The Circulatory System

Abstract

The circulatory system of insects, like that of all arthropods, is of the “open” type; that is, the fluid which circulates is not restricted to a network of conducting vessels as, for example, in vertebrates, but flows freely among the body organs. An open system results from the development, in evolution, of a hemocoel rather than a true coelom. A consequence of the open system is that insects have only one extracellular fluid, hemolymph, in contrast to vertebrates which have two such fluids, blood and lymph. The occurrence of an open system does not mean that hemolymph simply bathes the organs it surrounds because usually thin granular membranes separate the tissues from the hemolymph itself. Insects generally possess pumping structures and various diaphragms to ensure that hemolymph flows through the body along a definite route. As the only extracellular fluid, it is perhaps not surprising that hemolymph, in general, serves the functions of both blood and lymph of vertebrates. Thus, plasma is important in providing the correct milieu for body cells and is the transport system for nutrients, hormones, and metabolic wastes, while hemocytes provide the major defence mechanism against foreign organisms which enter the body and are important in wound repair and in the metabolism of specific compounds.