The Circulatory System and Associated Tissues

Abstract

In the insect body there is only one tissue fluid, the blood or haemolymph, occupying a single cavity, the haemocoel, and separated from the tissue cells only by the delicate but continuous basement membranes. With few exceptions, there is only one blood-vessel, which runs along the midline of the back. The posterior segment of this pulsating vessel, the ‘heart’, is provided with a series of valved openings or ostia through which the blood can enter; the anterior segment, the ‘aorta’, is a uniform contractile tube. After passing through the brain above the oesophagus, the aorta ends more or less abruptly; and from this point the blood simply percolates slowly backwards through the tissues. But in many insects it is still subject to some direction: the aorta may discharge into definite vessels carrying the blood in different directions; the antennae and limbs are often divided by longitudinal membranes, the blood entering the limb on one side and leaving on the other; and the abdominal cavity in some insects is divided by two horizontal membranes, the dorsal and ventral diaphragms, into three sinuses: the pericardial sinus containing the heart, the perineural sinus containing the nerve cord, and the visceral sinus between. The perineural sinus tends to conduct the blood to the posterior end of the abdomen before allowing it to return to the heart; but it is perforated and incomplete laterally, so that some of the blood circulates transversely across the abdomen below it. The dorsal diaphragm, when present, is also fenestrated, and thus allows the blood to enter the pericardial sinus throughout its length (Fig. 9).