Fundamentals of Entomology (2+1)

Circulatory system in insects

• 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. 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 up to three pairs of thoracic ostia and nine pairs of abdominal ostia. The dorsal vessel lies in the pericardial sinus, a compartment above a dorsal diaphragm (a fibromuscular septum - a separating membrane) formed of connective tissue and segmental pairs of alary muscles. The alary muscles support the dorsal vessel but their contractions do not affect heartbeat.
• Haemolymph enters the periocardial sinus via segmental openings in the diaphragm and then moves into the dorsal vessel via the ostia during a muscular relaxation phase. Waves of contraction start at the posterior end of the body, pump the haemolymph forward in the dorsal vessel and out via the aorta into the head. Next the appendages of the head and thorax are supplied with haemolymph as it circulates posteroventrally and finally returns to the pericardial sinus and dorsal vessel.
• Another important component of the insect circulatory system is the ventral diaphragm, a fibromuscular septum that lies in the floor of the body cavity associated with the ventral nerve cord. Circulation of the haemolymph is aided by active peristaltic contractions of the ventral diaphragm which direct the haemolymph backwards and laterally in the perineural sinus below the diaphragm. These movements are important in insects that use the circulation in thermoregulation. Ventral diaphragm also facilitates rapid exchange of chemicals between the ventral nerve cord and the haemolymph.
• Haemolymph is generally circulated to appendages unidirectionaly by various tubes, septa, valves and pumps. The muscular pumps are termed accessory pulsatile organs and occur at the base of the antennae and legs. Antennal pulsatile organs releases neurohormones that are carried to the antennal lumen to influence the sensory neurones. Circulation occurs in the wings of young adult. In wing circulation is sustained by influxes of air into the wing veins, rather than any pulsatile organs. Pulses of air in the fine tracheal tubes of the veins push the haemolymph through the enclosed space of the veins.
• The insect circulatory system shows high degree of co-ordination between dorsal vessel, fibro-muscular diaphragms and accessory pumps.