Ultrastructure of the atypic muscles associated with terminalial inversion in male Aedes aegypti (L).

Two sets of opposed, crossed muscles are present in the rotating region of the abdomen in male A. aegypti. These muscles undergo changes during rotation of the genitalia that suggest they function as the driving force for rotation. During this rotation, one muscle of each set contracts and the opposed one becomes elongated. The contracting muscles are atypic physiologically. They contract from 300 mum to about 69 mum, and this requires a period of 18 to 24 hours. They shorten only once and those muscle fibers still present after the completion of rotation remain in a contracted condition at least for two weeks. The elongated muscles never shorten; they become stretched to approximately 2.5 times their original length and disappear soon after rotation is completed.