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closed network of blood-conducting vessels seen in vertebrates.

3.4.1 Hemolymph

The volume of the hemolymph may be substantial (20-40% of body weight) in soft-bodied larvae, which use the body fluid as a hydrostatic skeleton, but is less than 20% of body weight in most nymphs and adults. Hemolymph is a watery fluid containing ions, molecules, and cells. It is often clear and colorless but may be variously pigmented yellow, green, or blue, or rarely, in the immature stages of a few aquatic and endoparasitic flies, red owing to the presence of hemoglobin. All chemical exchanges between insect tissues are mediated via the hemolymph - hormones are transported, nutrients are distributed from the gut, and wastes are removed to the excretory organs. However, insect hemolymph only rarely contains respiratory pigments and hence has a very low oxygen-carrying capacity. Local changes in hemolymph pressure are important in ventilation of the tracheal system (section 3.5.1), in thermoregulation (section 4.2.2), and at molting to aid splitting of the old and expansion of the new cuticle. The hemolymph serves also as a water reserve, as its main constituent, plasma, is an aqueous solution of inorganic ions, lipids, sugars (mainly trehalose), amino acids, proteins, organic acids, and other compounds. High concentrations of amino acids and organic phosphates characterize insect hemolymph, which also is the site of deposition of molecules associated with cold protection (section 6.6.1). Hemolymph proteins include those that act in storage (hexamerins) and those that transport lipids (lipophorin) or complex with iron (ferritin) or juvenile hormone (JH-binding protein).

The blood cells, or hemocytes (haemocytes), are of several types (mainly plasmatocytes, granulocytes, and prohemocytes) and all are nucleate. They have four basic functions:

1 phagocytosis - the ingestion of small particles and substances such as metabolites;

2 encapsulation of parasites and other large foreign materials;

3 hemolymph coagulation;

4 storage and distribution of nutrients.

The hemocoel contains two additional types of cells. Nephrocytes (sometimes called pericardial cells) generally occur near the dorsal vessel and appear to function as ductless glands by sieving the hemolymph of certain substances and metabolizing them for use or excretion elsewhere. Oenocytes may occur in the hemocoel, fat body, or epidermis and, although their functions are unclear in most insects, they appear to have a role in cuticle lipid (hydrocarbon) synthesis and, in some chironomids, they produce hemoglobins.

3.4.2 Circulation

Circulation in insects is maintained mostly by a system of muscular pumps moving hemolymph through compartments separated by fibromuscular septa or membranes. The main pump is the pulsatile dorsal vessel. The anterior part may be called the aorta and the posterior part may be called the heart, but the two terms are inconsistently applied. The dorsal vessel is a simple tube, generally composed of one layer of myocardial cells and with segmentally arranged openings, or ostia. The lateral ostia typically permit the one-way flow of hemolymph into the dorsal vessel as a result of valves that prevent backflow. In many insects there also are more ventral ostia that permit hemolymph to flow out of the dorsal vessel, probably to supply adjacent active muscles. There may be up to three pairs of thoracic ostia and nine pairs of abdominal ostia, although there is an evolutionary tendency towards reduction in number of ostia. The dorsal vessel lies in a compartment, the pericardial sinus, 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. Hemolymph enters the pericardial sinus via segmental openings in the diaphragm and/or at the posterior border and then moves into the dorsal vessel via the ostia during a muscular relaxation phase. Waves of contraction, which normally start at the posterior end of the body, pump the hemolymph forwards in the dorsal vessel and out via the aorta into the head. Next, the appendages of the head and thorax are supplied with hemolymph as it circulates posteroventrally and eventually returns to the pericardial sinus and the dorsal vessel. A generalized pattern of hemolymph circulation in the body is shown in Fig. 3.9 a; however, in adult insects there also may be a periodic reversal of hemolymph flow in the dorsal vessel (from thorax posteriorly) as part of normal circulatory regulation.

Another important component of the circulation of many insects is the ventral diaphragm (Fig. 3.9b) - a

Fig. 3.9 Schematic diagram of a well-developed circulatory system: (a) longitudinal section through body; (b) transverse section of the abdomen; (c) transverse section of the thorax. Arrows indicate directions of hemolymph flow. (After Wigglesworth 1972.)

fibromuscular septum that lies in the floor of the body cavity and is associated with the ventral nerve cord. Circulation of the hemolymph is aided by active peristaltic contractions of the ventral diaphragm, which direct the hemolymph backwards and laterally in the perineural sinus below the diaphragm. Hemolymph flow from the thorax to the abdomen also may be dependent, at least partially, on expansion of the abdomen, thus "sucking" hemolymph posteriorly. Hemolymph movements are especially important in insects that use the circulation in thermoregulation (some Odonata, Diptera, Lepidoptera, and Hymenoptera). Another function of the diaphragm may be to facilitate rapid exchange of chemicals between the ventral nerve cord and the hemolymph by either actively moving the hemolymph and/or moving the cord itself.

Hemolymph generally is circulated to appendages unidirectionally by various tubes, septa, valves, and pumps (Fig. 3.9c). The muscular pumps are termed accessory pulsatile organs and occur at the base of the antennae, at the base of the wings, and sometimes in the legs. Furthermore, the antennal pulsatile organs may release neurohormones that are carried to the antennal lumen to influence the sensory neurons. Wings have a definite but variable circulation, although it may be apparent only in the young adult. At least in some Lepidoptera, circulation in the wing occurs by the reciprocal movement of hemolymph (in the wing vein sinuses) and air (within the elastic wing tracheae) into and from the wing, brought about by pulsatile organ activity, reversals of heartbeat, and tracheal volume changes.

The insect circulatory system displays an impressive degree of synchronization between the activities of the dorsal vessel, fibromuscular diaphragms, and accessory pumps, mediated by both nervous and neurohormonal regulation. The physiological regulation of many body functions by the neurosecretory system occurs via neurohormones transported in the hemolymph.

3.4.3 Protection and defense by the hemolymph

Hemolymph provides various kinds of protection and defense from (i) physical injury; (ii) the entry of disease organisms, parasites, or other foreign substances; and sometimes (iii) the actions of predators. In some insects the hemolymph contains malodorous or distasteful chemicals, which are deterrent to predators (Chapter 14). Injury to the integument elicits a wound-healing process that involves hemocytes and plasma coagulation. A hemolymph clot is formed to seal the wound and reduce further hemolymph loss and bacterial entry. If disease organisms or particles enter an insect's body, then immune responses are invoked. These include the cellular defense mechanisms of phagocytosis, encapsulation, and nodule formation mediated by the hemo-cytes, as well as the actions of humoral factors such as enzymes or other proteins (e.g. lysozymes, propheno-loxidase, lectins, and peptides).

The immune system of insects bears little resemblance to the complex immunoglobulin-based vertebrate system. However, insects sublethally infected with bacteria can rapidly develop greatly increased resistance to subsequent infection. Hemocytes are involved in phagocytosing bacteria but, in addition, immunity proteins with antibacterial activity appear in the hemolymph after a primary infection. For example, lytic peptides called cecropins, which disrupt the cell membranes of bacteria and other pathogens, have been isolated from certain moths. Furthermore, some neuro-peptides may participate in cell-mediated immune responses by exchanging signals between the neuroendocrine system and the immune system, as well as influencing the behavior of cells involved in immune reactions. The insect immune system is much more complicated than once thought.

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