The tracheae and tracheoles are respiratory surfaces in insects, in the same sense that lungs are the respiratory surfaces in mammals and gills are the respiratory surfaces in fish. Because water readily traverses these surfaces, respiratory surfaces are major avenues of water loss in terrestrial organisms. Water loss is reduced when the respiratory surfaces are somehow covered. Internalization of the respiratory surfaces is regarded as a major adaptation for terrestrial life. Although their respiratory surfaces are internalized, insects rapidly desiccate when their spiracles are open.
Many insects regulate spiracle opening in response to respiratory requirements.
This is best studied in a large insect, the desert locust, Schistocerca gregaria . There are two modes of ventilation in this grasshopper, one when the insect is at rest, and another during flight. During rest spiracles 1 and 4 on both sides open during abdominal expansion, which results in air inspiration. Then spiracles 1 and 4 close, and spiracle pair 10 opens while the abdomen contracts, and air is expelled. Spiracles 2 and 3 are open continually during flight.