Circulatory System

Thomas A. Miller

University of California, Riverside

Insects have an open circulatory system. This means that the internal organs and tissues are bathed in hemolymph, which is propelled actively to all internal surfaces by specialized pumps, pressure pulses, and body movements and is directed by vessels, tubes, and diaphragms. Without such constant bathing, tissues would die. The internal organs and tissues depend on the circulatory system for the delivery of nutrients, both to carry away excretion products and as the chemical communication pathway by which hormone messengers coordinate development and other processes. So vital is this function that it can be equated to the umbilical cord of a human fetus during development (Fig. 1).

Gas exchange in insects occurs via the tracheal system, which supplies all internal organs with tracheole tubules from spiracular openings in the body wall of terrestrial insects or from gill structures in aquatic insects. However, the hemolymph has the capacity to dissolve carbon dioxide gas in the form of bicarbonate ions. A few insects live in low oxygen environments and have a type of hemoglobin that binds oxygen at very low partial pressures, but for the most part oxygen is supplied and carbon dioxide is removed by ventilation through the tracheal system via the same system.

Besides the functions already mentioned, the circulatory system provides a medium in which battles are fought between the insect host and a myriad of invading disease microorganisms, including viruses, bacteria, fungi, and insect parasites. Principal participants in these interactions are the blood cells or hemocytes.

FIGURE 1 Delivery of the hemolymph to all tissues is so vital that a number of structures have evolved to ensure complete circulation including accessory pulsatile organs (APO), aorta (A), dorsal vessel (DV), dorsal diaphragm (DD), ostial valves (OS), and septa (S). Perfusion of the abdominal ventral nerve cord (N) is associated with a ventral diaphragm (VD). [Modified from Wigglesworth, V. B. (1972). "Principles of Insect Physiology." 7th ed. Chapman & Hall, New York, London, with kind permission of Kluwer Academic Publishers.]

FIGURE 1 Delivery of the hemolymph to all tissues is so vital that a number of structures have evolved to ensure complete circulation including accessory pulsatile organs (APO), aorta (A), dorsal vessel (DV), dorsal diaphragm (DD), ostial valves (OS), and septa (S). Perfusion of the abdominal ventral nerve cord (N) is associated with a ventral diaphragm (VD). [Modified from Wigglesworth, V. B. (1972). "Principles of Insect Physiology." 7th ed. Chapman & Hall, New York, London, with kind permission of Kluwer Academic Publishers.]

While maintaining the body tissues, the circulatory system is the medium in which homeostasis is ensured, including the regulation of pH and inorganic ions, as well as the maintenance of proper levels of amino acids, proteins, nucleic acids, carbohydrates, and lipids. Any change in the hemolymph immediately affects all organs bathed. The time for complete mixing of the hemolymph depends on the size of the insect, but it can be up to 5 min in a resting adult cockroach weighing about a gram. Any substance injected into a healthy insect will eventually appear at the extreme ends of all appendages in a few minutes, emphasizing the efficiency of the delivery mechanisms, which can be marvels of microhydraulic engineering.

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