…here's a real sex difference, reported in Science.

The odd group of insects called twisted-wing parasites, or more formally Strepsiptera, is easily overlooked. Spending most of their lives hidden inside other insects, the majority of the 596 known species have been identified only from adult males caught during their brief mate-seeking flight. "These are really, truly enigmatic insects," says David Grimaldi, entomology curator at the American Museum of Natural History in New York City. "They break all the rules."

The differences between males and females of the same strepsipteran species are extreme. Adult males are small, flylike creatures, whereas most adult females resemble grubs and remain inside their host, merely protruding their fused heads and thoraces when ready to receive a male's sperm. In one strepsipteran family, males and females actually parasitize different kinds of insects. "Everything you find about them is like they came from outer space," says population geneticist J. Spencer Johnston of Texas A&M University in College Station.

Don't take the "came from outer space" bit literally—they are terrestrial arthropods in good standing, even if their relationships to other arthropods are a little bit obscure. But they are definitely bizarre. They are parasites of other arthropods with an extreme sexual dimorphism—the ladies look rather like Jabba the Hutt, to me. Here are a couple of examples:

Adult male, left, and female, right.

The females lurk inside their hosts, with their heads and thoraces bulging out through the intersegmental membranes, and secrete pheromones that attract the more typically insectile adult males, which flit to the females and mate with their heads. The females have a specialized opening between their antennae/mouthparts called the brood passage that channels sperm back to their more conventionally located reproductive tract.

Adult female Eoxenos laboulbenei, left, and abdomen of a wasp infested with Xenos vesparea, right.

The females retain a swarm of eggs (on the order of a thousand) and allow them to develop internally, and then release the motile larvae, called triungulins, back through the brood passage. That's right, they mate through the "face", and give birth back through the same place. The larvae then crawl or spring to new hosts by curving their abdomens ventrally, then snapping it back to hop forward. When they find an insect host, they secrete a digestive enzyme that dissolves a patch of host cuticle, and they burrow inward to molt into a grub that lives within the body cavity.

Adult male Eoxenos laboulbenei, left, and larval (also called a triungulin) Halictophagus tettigometrae, right.

Strepsipteran parasites also do odd things to their hosts. Some species modify the host's secondary sexual features, flipping them into the form carried by the opposite sex, basically transgendering them. Others may secrete hormones that cause the gonads to atrophy, causing parasitic castration. They may also increase the life expectancy of their hosts, either by behavioral changes that reduce predation or directly, by physiological changes that reduce the host's sexual activity—sex is risky and exhausting, so minimizing the activity may boost one's lifespan (whether the host thinks it's worth it is an unanswered question.)

Proffitt F (2005) Twisted Parasites From "Outer Space" Perplex Biologists. Science 307(5708):343.