• Breathing—The Bird and the Insect Ways
    Awake!—1982 | May 8

    • Insect Breathing

      Have you ever considered the possibility of an ant the size of an elephant? Imagine the power it would have! An ant can carry twice its own weight. And small though insects are (the biggest, the Atlas moth, is only 10 to 12 inches [25 to 30 cm] from wingtip to wingtip) they have enormous appetites. Why, in North Dakota grasshoppers caused $1,714,000 (U.S.) worth of crop and rangeland damage in just one year! What would the damage have been if grasshoppers were the size of horses?

      Well, there is no cause for alarm. The insect’s respiratory system keeps him in his place​—sizewise. According to Scientific American, the insect’s respiratory system, which the magazine called “a refinement of biological engineering almost past belief,” has a built-in size-limiting factor! Furthermore, just as the bird’s respiratory system is ideal for flying, the insect’s is ideal for his way of life. How so?

      Insects are energy factories. For their size, they carry out truly Herculean tasks. So their demand for oxygen is very high. However, insects do not have lungs. Nevertheless, it is indeed doubtful that you will ever come across a breathless insect! Why? Because they have a respiratory system that is designed to cope with an unlimited demand.

      During the embryonic stage, the skin of an insect pushes inward at many points to form hollow tubes, which open out to the atmosphere. As these tubes grow deeper and deeper into the insect’s body, they branch many times, each branch becoming narrower and narrower. Finally, one or more of these tubes come in contact with each cell. Thus, each cell has a direct pipeline to the atmosphere, which means that oxygen is immediately available for use without its having to travel through a blood circulation system. And that is just what the insect needs to carry on his high-energy activity!

      But the problem with a system of tubes to breathe through is that you need a two-way flow​—oxygen going in and carbon dioxide going out. The tubes in the insect can bring oxygen in, but what happens to the carbon dioxide? Well, unlike oxygen, carbon dioxide diffuses more easily through tissue. So it doesn’t try to get out back through the tubes. Rather, it passes out of the insect through its skin.

      Although they are dependent upon the atmosphere for their supply of oxygen, some insect larvae live under water. How do they breathe there? Some send up a “snorkel” tube​—at times equipped with a valve in case the water gets rough and threatens to get into the tube. Others live in a “diving bell,” that is, a bubble of air. Of course, as they use up oxygen in the bubble, it must be replaced. Researchers were long puzzled by the fact that the insect could stay under water long after it should have used up the supply of oxygen in the bubble. How was this possible?

      The process of diffusion comes into play. As the oxygen pressure in the bubble drops below the oxygen pressure in the surrounding water, the oxygen in the water rushes into the bubble. (Remember, water is made up of two atoms of hydrogen and one of oxygen.) ‘But why doesn’t the bubble collapse?’ you may be wondering. Well, there is nitrogen in the air of the bubble, and it does not diffuse into water; it prefers to stay in the bubble. So while the insect larva may not need nitrogen for his metabolism, his “life-support system” certainly is dependent upon it!