Flying with eight wings: Inter-sex differences in wingbeat kinematics and aerodynamics during the copulatory flight of damselflies (ischnura elegans)

Copulation in the blue-tailed damselfly, Ischnura elegans, can last over 5 hours, during which the pair may fly from place to place in the so-called Bwheel position. We filmed copulatory free-flight and analyzed the wingbeat kinematics of males and females in order to understand the contribution of the two sexes to this cooperative flight form. Both sexes flapped their wings but at different flapping frequencies resulting in a lack of synchronization between the flapping of the two insects. Despite their unusual body posture, females flapped their wings in a stroke-plane not significantly different to that of the males (repeated-measures ANOVA, F_{1, 7} = 0.154, p = 0.71). However, their flapping amplitudes were smaller by 42 ± 17% compared to their male mates (t test, t₇ = 9.298, p < 0.001). Thiswas mostly due to shortening of the amplitude at the ventral stroke reversal point. Compared to solitary flight, males flying in copula increased flapping frequency by 19%, while females decreased flapping amplitude by 27%. These findings suggest that although both sexes contribute to copulatory flight, females reduce their effort, while males increase their aerodynamic output in order to carry both their own weight and some of the female’s weight. This increased investment by the male is amplified due to male I. elegans being typically smaller than females. The need by smaller males to fly while carrying some of the weight of their larger mates may pose a constraint on the ability of mating pairs to evade predators or counter interference from competing solitary males.