To keep body weight to a minimum, large flying insects decrease the amount of water in the hemolymph. The remaining amount of hemolymph is first delivered into the thorax and directed into the wing veins. To make room for the hemolymph in the wing veins, an accompanying tracheal tube collapses as the hemolymph is pumped into the vein. Thus, the space in the wing veins is first taken up by hemolymph at the expense of air in the tracheal tube and then, as the hemolymph is pumped out, the air reenters the wing vein. Lutz Wasserthal called this periodic exchange of air and hemolymph a "tidal flow" of hemolymph.
The entire circulatory system of the insect is recruited to operate the tidal flow of hemolymph in large flying insects. Thus both the dorsal vessel and the ventral diaphragm are recruited to direct the hemolymph in the proper direction during each tidal cycle. This implies a very sophisticated control mechanism that must operate from the central nervous system.
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