Introduction

Introduction
The circulatory system in insects is composed of the hemolymph and all of its contents, plus all accessory pumps and diaphragms that move or direct the hemolymph. All digestion and excretion products are transported to their destinations via the hemolymph. All products and substrates of intermediary metabolism, plus amino acids, lipids or carbohydrates needed for energy supply or biosynthesis are moved via the hemolymph. Similarly, hormones and neurohormones use this same route to act as chemical communication triggers that synchronize physiological and behavioral events.

The hemolymph is modified to act as an antifreeze in many hibernating insects in the intemperate regions away from the equator; it acts as a coolant for temperature control in some adults, and under hydrostatic pressure can be used to force open exuvia or puparia during eclosion. When circulation of the hemolymph to the extremities is blocked, the epidermal tissues no longer bathed with a fresh supply of hemolymph soon dry up and become brittle. Hemocytes provide the first line of defense against infection by microorganisms or parasites. Microcirculation to appendages and other peripheral structures is assisted by accessory pulsatile organs as well as fibromuscular septae in the body of the insect. The accessory organs are usually located at the bases of extremities, such as the better known antennal or wing accessory pulsatile organs (APOs). Both ventral and dorsal diaphragms, when present, can assist in the local movement of hemolymph.

Recent advances:
Four major research advances in the past twenty have changed fundamentally the way in which we understand the circulatory system of insects. These include Heinrich's descriptions (Heinrich, 1970a, 1970b, 1971, 1976, 1993) of the role of circulation in temperature control of large adult insects and will be described in more detail below. The second advance is the description of the tidal flow of hemolymph from work by Wasserthal (1980, 1981, 1982a). And the third is the discovery of pressure pulses involved in the active exchange of gases during insect respiration by Karel Slama (1988).

These three advances, precise central nervous control of circulation during temperature regulation, tidal flow of hemolymph in large adult flying insects, particularly holometabola, and active ventilation, all suggest a nervous control of not just dorsal vessel, but of diaphragm muscles in insects and circulation in appendages. In addition, it also suggests a high degree of organization and coordination between circulation and respiration, not appreciated before.

The fourth advance came from neuroanatomical and electrophysiological studies on cardiac reflexes in larvae of the silkworm, Bombyx mori. Five types of cardiac reflex responses were found and their underlying neural pathways were revealed. Identification of a motor neuron involved in heartbeat reversal initiation was included (Ai, 1994).