Loaded flight in male Ischnura elegans and its relationship to copulatory flight.

Copulation in the blue-tailed damselfly (Ischnura elegans) can last several hours, during which the pair may fly together in the 'wheel position' with both insects flapping their wings. Previous studies have suggested that during flight in copula, the male increases its power output while the female decreases it. Consequently, the male must support some of the female's body weight in the air. We tested the hypothesis that female body mass places a biomechanical constraint on the ability of smaller males to mate with larger females by attaching weights to male damselflies and analyzing their wing motion and force exerted using high-speed cameras. Males flying with an added load exerted extra forces equivalent to 157% of their body weight. Males flying in the mating wheel position with females whose wings were clipped bore a similar weight and were barely able to fly. To fly with an added load, males increased their wing-flapping frequency and amplitude, reaching values of mean wing tip flapping speed that were 1.9-fold higher than that in solitary flight. Our experiments indicate that although males would be able to fly briefly with the added weight of a non-responsive female, the flight performance of the pair would be severely compromised without the female contributing effort to the joint flight.