Physical restraint or stimulation?: The function(s) of the modified front legs of male Archisepsis diversiformis (Diptera, Sepsidae)

Four hypotheses that could explain the elaborate species-specific morphology of the clasping organs on the front legs of male Archisepsis diversiformis flies were tested: direct male-male combat, mechanical fit, male-female conflict of interests, and male stimulation of the female. Experimental modification of the shape of the male clasper and of the female wing base where the male clasped the female both strongly reduced the chances that a mount would result in copulation. This reduction was not predicted by the male-male combat hypothesis but was predicted by the others. Males in the field did nave to fight other males to remain mounted on females, as expected by the male-male combat hypothesis. Reduced male copulatory success was not due to inferior male ability to grasp and hold onto the female's wings, as predicted by the mechanical fit and male-female conflict hypotheses but to a reduction in the likelihood that the female would allow intromission, as predicted by the stimulation hypothesis. By a process of elimination, and in combination with data from a previous morphological study, the data support the hypothesis that the species-specific aspects of grasping organs in these flies function to stimulate females. Further behavioral data will be needed to test alternative possibilities.