As the muscles of the thorax contract during the wing stroke, the thoracic box alternately increases and decreases in volume. This action presses on air sacs in the thorax, causing them to experience volume changes during each wing stroke. Many insects actively ventilate the thorax by compressing large abdominal air sacs. This is known as abdominal pumping, and is done by contracting intersegmental muscles in the abdomen. 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.