The Midcoxal Prong in Flies (Diptera: Brachycera)

Abstract

The midcoxal prong [MCP] is a fine outgrowth from the distal sclerite of the coxa into the space between the coxa and the trochanter. It is present in members of all but a few families of 'higher flies'. The morphology of MCP was investigated in SEM preparations. Walking in Eristalis and tethered flight in Calliphora were videorecorded in intact flies and after removal of one or both MCPs. Muscle potentials were recorded in tethered Lucilia. MCP morphology is the same in males and females. It lacks any sensilla. No reflexes in leg muscles were recorded after touching MCP. It presumably performs some mechanical function. During walking as well as grooming, start, landing reflex etc the prong glides freely over the trochanter. During flight, the leg is placed in its peculiar flight position by supination of the trochanter, and the prong is clumped between the coxa and the trochanter. The trochanter presses the anterior hairy face of MCP against a hairy field on the anterior coxal sclerite. The morphology and biomechanical properties of the middle coxa suggest that MCP acts as a sort of a Velcro fastener, locking the middle leg in its flight position. However, excision of MCP did not impair either walking or the ability to place the middle leg in a proper flight position and to hold it against a strong airstream. So the function of this strange organ is still unknown.