It seems an obvious result: researchers lop off part of an insect’s penis and find it doesn’t work so well. But is this as cut and dried as it seems?

Scientific papers are not known for their brilliant titles. But today, the Royal Society has published a wonderful exception.

“Experimental reduction of intromittent organ length reduces male reproductive success in a bug”

It’s a good title on so many levels. There is the initial surprise of translation. “Experimental reduction” becomes “chopping off”, “intromittent organ” is the technical term for the genitalia of male insects and “reproductive success” means productive sex. In essence, researchers have chopped off an insect’s penis and find it doesn’t work so well.

The shock that results from translation is quickly overwhelmed a second, crashing wave of bewilderment at the seemingly self-evident nature of this observation. The discovery that lopping off a significant chunk of a male’s genital apparatus does not help his sex life is so unsurprising it is surprising.

By now, the title has more than done its job. It has amused. It has bewildered. Above all, it it begging questions (notably “why?” and “what?”) and the only way to get some answers is to go to the paper itself, published today in Proceedings of the Royal Society B.

Here, I discover several things. The experimental insect – the seed bug Lygaeus simulans – has an intromittent organ around 7 mm long. This might not sound like much, except that the insect itself is only 11 mm, so it has a penis about two-thirds of his body length. Whilst this sounds unwieldy, it is not unusual amongst insects. “Such an extremely long male intromittent organ is common in the Heteroptera and is also found in several other insect groups,” the researchers note.

It seems pretty obvious that such extraordinary genital morphology is principally the result of sexual selection, the iterative, generation-after-generation struggle to leave more descendants than your competitors. An alternative possibility is that a penis two-thirds your body length could confer some survival advantage, though this seems unlikely. Or perhaps God personally shaped all these penises, but if he did he is not loving. In many species, males have genitals that both resemble and are used like weapons.

In many species, males have genitals that both resemble and are used like weapons, as in the male cowpea weevil Callosobruchus maculatus Photograph: Johanna Rönn/CC-BY-SA

But what is the precise advantage that a male gets from having such a protracted organ? It is this that the researchers seek to explore through experimentation. In most species, cutting the penis would result in a catastrophic loss of function, not to mention serious trauma. But in the seed bug, the intromittent organ is a long, thin tube with no obvious musculature or vascularization, a straight-forward, pipe-like structure that makes it amenable to experimental reduction. “We can show that males that had the penis cut, but not reduced in length significantly, had the same reproductive success as males with normal penises,” says Liam Dougherty, a postgraduate researcher at the University of St Andrews and the lead author on the paper.

But when researchers made more significant experimental reductions, it had a clear effect upon reproductive success. Males with a penis shortened by just 5% had significantly fewer offspring than intact males. A more radical snip of 30% cut the time spent copulating in half and drastically reduced insemination success and number of offspring. “By showing that the length of the penis influences reproductive success, we can say that it has been sexually selected to some extent,” says Dougherty.

Interestingly, the male seed bug’s penis is around three times the length of the female’s spermathecal duct, the narrow tube through which it must be threaded if it is to get to the the female’s sperm storage organ. The long penis is obviously up to something other than simply reaching directly for the spermatheca. Indeed, micro-CT scans of copulating bugs reveal that the male’s penis gets up to some impressive coiling antics inside the female. It is this coiling, suggests Dougherty, that may provide the ultimate explanation for the seed bug’s extraordinary penis.



Dougherty, L.R. et al. (2015) Experimental reduction of intromittent organ length reduces male reproductive success in a bug. Proc. R. Soc. B 20150724