Effects of load-lifting constraints on the mating system of a dance fly

Load lifting was studied in nature for Hilara sp.nov. (Diptera: Empididae). During mating, male Hilara sp. flies present insect prey to females and then fly in tandem while the female consumes the prey. During prey transfer and initiation of copulation, males lift their own mass, the mass of the female, and the prey. The maximum mass that males carried was consistent with limitations predicted from laboratory experiments (maximum lift force of 54-63 N/kg of flight muscle). Load-lifting limitations must frequently constrain the size of females and prey that a male can carry in flight. Observed consequences of this constraint were that males in copula had significantly greater body mass and flight muscle ratio than randomly captured males; males and females mated assortatively for body size; and larger females were presented with smaller prey when male size was held constant. -from Author