Sex, lies, and sperm competition: the fiendish world of insect sex

When it comes to sex, evolution will select whichever strategy works best. It cares nothing for propriety, and doesn’t mind offending your delicate sensibilities ‒ as long as sperm meets egg in most efficient way possible. Our very own Mindboggler Ella has already described some of the weird and wonderful sexual behaviours you’ll find in nature, but that’s not the worst of it. So take a deep breathe and cross those legs nice and tight, because you’re about to enter the gruesome world of the insect.

Bean weevils have a terrifying penis

Your eyes have already been drawn towards the next image, so let’s not beat about the bush: yes, those are real spines, and, yes, this story is going to be as horrifying as you think it is. The bean weevil mating process starts off just like any other sexual encounter, with the male penetrating the female and ejaculating his sperm. However, on its way out, the male’s spiny penis drags inside the female, shredding up her reproductive tract like an ice-pick scraping against a wall of cheese.


I was so aghast by this image that I couldn’t think up a decent metaphor. Source: Wikimedia Commons

What possible reason could the male have for maiming his sexual partner in this way?

In the animal kingdom, males are always competing against each other, both before mating (intrasexual selection) and after it (sperm competition). Once a male has mated, it’s in his best interests to ensure that she doesn’t mate with anybody else. There are several possible approaches for this:

  • mate guarding, the male just hangs around the female and thwarts any future mating attempts
  • copulatory plugs, chemicals released into the female that block any extra sperm

For a long time, scientists thought that male bean weevils were taking this selfish behaviour to new extremes, vandalising the female’s reproductive tract to make damn sure she doesn’t mate again.

Bean weevil full

Yes, you do well to hang your head in shame. Source: Wikimedia Commons

But more recent research suggests a slightly different theory. Two researchers in Sweden conducted an experiment comparing the size of a male’s penis spines with his success in fertilising a female’s eggs. They found that larger spines equalled greater success, but we’re still not sure why ‒ perhaps because longer spines allow the male to anchor or better position himself during sex. Any damage caused to the female seems to be an unfortunate by-product of this male competition.

Female bean weevils, of course, weren’t going to take all this abuse lying down. Their reproductive tracts have evolved large amounts of connective tissue to resist (at least partly) the male’s spiny penises, and, when they’ve had enough of sex, they just start kicking until he lays off.

Bed-bugs just have a stab

If you’re a male bed-bug, you don’t concern yourself with details. For example, female bed-bugs have a perfectly good reproductive tract, but males pretend not to notice it.

Rather than inserting their reproductive organs into the normal place, male bed-bugs have evolved a needle-like penis that they simply thrust into the female’s abdomen, forcibly injecting their sperm directly into her abdominal cavity (or haemocoel).

Bed bugs

Whether she likes it or not. Source: Wikimedia Commons

This horrific behaviour is called traumatic insemination, and, once again, is probably the result of male sperm competition. In the ever-mounting race to deposit sperm as close as possible to the ovaries, some males started to bypass the female reproductive tract entirely.

The practice is as damaging as it sounds; mating wounds can often become infected or trigger unwanted immune reactions. In response, females have evolved a special-purpose organ called the spermalege, which provides a target for sexual penetration and limits the amount of damage caused to the rest of the body (I usually avoid linking to Wikipedia articles, but I wrote most of this one myself, so it’s all good).

However, it’s not only females who should be worried. Bed-bugs are pretty much unable to tell the difference between sexes, and instead use size as an indicator of attractiveness. That means any recently-fed bed-bug ‒ male or female ‒ is liable to be mounted and subjected to the rough treatment. To avoid unwanted homosexual harassment, male bed-bugs release an alarm pheromone whenever they’re mounted by another male. This pheromone is traditionally a tool for dissuading predators, but here it’s found a new use in what basically amounts to the cry, “Don’t do that ‒ I’m a dude!”


This is about as attractive as bed-bugs get. Source: Wikimedia Commons

Intriguingly, females never release these pheromones in mating contexts, even though they’re perfectly capable of doing so. Perhaps that’s a sign that we shouldn’t feel too sorry for them. Bed-bugs are seemingly quite happy doing whatever it takes to spawn the next generation, and who are we to argue with that?

Andrew Katsis is an MSc candidate in zoology at the University of Melbourne. For more of his biological musings on the evolution of sex, you can click here (kissing), here (sexual attraction) or here (human pheromones).

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