It’s a curious fact of evolutionary history that Darwin did not quite reel in the catch he had hooked with his theory of sexual selection. He applied it to all kinds of external ornaments (bird plumage, dung beetle horns), but fell short of including elaborately-formed genitalia–leaving that one up for grabs; and it took until the 1980s for Bill Eberhard to do the grabbing. But this was not for lack of inspiration. In his monograph on barnacles, Darwin described what still counts as the longest (relatively speaking) penis of the animal kingdom: the burrowing barnacle’s. This video by Stefan Siebert shows a similarly sizeable member in another barnacle species. It is this video that was the basis for the barnacle illustration in my upcoming book.