Insects breathe air through tracheae.

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With the evolution of life on land, animals that breathe air were able to increase their uptake of O2 and achieve higher rates of metabolism. Oxygen uptake increased for the following reasons:

  1. The O2 content of air is typically 50 times greater than that of a similar volume of water. Fresh air contains approximately 210 mL O2 per liter of air (21%), whereas the concentration of O2 in well-mixed freshwater or salt water rarely exceeds 4 mL O2 per liter of water (0.4%).

  2. Oxygen diffuses about 8000 times faster in air than in water. For this reason, the O2 content can be close to zero a few centimeters beneath the surface of a stagnant pond.

  3. Water is 800 times denser and 50 times more viscous than air. Thus, it requires more energy to pump water than to pump air past a gas exchange surface.

Because of their small size, insects can use a direct pathway of air transport that gets air right to their tissues (see Fig. 39.4b). In contrast to terrestrial vertebrates, insects rely on a two-step process of ventilation and diffusion to supply their cells with O2 and eliminate CO2. First, air enters an insect through openings, called spiracles, along either side of its abdomen. The spiracles can be opened or closed to limit water loss and regulate O2 delivery, much like the stomata of leaves in plants (Chapter 29). Inside the insect body, air is ventilated through a branching series of air tubes—the tracheae and tracheoles—directly to the cells. Second, diffusion occurs at the cell: O2 supplied by the fine airways diffuses into the cells, and CO2 diffuses out and is eliminated through the insect’s tracheae.

The mitochondria of the flight muscles and other metabolically active tissues of insects are located within a few micrometers of tracheole airways. Because O2 and CO2 diffuse rapidly in air, the tracheal system of insects delivers O2 to cellular mitochondria at high rates. Active insects also have an air sac system connected to their tracheae. Powered by contracting abdominal muscles, the air sacs act like bellows to pump air through the tracheae, speeding the movement of air to tissues so that gas exchange is faster.