WAYS AND MEANS OF LIVING
along the sides of the body (Fig. 70), and from these trunks are given off branches into each body segment and into the head, which go to the alimentary canal, the heart, the nervous system, the muscles, and to all the other organs, where they break up into finer branches that terminate in minute end tubes going practically to every cell of the body.
Many insects breathe by regular movements of expansion and contraction of the under surface of the abdomen, but experimenters have not yet agreed as to whether the air goes in and out of the same spiracles or whether it enters one set and is expelled through another. It is probable that the fresh air goes into the smaller tracheal branches principally by gas diffusion, for some insects make no perceptible respiratory movements.
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Fig. 70. Respiratory system of a caterpillar.
The external breathing apertures, or spiracles (Sp, Sp), along the sides of the body open into lateral tracheal trunks (a, a), which are connected crosswise by transverse tubes (b, b) and give off minutely branching tracheae into all parts of the head (H) and body
The actual exchange of oxygen from the air and carbon dioxide from the tissues takes place through the thin walls of the minute end tubes of the tracheae. Since these tubes lie in immediate contact with the cell surfaces the gases do not have to go far in order to reach their destinations, and the insect has little need of an oxygen carrier in its blood—its whole body, practically, is a lung. And yet some investigations have made it appear likely that the insect blood does contain an oxygen carrier that functions in a manner similar to that of the hemoglobin of vertebrate blood, though the importance of oxygen transportation in insect physiology has
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