44 THE BIOLOGY OF INSECTS 



The combustion processes that go on in the tissues of 

 insects tend to maintain a fairly high body temperature. 

 This becomes evident when a number of active insects are 

 crowded together ; the summer temperature of a beehive 

 is over 90" F. and the winter temperature nearly 80"^. 

 Experiments on the gaseous exchanges of a number of bees 

 whose thoracic air-tubes were choked with small parasitic 

 mites {Acarapis, see J. Rennie, 1921) show that the output 

 of carbon dioxide is in such cases very much below the 

 normal, while the wing- muscles, largely deprived of their 

 needed oxygen, are often incapable of normal action so 

 that the insects cannot fly. 



In insects of feeble or occasional flight, such as cock- 

 roaches, the whole tracheal system may be described as 

 tubular ; but in many insects of great activity and continual 

 and powerful flight, capacious air-sacs of rotund or oval 

 shape, are developed as enlargements of the main trunks or 

 branches. Grasshoppers, flying beetles such as chafers, 

 dragonflies, and bees are examples of insects with extensive 

 air-sacs (Fig. 12, ts). The walls of these expansions are 

 without the spiral thickening of the Hning that charac- 

 terises the tracheal system generally. It may be concluded 

 that they serve as reservoirs of air from which the finer 

 branches may be replenished. 



Many and great modifications of the typical insectan 

 breathing system are found to be correlated with special 

 modes of hfe, particularly in the immature stages of those 

 insects which pass through marked transformation in their 

 life-history. These will be more appropriately considered 

 in later chapters. But in this general account of the function 

 of breathing an introductory reference is necessary to the 

 means whereby insects living submerged in water obtain 

 their supply of air. Many adult insects that frequently 

 dive and swim under water — some aquatic beetles and bugs 

 for example — carry down with them a supply of air, sealed 

 beneath their firm forewings or as a bubble surrounding 

 some part of the body giving access to the spiracles. Some 

 aquatic larvae — like the familiar gnat-grub — have their hind- 



