18 INSECT PHYSIOLOGY 



an essential element in the supply of oxygen; and where they lie in 

 rigid portions of the body, such as the head, they are ventilated, as 

 Graber (1877) pointed out, by the transmitted pressure of the blood. 



The result of this process is that the greater part of the tracheal 

 system is kept filled with a gas which approximates in composition 

 to the outside air ; but the actual supply of oxygen to the tissues again 

 takes place by diffusion along the tracheal branches given off by the 

 air sacs. This, of course, is analogous to what happens in vertebrates, 

 where only the upper parts of the respiratory tract are ventilated 

 mechanically, exchanges in the alveoli of the lung being dependent 

 on diffusion. The extent to which the tracheal system is ventilated 

 mechanically varies enormously in different insects, and in the same 

 insect according to its physiological state; but in many cases the 

 efficiency of the process compares very favourably with the ventila- 

 tion of the human lungs. For instance, in Melolontha the respiratory 

 system may be emptied during expiration of about one-third of its 

 total capacity (the total capacity being 39 per cent, of the body 

 volume !), and in the larva of Dytiscus of about two-thirds. (In quiet 

 respiration in man, the lungs are emptied of about one-seventh of 

 their total capacity; the most extreme degree of ventilation empties 

 about two-thirds.) 



In addition to their ventilating function, the tracheal air sacs of 

 insects sometimes serve a purpose like the air-containing sinuses in 

 the facial skeleton of mammals, or the pulmonary air sacs in the 

 bones of birds; thus, by extending into the massive head and man- 

 dibles of certain beetles, they permit an increase in the skeletal bulk 

 without adding to the insect's weight. They also allow for changes in 

 size of the internal organs, such as the ovaries or the gut, without 

 affecting the external form of the body. That is well seen in Muscid 

 flies. And they serve to reduce the volume of the circulating blood 

 and thus aid the circulation of sugar and other fuels to the active 

 muscles. 



The respiratory movements of insects are brought about by a 

 special musculature which varies greatly from one group to another. 

 As a general rule the movements are confined to the abdomen, but 

 in Dytiscus and Hydrophilus ventilation is maintained by aspirating 

 movements of the metathorax, the abdominal movement apparently 



