Tracheal System

This chapter describes tracheal system of insects. Insects have a tracheal respiratory system in which oxygen and carbon dioxide travel primarily through air-filled tubes called tracheae. Usually the tracheal system penetrates the cuticle via closeable valves called spiracles and ends near or within the tissues in tiny tubes called tracheoles. The tracheae primarily serve as pipes that transport gases between the spiracles and the tracheoles, whereas the thin-walled tracheoles are thought to be the main sites of gas exchange with the tissues. However, in many insects, the tracheae are compressible, and dilations of the tracheae form thin walled air sacs that together serve as bellows for enhancing the flow of gases through the tracheal system. In general, the size of the tracheal system increases with age in order to support the increased gas exchange needs of the larger insect. Major changes in tracheal structure, including changes in spiracle number and tracheal system organization, can occur at each molt and during the pupal period for endopterygote insects. The organization of the tracheal system varies dramatically among insects, with spiracle number ranging from 0 to 20 and with tracheal branching patterns varying widely across species, between body regions, and during the developmental stages of holometabolous insects.