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Living and dying by the sword

by Rowan Hooper

Alfred, Lord Tennyson famously drew attention to the rigors of the natural world when he wrote of “Nature red in tooth and claw.” His poem, “In Memoriam,” was published in 1859 (the same year as “The Origin of the Species”). But had Tennyson known of the sexual habits of the common bedbug, and if he could have swallowed his Victorian sensibilities, he might have written, “Nature red in tooth, claw . . . and penis.”

Instead of becoming poet laureate, Tennyson might then have become the Bret Easton Ellis of the 19th century, for the story of the bedbug is among the most gruesome in the insect world, a world already renowned for its stomach-churning weirdness. The squeamish and those who happen to be eating as they read this are hereby warned.

Bedbugs are in some ways pretty standard insects. The female has ovaries and a reproductive tract and lays eggs. The male has testes and a penis. So far, so normal. Their feeding habits are fairly repulsive to humans, but more of that later. It’s how they copulate that is really different.

The male’s penis is shaped like a samurai sword: sharp, curved and pointed. With it he hacks through the body wall of the female, entirely bypassing the vagina, and inseminates sperm directly into her body cavity. Sperm then migrate to the ovaries and fertilize the eggs.

Traumatic insemination, as it is dryly referred to by entomologists, is rare but actually fairly widespread in the invertebrate world. It is now the subject of a paper in the journal Proceedings of the National Academy of Science. The authors, Alastair Stutt and Mike Siva-Jothy from the University of Sheffield, U.K., describe how this battle of the sexes exacts serious costs to females.

The male’s penis (“intromittent organ” in entomological parlance) pierces the female at a predetermined point on the body wall. (The original female genitals are not used by any species of bedbug.)

Have females done anything to limit the damage?

This is what the researchers were interested in, since evolutionary theory predicts that whenever there is a conflict of interest, there will be an “arms race.” This means, for example, that when the cheetah evolved stronger muscles to enable it to run faster, the antelope countered (over evolutionary time) with its own increase in speed. Researchers have only recently realized that the conflict of interest applies equally to males and females of the same species as it does to predator and prey. What is good for males, after all, may not be the same as what is good for females.

To see if counter-adaptations have evolved to reduce the cost of copulation, Stutt and Siva-Jothy first had to show that traumatic insemination does harm females. To do this, they determined how many times a female would need to copulate to get enough sperm to fertilize all her eggs. If the bugs had sex more often than they needed to for maximum fertility, and if their life spans became shorter because of the violent nature of the act, it would suggest that females were losing the battle of the sexes.

This is exactly what the researchers found. Traumatic insemination reduces the females’ life span and reproductive output, bringing a new meaning to the phrase “live by the sword, die by the sword.”

Why haven’t females evolved a counter-adaptation? They have, say the researchers. The female has a “paragenital tract” — a secondary genital tract that underlies the region where the male pierces the body wall, and, like armor, buffers the wall against damage. “This seems to function to diminish the potential effects of wounding and infection caused by traumatic insemination,” the authors write.

On the off-chance that this has generated sympathy for the traumatically inseminated female bug, remember how they live and feed. They are blood-feeding parasites of humans, bats and chickens. They live in crevices and mattresses during the day and bite at night.