Isometric patterns for male genital allometry in four damselfly species

Recent studies have found that insect genitalic traits show negative allometry, i.e., are relatively small in relation to body size. One interpretation of this is that males use their genitalia to stimulate females. Thus, given the nature of damselfly copulation in which males physically reach the rival sperm that females have stored from previous matings, male genitalic traits are not expected to show negative allometry. To test this idea, we assessed (a) the rival sperm displacement function by the mating male and (b) allometry of aedeagal length of four damselfly species (Argia anceps, Argia tezpi, Argia extranea, and Enallagma praevarum). Sperm displacement was assessed by inspecting whether the aedeagus reached the rival sperm during copulation in mating pairs for the four species. To have a standard for comparing allometric patterns, allometry of aedeagal was compared to that of two non-genital traits, tibial, and fourth abdominal segment length. In all cases, the aedeagus was found to reach the rival sperm which supports the idea that stimulation is not the mechanism for sperm displacement but physical displacement. Aedeagal length was isometric, and its slope was lower in general compared to that of tibial length and fourth abdominal segment. Given that this isometric pattern is not common for other odonate species, our interpretation of these varying aedeagal scaling patterns in this insect order is that males’ and females’ sexual interests are in conflict (males are evolving an elongated aedeagus to reach rival sperm while females are evolving unreachable sperm storage organs to prevent displacement of stored sperm). This sexual conflict scenario would favor varying scaling patterns for aedeagal length in odonates. A final interpretation is that the risk of interspecific matings in damselflies, may also explain different species-specific, aedeagal allometries.