It's age-old wisdom: if you have anything that's really important, you should keep a spare, too.
Thus males among the Labidura riparia species prudently have two penises, which are symmetrically paired. The insect generally uses the right-side penis when having sex. But in the event that that that penis has become damaged or lost, it can make do tolerably well with its backup, left-side penis. It does, however, have more sexual success with the right-side penis.
Researchers Yoshitaka Kamimura, Chin‐Cheng Scotty Yang, and Chow‐Yang Lee recently published their findings in the Journal of Evolutionary Biology. From the abstract:
Males of the earwig Labidura riparia (Insecta: Dermaptera: Labiduridae) have morphologically similar laterally paired penises, only one of which is used for inseminating the female during a single copulation bout, and thus provide a rare opportunity to address how selection pressure may shape the evolution of population‐level laterality. Our population studies revealed that in 10 populations, located at 2.23–43.3° north, the right penis is predominantly used for copulating (88.6%). A damaged penis was found in 23% of rare left‐handers, suggesting that the left penis can function as a spare when the right one is damaged. By pairing L. riparia females with surgically manipulated males, we found that males forced to use the right penis outperformed left‐handed males in copulation (the probability of establishing genital coupling during the 1‐hr observation period: odds ratio [OR] of 3.50) and insemination (probability of transferring a detectable amount of sperm: OR of 2.94). This right‐handed advantage may be due to the coiled morphology of the sperm storage organ with a right‐facing opening. Thus, female genital morphology may play a significant role in the evolution of handedness and may have acted as a driving force to reduce penis number in related taxa.
Citation:
Kamimura, Yoshitaka, et al. “Fitness Advantages of the Biased Use of Paired Laterally Symmetrical Penises in an Insect.” Journal of Evolutionary Biology, vol. 32, no. 8, Aug. 2019, pp. 844–855. Academic Search Complete, doi:10.1111/jeb.13486.
Photo: Donald Hobern