TheaperC (abnormal proboscis extension reflex C)mutation ofDrosophilacauses the degeneration of a particular pair of head muscles involved in the proboscis extension. To define the mechanisms underlying the muscle degeneration, we studied the influence of use or disuse on the muscle degeneration by two kinds of immobilization experiments. In the first experiment, the rostrum was fixed onto the cranium and the extension of the proboscis was mechanically prevented. In the second experiment, we created anaperCaperAdouble mutant, genetically preventing proboscis extension with theaperAmutation. Both immobilizations suppressed the muscle degeneration inaperCmutant flies.