The spirmular gill of Deuterophlebia is the chief respiratory organ of the pupa and pharate adult. It is formed from the body wall adjacent to the spiracle. Its surface everywhere supports a plastron that opens into the atrial chamber of the spiracle at the base of the gill. Blood and epidermis are isolated in the spiracular gill after the larval-pupal ecdysis but before the completion of the pupal-adult moult. After the pupal-adult moult, but before the pupal-adult ecdysis, the epidermal cells of many parts of the gill dissociate, become rounded, and separate away from one another and the cuticle. In due course many of these cells form loose clumps in the lumen. The epidermal cells do not degenerate during the life of the pharate adult. The structure and function of the spiracular gills of the Deuterophlebiidae and other Diptera are compared.