Correction of negative buoyancy in the phantom larva, Chaoborus americanus

Fourth instar larvae of Chaoborus americanus correct a condition of negative buoyancy by increasing the amount of gas held by two pairs of tracheal sacs. Mass spectrometric analysis of sac gas during the period following the addition of weights to larvae, reveal that the added gas is derived from the dissolved gases present in the surrounding medium and that it enters by physical means. The response is characterized by a transient decrease in the rigidity of the sac wall. Electron and light microscopy reveal that muscular tissue and motor terminals are associated with the sac wall and that a resilin-like protein may be present. These results suggest that the increase in the amount of gas held by the sacs is a secondary effect of an initial expansion of the sac wall itself and that the cuticular lining of the sacs is an active participant in the adjustment process.