Traumatic insemination and sexual conflict.

PZ Myers over at Pharyngula has a great picture of a spiny beetle phallus. It’s a wicked, mace-like thing that is used to hold onto the female during mating. It’s also a great example of how we as humans tend to be biased by our own experiences.

You see, sex for us is incredibly fun and something like this seems weird to us. For many insects, not so much. Insect sex can be crazier than anything we can imagine. I’m not talking like drunken ex-girlfriend hotel room crazy sex. It’s common for females to fight off males and for males to need to be able to hold on to their mates or to force them into submission. Competition for mates can be fierce in the insect world and in addition to this, many females only mate once.

There’s a way that many insects have evolved to get around this…one is by something called traumatic insemination.

And yes…that’s exactly what it sounds like.

Traumatic insemination is the term for when the male of a species peirces the female’s cuticle outside the body wall with his genetals and introduces sperm into the body cavity. It’s extremely common in bed bugs, and has evolved in other insects as well.

The reason this works is because insect bodies are set up a bit differently than ours. Instead of a vascular system, they just circulate nutrients in their interstitial fluid (hemolymph) that is kept circulating by a heart on the top of their body. After sperm is introduced straight into the body cavity, the sperm just kind of migrate and find the ovaries.

Here’s a simplified diagram of what I’m talking about:

Bed bugs have actually evolved a paragenital system where the male squeeges into something that isn’t at all related to the reproductive system. This is their main mode of reproduction and the reproductive tract mainly functions in the laying of the eggs rather than the combined function of introducing sperm and laying eggs.

It’s an interesting example of sexual conflict. You’d think that the males and females have this one common goal of reproduction, but that isn’t the case. Successful reproductive strategies for males and females can often be quite different, and this can set the stage up for an arms race between males and females which can lead to some pretty spectacular adaptations. Let’s take this bed bug genetalia below, for example.

Here, you can see how bed bugs mate. A is the organ the males use to copulate. When he’s not doing the nasty (stabbing his current girlfriend in the stomach), he stores it in the structure C. These guys litterally have to sheath their penises!

In picture B, you can actually see the wound caused by the male during insemination. It’s not exactly a small wound and you can see it even in this tiny picture.

Even though this offers an alternate way for the male to inseminate the female, this also has trade-offs. Healing wounds can be kind of expensive in terms of resources, and this offers a nice route for the female to get infected by various pathogens. She has to spend energy repairing the damage from her rough night out, as well as fighting infections. It’s not beneficial for females, and females exposed to multiple mating attempts die at greater rates than females who don’t mate very much. Females who mate more than once also lay fewer eggs in a lifetime than do females who mate many times. It seems counter-intuitive that females who have more sex would lay fewer eggs, but you also have to remember that females need to patch the damage caused by their boyfriends. The current thinking on this is that the females need to shunt resources to things like fighting infections and wound healing in order to patch themselves up after mating.

It may be incredibly brutal, but it works quite nicely. For the males, anyways.
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A. D. Stutt (2001). Traumatic insemination and sexual conflict in the bed bug Cimexlectularius Proceedings of the National Academy of Sciences, 98 (10), 5683-5687 DOI: 10.1073/pnas.101440698