The change of air is produced in the tracheal system by the enlargement and contraction, or bellows-action, of the abdomen, the segments sliding in and out with a telescopic movement, very perceptible in the larger insects.
Many insect larvæ live in water, but are still air-breathers. The mosquito is an example. Here the eighth joint of the abdomen sends off a long tube, crowned at the extremity with feathery bristles, which are brought to the surface of the water to seize a. drop of air. With this the larva descends and the repellent action of the setæ prevents the water from dissolving it. Some water-larvæ have tubes several inches long. Other larvæ, destitute of tubes, have the spiracles at the posterior end of the abdomen. In the pupa state the mosquito has two respiratory tubes on the back of the thorax.
The aquatic larvæ of the dragon-flies (Libellulidœ) have, as already mentioned, hair-like processes, "villi," of the lining of the rectum, which absorb air from the water and convey it into the tracheal system. It is difficult to determine if such larvæ should be regarded as aquatic or aërial. Certainly in their habits, and in their method of procuring oxygen, they are aquatic and possess gills. But the oxygen is not immediately given to the blood in the villi, as in true gills, but is conveyed through tracheæ just as in adult insects. On the latter account these larvæ are generally considered aërial.