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Grasshopper and insect respiratory system, illustration. Structures called tracheae and spiracles form an insect's respiratory system. The spiracles (see inset at left) are the openings to the tracheae (airways), which branch throughout the insect's body. Tracheae are continuous with the epidermis, and are thus made of chitin, and reinforced with bands of chitin. The tracheae branch into smaller units (tracheoles) to reach the cells (see inset at right), allowing oxygen to diffuse in and carbon dioxide to diffuse out. Internal air sacs also help bring oxygen to body tissues. For this illustration with labels, see image C048/2741. (KEYSTONE/SCIENCE PHOTO LIBRARY/MIKKEL JUUL JENSEN / SCIENCE PHOTO LIBRARY)