Thoracoabdominal Shunting

In some active insects, butterflies, beetles and flies, hemolymph shunts back and forth between the thorax and abdomen as a flap of fatty tissue or large air-filled sacs reverse periodically near the pedicle when pressures in the abdomen and thorax shift back and forth, alternately opening and closing a tubular connector in the pedicle between the thorax and abdomen. When the abdomen contracts, abdominal pressure rises to exceed hydrostatic pressure in the thorax so it pumps hemolymph forward into the thorax and head. When the heart reverses and pumps hemolymph backwards, this new volume actively expands the abdomen, and the expansion draws hemolymph past the flap of fatty tissue. In such active insects, movements of the hemolymph correlate with movements of the tracheae. These reversals of pressures and flows serve to control oxygen flow to the muscles of the wings and legs in the thorax and to exchange heat from these same muscles through the abdomen (Chapter 10).

Some insects circulate hemolymph through their wings. This circulation may occur in young adults as wings stiffen and become flight-ready. In the absence of circulation through the wings, the tracheae in the wings collapse and the wings turn dry and brittle. In forms having hemolymph moving in the wings, the thoracic pulsatile organs control this flow.

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