When mayflies have an erection: functional morphology of the genitalia in Ecdyonurus (Ephemeroptera: Heptageniidae)

The aerial copulation of the mayfly Ecdyonurus venosus (Fabricius, 1775) and Ecdyonurus submontanus Landa, 1969 is described based on SEM, dissections, histological serial sections, and µ-CT. The abdomina of a pair in copula and a resting male specimen are reconstructed, with emphasis on sclerites, musculature and associated muscles of the genitalia and the male gonopods. The resulting models reveal a change in penis configuration while the penis is located inside the female vestibulum. This change is facilitated by a pair of intrinsic penis muscles. Its action leads to a depression of the penis shaft sclerite, which is associated with an upward rotation of the penis lobes and the eversion of a pair of spike-like titillators. Once exposed inside the female vestibulum, the penes are slightly moved dorsally. This movement is facilitated by a pair of extrinsic penis muscles. Thereby, the everted titillators are pricking into the female intersegmental membrane between sternites VII and VIII. In this way, the titillators act as an anchoring device, fixing the penes within the female vestibulum and enlarging the latter during copulation. The evolutionary implications of these findings are discussed, and a new nomenclature for the copulatory organs is proposed.