The spiracular gill if the fly Orimargula astraliensis and its relation to those of other insects.

Abstract

The spiracular gills of the aquatic pupa of the fly Orimargula australiensis are the chief respiratory organs of the pharate adult before it sheds the pupal cuticle. Although the spiracular gills are pupal structures, they do not function in the pupal stage because the pupal-adult moult occurs before the larval-pupal ecdysis. Each of the two spiracular gills is five-branched, and a branch of the spiracular atrium is contained within each gill branch. The surface of each gill supports about 700 plastron lines, each of which opens into a cuticular tube or aeropyle that in turn opens into the spiracular atrium. At the pupal-adult moult the layer of epidermis that secreted the gill wall and the layer that secreted the spiracular atrium are left behind in the gill lumen as the body wall epidermis retracts from the pupal cuticle. At this moult a thin sheet of cuticle, the basal membrane, is secreted across the opening into the gill lumen. The blood and epidermis in the gill lumen are thus completely isolated from the living animal by the basal membrane, a layer of moulting fluid, and the adult cuticle. The blood and epidermis discarded and isolated in the lumen of the gill at the pupal-adult moult are essential for the proper functioning of the gill during the pharate adult stage. Absorption of water by the isolated tissue maintains the turgidity of the gills. When the gills are fully stretched and there is no net transferance of water into them, the osmotic pressure of the isolated tissue is 4.5-4.8 atmospheres. The internal hydrostatic pressure or turgor pressure of the gill is thus equivalent to its osmotic pressure less the osmotic pressure of the stream water, which is probably not more than about 0.1 atmospheres. The isolated tissue is competent to repair breaks in the gill wall with sclerotin during the life of the pharate adult. This competence is retained even after the pupal cuticle is shed and the adult has flown away. The structure and function of the spiracular gills of Orimargula are compared with those of other Diptera. The selective value of the tissue isolated in the gills of different species is very diverse, and in some groups the isolated tissue appears to have no selective value. The significance of the reduction or total suppression of the nonpharate pupal stage in the Diptera is discussed.