Video: (Ground) lousy sex

Giant sperm come in small packages (Image: Romano Dallai/University of Siena)

Species: Zorotypus impolitus

Habitat: The rainforests of Peninsular Malaysia

If it exists, then so does porn featuring it. So says “rule 34” of the internet. It seems unlikely, however, that anyone has made a pornographic film inspired by the ground louse Zorotypus impolitus. It doesn’t even have a Wikipedia page (yet).

More to the point, it would be surprising if anyone wanted to replicate its peculiar sex life. The male produces a single sperm that is almost as long as he is, wraps it in a package called a spermatophore, and sticks it on the female’s abdomen. It’s then up to the female to transfer this lone giant sperm into her genital tract. While most insects abandoned this primitive mating system millions of years ago, Z. impolitus has stuck with it.


Lousy

Z. impolitus belongs to a rare and obscure order of insects called Zoraptera. They are commonly called ground lice or, for no good reason, angel insects. Just a few millimetres long, the creatures look rather like termites and spend their lives in piles of rotting wood, where they feed mostly on fungi.

Obscure they may be, but the ground lice belong to the most successful group of animals on the planet: the winged insects, or Pterygota. Despite their vast diversity, all winged insects mate in more or less the same way: the male transfers his sperm directly into the female’s genital opening. “In general it is copulation,” says Romano Dallai of the University of Siena in Italy.

But Z. impolitus is the exception to that rule. Dallai and colleagues kept colonies of Z. impolitus in their lab, and monitored how they mated. To their surprise, the males didn’t bother to place their sperm inside the female’s genital tract.

“This is the first time we have described external transfer of sperm in a Pterygote insect,” says Dallai. In insects, external transfer is only found in ancient wingless groups like springtails, but they use yet another system. “The male deposits a spermatophore on the soil and then the female picks it up,” says Dallai.

Z. impolitus is a bit more advanced than the springtails, and may represent an intermediate stage in the evolution of true copulation. Male crickets are a step further along. Although they place their sperm in the female’s genital opening, “part remains outside and the female eats it”, says Dallai.

Hello sexy

Everything about Z. impolitus mating is strange. The female starts the process, approaching the male and stroking him with her antennae. If the male wants to mate, he moves behind her and performs a simple dance: he walks forwards and backwards, lowers his head and vibrates his antennae.

The climax of the process is when the male slips underneath the female for a few seconds and attaches a spermatophore to the female’s abdomen: a tiny package with a large surprise. “It [the sperm package] is the smallest we have seen in all insects,” he says. Whereas other species make spermatophores up to 2 millimetres across, those of Z. impolitus are just 0.1 millimetres across.

When Dallai dissected some of these spermatophores, however, he found that each one contained a single sperm about 3 millimetres long – about as long as the female. This seems strange. Males generally want to maximise their chances of fertilising the female’s eggs, so why produce only one, giant sperm?

Dallai thinks it may be a way of outcompeting other males. “The sperm is so large, it can fill the space in the female’s [genital tract],” he says. That plugs it up, so no other male can mate with her. Also, by using only one sperm at a time, the male ensures he gives each female just enough to fertilise her, while leaving him plenty to fertilise other females.

Journal reference: Naturwissenschaften, DOI: 10.1007/s00114-013-1055-0