Functional morphology of sexually selected gentalia in the water strider Aquarius remigis

In the water strider Aquarius remigis (Say), sexual selection favours males with longer genitalia. We used video analysis plus light and scanning electron microscopy to examine the structure, movement, and articulation of the genitalia as a first step in determining the functional basis of this selection. Male A. remigis are characterized by long, robust genital segments; a marked ventral median notch in the posterior margin of the pregenital segment; an enlarged phallus tipped with a uniquely prolonged sclerotized plate; and a large, spine-covered, membranous lobe that inflates within the female's reproductive tract. Detailed examinations of genital interactions prior to and during copulation allow us to deduce the functional significance of these distinguishing traits. We postulate that by increasing the length, mobility, strength, and rigidity of the phallus, the first three traits increase the ability of males to achieve intromission in spite of active female resistance, and to maintain intromission during the prolonged copulations characteristic of this species. Inflation of the large, spinous lobe probably contributes to the latter function, and may also directly affect fertilization success by displacing or damaging sperm from previous males. We discuss these interpretations in the context of current theories of genitalic coevolution and sexual conflict in the Gerridae.