insects. Illustrations of the private parts of insects look like something produced by the love child of Hieronymus Bosch and M. C. Escher, with a hefty measure of Rube Goldberg thrown in. How similar the sexual apparatus of two species looks is often a key to how recently they shared a common ancestor, or indeed to whether they are really two species at all.

The elaboration of insects’ genitalia also has a more diabolical side. We may see two damselflies forming a heart shape with their bodies as they couple, abdomens elaborately curled as they perch on a stem, but if you could look inside them, you would see what appears to be a minuscule Swiss Army knife with its attachments unfolded. That would be the male’s penis. A female damselfly may mate with more than one partner, but from each male’s perspective, the more of her eggs he can keep from rivals and fertilize himself, the better. The spines and scoops on his penis serve to remove the prior male’s sperm so he can replace it with his own. Nature may be red in tooth and claw, but she is utterly ruthless when it comes to genitalia.