Respiratory organs of invertebrates

Two common respiratory organs of invertebrates are trachea and gills. Diffusion lungs, as contrasted with ventilation lungs of vertebrates, are confined to small animals, such as pulmonate snails and scorpions.

Trachea

This respiratory organ is a hallmark of insects. It is made up of a system of branching tubes that deliver oxygen to, and remove carbon dioxide from, the tissues, thereby obviating the need for a circulatory system to transport the respiratory gases (although the circulatory system does serve other vital functions, such as the delivery of energy-containing molecules derived from food). The pores to the outside, called spiracles, are typically paired structures, two in the thorax and eight in the abdomen. Periodic opening and closing of the spiracles prevents water loss by evaporation, a serious threat to insects that live in dry environments. Muscular pumping motions of the abdomen, especially in large animals, may promote ventilation of the tracheal system.

Although tracheal systems are primarily designed for life in air, in some insects modifications enable the tracheae to serve for gas exchange underwater. Of special interest are the insects that might be termed bubble breathers, which, as in the case of the water beetle Dytiscus, take on gas supply in the form of an air bubble under their wing surfaces next to the spiracles before they submerge. Tracheal gas exchange continues after the beetle submerges and anchors beneath the surface. As oxygen is consumed from the bubble, the partial pressure of oxygen within the bubble falls below that in the water; consequently oxygen diffuses from the water into the bubble to replace that consumed. The carbon dioxide produced by the insect diffuses through the tracheal system into the bubble and thence into the water. The bubble thus behaves like a gill. There is one major limitation to this adaptation: As oxygen is removed from the bubble, the partial pressure of the nitrogen rises, and this gas then diffuses outward into the water. The consequence of outward nitrogen diffusion is that the bubble shrinks and its oxygen content must be replenished by another trip to the surface. A partial solution to the problem of bubble renewal has been found by small aquatic beetles of the family Elmidae (e.g., Elmis, Riolus), which capture bubbles containing oxygen produced by algae and incorporate this gas into the bubble gill. Several species of aquatic beetles also augment gas exchange by stirring the surrounding water with their posterior legs.

The respiratory structures of spiders consist of peculiar “book lungs,” leaflike plates over which air circulates through slits on the abdomen. The book lungs contain blood vessels that bring the blood into close contact with the surface exposed to the air and where gas exchange between blood and air occurs. In addition to these structures, there may also be abdominal spiracles and a tracheal system like that of insects.

Since spiders are air breathers, they are mostly restricted to terrestrial situations, although some of them regularly hunt aquatic creatures at stream or pond edges and may actually travel about on the surface film as easily as on land. The water spider (or diving bell spider), Argyroneta Aquatica—known for its underwater silk web, which resembles a kind of diving bell—is the only species of spider that spends its entire life underwater. Using fine hairs on its abdomen, where its respiratory openings lie, the water spider captures tiny bubbles of air at the water’s surface, transports them to its silk web, which is anchored to underwater plants or other objects, and ejects them into the interior, thereby inflating the underwater house with air. Research has shown that the inflated web serves as a sort of gill, extracting dissolved oxygen from the water when oxygen concentrations inside the web become sufficiently low to draw oxygen in from the water. As the spider consumes the oxygen, nitrogen concentrations in the inflated web rise, causing it to slowly collapse. Hence, the spider must travel to the water’s surface for bubble renewal, which it does about once each day. Most of the life cycle of the water spider, including courtship and breeding, prey capture and feeding, and the development of eggs and embryos, occurs below the water surface. Many of these activities take place within the spider’s diving bell.

Many immature insects have special adaptations for an aquatic existence. Thin-walled protrusions of the integument, containing tracheal networks, form a series of gills (tracheal gills) that bring water into close contact with the closed tracheal tubes. The nymphs of mayflies and dragonflies have external tracheal gills attached to their abdominal segments, and certain of the gill plates may move in a way that sets up water currents over the exchange surfaces. Dragonfly nymphs possess a series of tracheal gills enclosed within the rectum. Periodic pumping of the rectal chamber serves to renew water flow over the gills. Removing the gills or plugging the rectum results in lower oxygen consumption. Considerable gas exchange also occurs across the general body surface in immature aquatic insects.

The insect tracheal system has inherent limitations. Gases diffuse slowly in long narrow tubes, and effective gas transport can occur only if the tubes do not exceed a certain length. It is generally thought that this has imposed a size limit upon insects.

Gills of invertebrates

Gills are evaginations of the body surface. Some open directly to the environment; others, as in fishes, are enclosed in a cavity. In contrast, lungs represent invaginations of the body surface. Many invertebrates use gills as a major means of gas exchange; a few, such as the pulmonate land snail, use lungs. Almost any thin-walled extension of the body surface that comes in contact with the environmental medium and across which gas exchange occurs can be viewed as a gill. Gills usually have a large surface area in relation to their mass; pumping devices are often employed to renew the external medium. Although gills are generally used for water breathing and lungs for air-breathing, this association is not invariable, as exemplified by the water lungs of sea cucumbers.

The marine polychaete worms use not only the general body surface for gas exchange but also a variety of gill-like structures: segmental flaplike parapodia (in Nereis) or elaborate branchial tufts (among the families Terebellidae and Sabellidae). The tufts used to create both feeding and respiratory currents, offer a large surface area for gas exchange.

In echinoderms (starfish, sea urchins, brittle stars), most of the respiratory exchange occurs across tube feet (a series of suction-cup extensions used for locomotion). However, this exchange is supplemented by extensions of the coelomic, or body-fluid, cavity into thin-walled “gills” or dermal branchiae that bring the coelomic fluid into close contact with seawater. Sea cucumbers (Holothuroidea), soft-bodied, sausage-shaped echinoderms that carry on some respiration through their oral tentacles, which correspond to tube feet, also have an elaborate “respiratory tree” consisting of branched hollow outpouchings off the cloaca (hindgut). Water is pumped in and out of this system by the action of the muscular cloaca, and it is probable that a large fraction of the animals’ respiratory gas is exchanged across this system.