Spiracular Gills in the Marine Fly Aphrosylus and their Relation to the Respiratory Horns of Other Dolichopodidae

Abstract

The larva of the dolichopodid fly Aphrosylus celtiber Haliday is an omnivorous carnivore in the intertidal zone. It pupates in an air-filled cocoon of debris and a secretion of the labial glands. The pupa has plastron-bearing spiracular gills that have been evolved from respiratory horns similar to those of recent terrestrial Dolichopodidae.Although the spiracular gills are pupal structures, they are the chief respiratory organs of the young adult, i.e. the pharate adult, before it emerges from the cocoon. At the pupal-adult apolysis the layer of epidermis that secreted the gill wall and the layer that secreted the spiracular atrium are left behind in the lumen of the gill, together with blood, as the body wall epidermis retracts away from the pupal cuticle. A thin sheet of cuticle, the basal membrane, is secreted across the opening into the gill lumen at this time. The epidermis and blood in the gill lumen are thus completely isolated from the haemocoele of the living animal by the basal membrane, a layer of moulting fluid, and the adult cuticle. The tissue contained in the respiratory horns of terrestrial Dolichopodidae is isolated in a precisely similar manner.The tissue isolated in the lumen of the gill of Aphrosylus does not degenerate until after the pupal-adult ecdysis. It seems to be discarded because, on balance, its loss is less damaging than the cost that would be incurred in other ways by its absorption or retraction into the body of the adult. For instance, in order to absorb the tissue in such long appendages, or to retract all of the epidermis from within them, would necessarily prolong the moulting period, a period when the insect is particularly vulnerable.