For centuries, taxonomists smugly leant back in the knowledge that the amazing diversity in genital forms could be explained with the Lock-and-Key principle. Or, as French fly specialist Léon Dufour conceived it in 1844, “L’armure copulatrice est un organe ou mieux un instrument ingénieusement compliqué, destiné à s’adapter aux parties sexuelles externes de la femelle pour l’accomplissement de l’acte copulatif; elle est la garantie de la conservation des types, la sauvegarde de la légitimité de l’espèce.” That is: “The copulatory apparatus is an organ, or rather an ingeniously constructed instrument, destined to adapt to the female sexual parts for achieving the act of copulation; it is the guarantee for the preservation of types, the safeguard of the legitimacy of the species.”

Sounds plausible, right? but in Chapter 2 of Nature’s Nether Regions, I explain why the lock-and-key hypothesis is now sinking to the bottom of the scientific sea.

The article by Dufour is: Dufour, L., 1844. Anatomie générale des Diptères. Ann. Sci. Nat., 1: 244-264. Unfortunately, this paper is not available online. But much of Dufour’s other work can be found via the goldmine that is the Biodiversity Heritage Library.