Blister beetles can be born in clutches containing 3,000 larvae. But these siblings do something most don't: work together. The larvae climb into a ball that, to a male bee, resembles the back of a female bee. They may even give off pheromones to attract the fellow.

When he climbs aboard and tries to mate, beetles cling to his underside, seen here. When he then mates with a female bee--a real one this time--they jump ship to her, hang out in her nest and steal the pollen she gathers for her young. There you have it: an insect that has evolved more or less into being an STD that affects other insects.