STRUCTURE AND FUNCTIONS OF CIRCULATORY SYSTEM
Circulation in insects is maintained by a system of muscular pumps moving haemolymph
through compartments separated by fibromuscular septa or membranes. The main pump is the
pulsatile
dorsal vessel (`heart')
. The anterior part may be called
aorta
and the posterior part the
heart. The dorsal vessel is a simple tube, generally composed of one layer of myocardial cells and
with segmentally arranged openings called
ostia
. The ostia permit the one-way flow of
haemolymph into the dorsal vessel due to valves that prevent backflow. There may be upto three
pairs of thoracic ostia and nine pairs of abdominal ostia. The dorsal vessel lies in the
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