TRACHEAL SYSTEMS

The problems associated with gas exchange in aquatic and terrestrial organisms are quite different. In water, where the oxygen supply is low but there is no problem in keeping the exchange surface moist, gills are a very efficient mechanism for exchange. But on land, where the oxygen concentration is high but water loss is a problem, lungs or a tracheal system are more efficient. For instance, fish devote nearly 20 percent of their total energy expenditure to ventilate their gills, whereas air breathers devote only 1 to 2 percent of their energy to ventilate their lungs, and essentially no energy for a tracheal system.

Most Land Arthropods Evolved A Tracheal System For Gas Exchange

One type of invaginated respiratory system evolved for air breathing is the tracheal system. It is one solution to the problem of bringing O₂ close to the cells, and carrying away the waste CO₂. Tracheal systems are typical of most terrestrial arthropods (e.g. insects). Here we find no localized gas exchange organs and little or no significant transport of gases by the blood. Instead, the system is composed of many small tubes, called tracheae, that open to the outside and branch throughout the body. The tracheae and the smaller tracheoles into which they branch carry air from the outside directly to the individual cells. But, in order to diffuse across cell membranes, the oxygen must first dissolve in the fluid at the end of the tracheoles. This is similar to other gas exchange systems.

Air enters the tracheae by way of the spiracles, openings in the body wall that can usually be opened and closed by valves. Some of the larger insects actively ventilate their tracheal systems by muscular contraction, but most small insects and some fairly large ones do not, although normal muscle movements do aid the movement of air. Calculations have shown that the rate of diffusion of oxygen in air through the tracheae is rapid enough to maintain at the tracheal endings an O₂ concentration only slightly below that of the external atmosphere. But diffusion, remember, is only effective over short distances (i.e. less than 1 mm), so this type of respiratory system, which depends on diffusion through the tracheal tubes, probably limits the size of insects.

The tracheal system of insects. The tracheal system consists of many small internal tubes called tracheae that branch throughout the insect body. The tracheae open to the outside through holes called spiracles. The tracheae, and the smaller tracheoles (insert) into which they branch, carry air directly to the individual cells, where diffusion across the cell membranes takes place.

Tracheae and spiracle of an insect. At top center is a spiracle (brown), from which numerous branching tubes – the tracheae – can be seen running to many parts of the insect’s body. The spiracles are usually located on the sides of the body segments, the number varying in different kinds of insects. (See previous figure for a drawing of terminal branches, or tracheoles.

Spiracles of two insects. (A) Scanning electron micrograph of a fully open ant spiracle. The pointed projections are sensory hairs that monitor external conditions and can trigger spiracle closing when necessary. (B) A nearly closed grasshopper spiracle; the black areas are the valves. Note the resemblance to the stoma of a leaf.