4 THE BIOLOGY OF INSECTS 



processes analogous to combustion or explosion. The 

 animal body requires therefore a supply of oxygen to 

 support this combustion-process, and a supply of food to 

 provide for the repair of waste and to act the part of fuel in 

 the Hving heat-engine. An animal that continues to live 

 must feed and breathe. Nov^ the organs and the method 

 of breathing in insects are among the most markedly 

 distinctive of the structural and vital features of this class 

 of animals. 



In several classes of the Arthropoda — the great compre- 

 hensive Group or Phylum to which insects belong — we find 

 that the outer skin with its firm cuticle is pushed into the 

 body in such a manner as to give rise to a series of branching 

 air-tubes of which the firm cuticle necessarily forms the 

 lining and prevents their channels from collapsing. As 

 these air-tubes open to the outside through a set of breathing- 

 holes or spiracles, air can pass in and come into touch with 

 organs inside the creature's body so that the process of 

 oxidation becom.es easy and direct ; the oxygen needs 

 not the circulating fluid or blood to carry it from special 

 breathing organs such as lungs or gills to the tissues 

 where the combustion processes are constantly going on 

 (Figs. 14, 15). 



Air-tubes like those just described are found in centi- 

 pedes, in certain spiders, and in many mites as well as in 

 insects ; but it is among the insects that we find them in 

 the highest grade of development. In a typical aerial flying 

 insect the spiracles are arranged segmentally in pairs along 

 the sides of the body, while the tubes branch repeatedly, 

 ending in minute ramifications with walls of exceeding 

 delicacy which allow free gaseous exchange between the 

 tissues and the atmosphere, oxygen being taken to support 

 the combustion-processes while waste products — carbon 

 dioxide and water- vapour — are given off. Such a flying 

 insect, therefore, while it lives in the air and is bathed in it — 

 using the aerial resistance for support and progress — is also 

 permeated with air inwardly, illustrating as perhaps no 

 other animal can do, the deeper meanings of that old-time 



