Many members of the earwig insect family have two penises, but you'd never know it from observing their sex lives. Most earwigs are exclusively "right-penised", which raises an important question: why bother having two penises if you only use one?

That's the question posed by a researcher at Japan's Rissho University. Noting that the current evidence suggests the single-penised earwigs evolved from double-penised ancestors, the question becomes whether earwigs ever really use both penises to begin with:

To date, examinations of double-penis earwig species have found that only a single penis is used per single copulation. These diversities in structural and behavioral aspects of genitalia raises the following intriguing questions: How are the two penises used? Why did a penis degenerate in several earwig families, and which one was lost?

Yes, those are certainly some intriguing questions. But the answers are way more intriguing:

Although there was no detectable morphological differentiation between the right and left penises, male L. riparia [earwigs] predominantly used the right one for insemination. This significant "right-handedness" developed without any experience of mating and was also manifested in the resting postures of the two penises when not engaged in copulation. However, surgical ablation of the right penis did not influence the insemination capacity of males. In wild-caught males, only about 10% were left-handed; within this group, abnormalities were frequently observed in the right penis.

Left-handed? Right-handed? Maybe I just believe in being accurate, but I believe the correct terms are left-penised and right-penised. Whatever the case, it's not looking good for the poor old left penis, as the researcher points out:

These lines of evidence indicate that the left penis is merely a spare intromittent organ, which most L. riparia males are likely never to use. Additional observations of five species of single-penis families revealed that the left penis degenerated in the common ancestor of this group. Considering the proposed sister relationship between the Labiduridae and the single-penis families, it is possible that such behavioral asymmetries in penis' use, as observed in L. riparia, are parental to the evolutionary degeneration of the infrequently used left penis.

So...if you don't use it, you lose it? I hope this will be a lesson to all male earwigs reading this: the left penis is a blessing, and you ignore it at your own risk.