Gas transport in insects is the function of the tracheal system. The circulatory system of insects, unlike that of vertebrates, usually plays only a minor role in this process.

The tracheal system (Figure1) is a system of cuticular tubes (the tracheae) that externally open at the spiracles (spr) and internally branch and extend throughout the body. They terminate in very fine closed branches, called tracheoles, that permeate and actually penetrate the living tissues (indenting but not actually breaking through the cell membranes). The tracheae are lined with a layer of cuticle, and in the larger branches this is thickened to form helical rings, called taenidia, that simultaneously give the tracheae strength (against collapse) and flexibility (to bend and twist). The tracheoles (also lined with cuticle) are minute intracellular tubes with thin walls, and they often contain fluid. It is across the walls of the tracheoles that gas exchange actually takes place.

Fig1. Diagram of a horizontal section of an in sect showing the arrangement of the principal tracheae. ant, antenna; com, commissural trachea; dtra, dorsal trachea; e, compound eye; l, legs; ltra, main longitudinal tracheal trunk; spr, spiracles; stra, spiracular trachea; vtra, ventral tracheae.

The spiracles are located laterally in the pleural wall and vary in number from 1 to 10 pairs (some species have no functional spiracles). There is typically a pair on the anterior margin of both the meso- and metathorax, and a pair on each of the first eight (or fewer) abdominal segments. They vary in size and shape and usually have some sort of valvelike closing device. These valves therefore play an important role in retaining body water.

In insects with an open tracheal system (that is, with functional spiracles), air enters the body through the spiracles, then passes through the tracheae to the tracheoles, and oxygen ultimately enters body cells by diffusion. Carbon dioxide leaves the body in similar fashion. The spiracles may be partly or completely closed for extended periods in some insects. Water loss through the spiracles may be minimized in this way.

Insects generally have longitudinal tracheal trunks (connectives, ltra) connecting the tracheae from adjacent spiracles on the same side of the body and trans verse commissures (com) connecting the tracheae on opposite sides of the body, so that the entire system is interconnected. The movement of air through the tracheal system is by simple diffusion in many small in sects, but in most larger insects this movement is augmented by active ventilation, chiefly by the abdominal muscles; the movements of the internal organs, or of the legs and wings, may also aid ventilation. Where ventilation occurs, air may move in and out of each spiracle, but generally enters through the anterior spiracles and leaves by the posterior ones. This flow of air through the tracheal system is effected by controlling which spiracles are open and when. Sections of the main tracheal trunks are often dilated to form air sacs, which help in ventilation.

Closed tracheal systems have the spiracles permanently closed but have a network of tracheae just under the integument, distributed either widely over the body or particularly below certain surfaces (the gills). Some aquatic and parasitic insects have closed systems. In these species, gases enter and leave the body by diffusion across the body wall between the tracheae and the external environment, and gases move through the tracheal system by diffusion.

A great many insects live in water; these get their oxygen from one (rarely both) of two sources: the oxy gen dissolved in the water or atmospheric oxygen. Gas exchange in many small, soft-bodied aquatic nymphs and larvae (and possibly some adults) occurs by diffusion through the body wall, usually into and out of a tracheal system. The body wall in some cases is un modified except perhaps for having a fairly rich tracheal network just under the integument. In other cases, there are special thin extensions of the body wall that have a rich tracheal supply and through which gas exchange occurs. These structures, called tracheal gills, come in a variety of shapes and may be located on different parts of the body. The gills in mayfly nymphs are in the form of leaflike structures on the sides of the first seven abdominal segments. In dragonfly nymphs, they are folds in the rectum, and water is moved into and out of the rectum and over these folds. In damselfly nymphs, the gills are three leaflike structures at the end of the abdomen as well as folds in the rectum. In stonefly nymphs, the gills are fingerlike or branched structures located around the bases of the legs or on the basal abdominal segments. Gas exchange may occur through the general body surface of these insects, and in some cases (such as damselfly nymphs), the exchange through the body surface may be more important than that through the tracheal gills.

Insects that live in water and get their oxygen from atmospheric air do this in one of three general ways: from air spaces in submerged parts of certain aquatic plants, through spiracles placed at the water surface (with the body of the insect submerged), or from a film of air held somewhere on the surface of the body while the insect is submerged. A few larvae (for example, those of the beetle genus Donacia and the mosquito genus Mansonia) have their spiracles in spines at the posterior end of the body, and these spines are inserted into the air spaces of submerged aquatic plants. Many aquatic insects (for example, waterscorpions, rattailed maggots, and the larvae of culicine mosquitoes) have a breathing tube at the posterior end of the body; which is extended to the surface. Hydrophobic hairs around the end of this tube enable the insect to hang from the surface film, and they prevent water from entering the breathing tube. Other aquatic insects (for example, backswimmers and the larvae of anopheline mosquitoes) get air through posterior spiracles placed at the water surface. These insects do not have an extended breathing tube.

The insects that get their oxygen from atmospheric air at the water surface do not spend all their time at the surface. They can submerge and remain underwater for a considerable period, getting oxygen from an air store either inside or outside the body. The air stores in the tracheae of a mosquito larva, for ex ample, enable the larva to remain underwater for a long time.

Many aquatic bugs and beetles carry a thin film of air somewhere on the body surface when they sub merge. This film is usually under the wings or on the ventral side of the body. The air film acts like a physical gill, with dissolved oxygen in the water diffusing into the bubble when the partial pressure of oxygen in the film falls below that of the water. The insect may get several times as much oxygen from this temporary structure as was originally in it, because of gas exchanges between the air film and the surrounding water. A few insects (for example, elmid beetles) have a permanent layer of air around the body surface, held there by a body covering of thick, fine, hydrophobic hairs; such a layer is called a plastron. The air reservoirs of aquatic insects not only play a role in gas exchange but also may have a hydrostatic function (like the swim bladder of fish). Two crescent-shaped air sacs in Chaoborus larvae (Diptera: Chaoboridae) are apparently used to regulate this insect's specific gravity: to hold it perfectly motionless or to enable it to migrate up or down in the water column.

Parasitic insects that live inside the body of their host get oxygen from the body fluids of the host by diffusion through their integument, or (for example, in tachinid fly larvae) their posterior spiracles may be extended to the body surface of the host or attach to one of the host's tracheal trunks.

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Protection and defence by the haemolymph in Insect

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