The very numerous supernumerary stigmata on the surface of the respiratory lobes of 3rd instar tsetse larvae open into outer air chambers, three in each lobe. Each stigma is guarded by a valve which allows the entry of air but prevents its exit. The outer air chambers communicate with inner air chambers through the so-called felt chambers, which may be regarded as perforated pistons; and the inner air chambers are continuous with the lateral tracheal trunks of the larva. Immediately in front of the base of the lobes they are connected by a posterior transverse commissure, and associated with this commissure are the respiratory muscles. The 2nd instar tracheal system is persistent and lies contained within the 3rd instar system except for the posterior part of the main lateral trunks which open separately between the respiratory lobes. Contraction of the respiratory muscle, by displacing the felt chamber anteriorly, causes intake of air through the supernumerary stigmata, at the same time forcing air forwards between 2nd and 3rd instar tracheae, backwards within the 2nd instar tracheae and out through the 2nd instar spiracles. REturn of the felt chamber to its original position does not involve a reversal of air flow, since egress of air through the supernumerary stigmata is prevented by closure of the respiratory valves. The mechanism described is functional only during the last third of intra-uterine life.