The basic insect respiratory system consists of a series of rigid tubes, called tracheae, connected to the outside via pairs of pores called spiracles (typically one pair per segment on the sides of the thorax and abdomen, lacking on certain segments). Air enters the system via the spiracles and the tracheae are air-filled. The spiracles can often be opened and closed and lead into short tracheae that enter a pair of longitudinal tracheal trunks, which are the main tracheal tubes. From these lateral tracheae branch smaller tracheae that supply the tissues with air. This supply is especially rich in the more active tissues, such as muscles, nervous tissues and the gut. Tracheae also extend into the wings, running inside the wing veins.
The tracheae branch until they reach a diameter of 2 to 5 micrometers and then often enter stellate tracheole cells (transition cells) from which they emerge as finer branches called tracheoles, with diameters less than one micrometer. These tracheoles terminate inside the tissues, almost always as open-ended or blind-ended tubes about 200 nanometers in diameter.
The outside cuticle of the insect extends inwards through the spiracles as spiral ridges (taenidia) to line the inside of the main tracheae, preventing the trachea from collapsing. These ridges may be rings (annular) or spirals. In the smaller tracheae this cuticle is reduced to a thin membrane lining.