At the hatching and moulting of Aedes aegypti the fluid present in the tracheal system is removed, apparently throughout the whole system, by an active absorption which takes place only when the respiratory siphon is open at the surface so that air can enter. The nervous system is apparently concerned in the initiation of this absorption.
First-stage larvae kept under water can retain fluid in the tracheal system for several days and absorb it when they get access to the air. This ability is gradually lost if filling is deferred for more than 3 days. The tracheal system then becomes filled with air for the first time after moulting to the next stage. Second-stage larvae must absorb the fluid within a few minutes of moulting or they are unable to do so.