The Anal Organ of Larvae of Musca autumnalis, M. domestica, and Orthellia caesarion (Diptera: Muscidae)1

The anal organ of 3 terrestrial dipterous larvae is described. This organ is difficult to see in unstained larvae, but when placed in acetocarmine or a dilute solution of silver nitrate it stands out. The organ is situated ventrally on the last larval segment and surrounds the anus. The cuticle of this organ is much thinner and appears more permeable than elsewhere. Underlying this cuticle are large epidermal cells containing polytene chromosomes. An infolding of the epicuticle surrounds the organ separating it from the surrounding cuticle and epidermis. The differences in the size of the anal organ for several members of the genus Musca are discussed with particular reference to its significance to the type of larval habitat and its importance as a taxonomic character.