Abdominal pumping causes increased hydrostatic pressure and organ movements which in turn press on the air sacs. Air is forced out of the sacs. Air is rhymthmically drawn into the air sacs by muscular or elastic expansion of the abdomen. True bugs and beetles move the turgum up and down. Dipterans and hymenopterans move both the turgum and sternum.
Some hymenopterans can also telescope the abdomen in and out.
In all of these ventilation mechanisms, it is the major tracheal trunks that are ventilated. We can identify the sites of gas exchange in animals. 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.