Several events which precede adult emergence in Aedes aegypti were studied, including changes in buoyancy, changes visible through the pupal cuticle and changes at the foregut-midgut junction. Our data suggest tht the gas which is present in the posterior midgut at the time of emergence originates in the tracheal system. During the process of emergence this gas moves into the exuvial space through the adult spiracles and then follows the exuvial fluid into the alimentary canal.