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.
