Carlos Cordero of the Autonomous University of Mexico, whose work on “caltrop cornuti” I mention in my book, sent me a few of his papers on the function of the “signa”. These are sharp structures inside the “corpus bursa” of female butterflies and moths, whose function has always been controversial, for lack of evidence for or against any of the seven (!) hypotheses. The corpus bursa is an organ in which the sperm package, or spermatophore, is deposited by the male before further use (which may involve digestion and/or fertilization of the eggs). It takes time for the spermatophore to be processed and all this while the female cannot be inseminated by other males. Therefore, it is in the male’s interest to increase the time it takes to process his spermatophore (for example, by providing it with a thick wall), while it is in the female’s interest to process the sperm bag as quickly as possible and to vacate the corpus bursa for new, possibly better, sperm. Cordero’s team studied the signa in several species of butterfly. They froze females after copulation and saw, frozen in time, the moment the signa actually pierced the envelope of the spermatophore. This suggests that the organ plays a role in sexually antagonistic coevolution (SAC): whether or not a male’s sperm is used for fertulization, is under female control. In another study, published in 2012 in PLOS ONE, he and his team found that in the evolutionary tree of butterflies, the signa were often lost when a species or group of species had evolved monandry: if a female mates with only a single male, the SAC setting disappears. Finally, in a paper in PeerJ earlier this year, the team demonstrate that in species of butterfly in which females mate with multiple males and use signa to rupture the spermatophore, the males have thicker spermatophore walls, to withstand the female’s “can opener”.