Gaseous exchange in insects occurs through a system of internal tubes, the tracheal system, the finer branches of which extend to all parts of the body and may become functionally intracellular in muscle fibers. Thus oxygen is carried directly to its sites of utilization and the blood is not concerned with its transport. In terrestrial insects and some aquatic species, the tracheae open to the outside through segmental pores, the spiracles, which generally have some closing mechanism reducing water loss from the respiratory surfaces. Other aquatic species have no functional spiracles, and gaseous exchange with the water involves arrays of tracheae close beneath the surface of thin, permeable cuticle.

Reviews: Grassé, 1976 – general; Mill, 1985 – general; Miller, 1981a – cockroaches

TRACHEAL SYSTEM

Tracheae

The tracheae are the larger tubes of the tracheal system, running inward from the spiracles and usually breaking up into finer branches, the smallest of which are about 2 μm in diameter. Tracheae are formed by invaginations of the ectoderm and so are lined by a cuticular intima which is continuous with the rest of the cuticle. A spiral thickening of the intima runs along each tube, each ring of the spiral being called a taenidium (Fig. 17.1). The intima consists of outer epicuticle with a protein/chitin layer beneath it. In the taenidia the protein/chitin cuticle is differentiated as mesocuticle or exocuticle. The chitin microfibrils in the taenidia run round the trachea, while between the taenidia they are parallel with the long axis of the trachea.