Respiratory System

Tiny air tubes called tracheae branch throughout the insect's body. Air enters the tracheae through holes in the body wall called spiracles, which are opened and closed by valves. In larger insects, air moves through the tracheae when the body muscles contract. The tracheae are invaginated—that is, folded into the body—and thereby kept moist. Thickened rings in the walls of the tracheae help support them. These vessels branch into smaller vessels called tracheoles, which lack the supportive rings. The tracheoles carry air directly to the surface of individual cells, where they branch further to deliver oxygen and pick up carbon dioxide. A fluid in the endings of tracheoles regulates how much air contacts the cells. If a cell needs oxygen, the fluid pulls back and exposes the cell membrane to the air.