THE LIFE PROCESS IN INSECTS. 95 



is mixed with the water, or held in solution. These are 

 delicate expansions of the body wall, and contain trachese, 

 which divide and subdivide until the smallest branches 

 are separated from the water only by exceedingly thin 

 membrane. Through this the air in the tracheae effects 

 exchange of gases with the air in the water outside, and 

 is thus purified and fitted for respiration. Tracheal gills 

 are usually situated on the abdomen, and must, of course, 

 project into the water. 



By either of these means, oxygen from the air is sup- 

 plied to all parts of the body; and that is the essential 

 thing in respiration. 



Assimilation of food is the business of each individual 

 cell. As no division of labor in society can relieve any 

 person of the necessity of eating, sleeping, and taking 

 exercise, so no physiological division of labor can exempt 

 any living cell in the body from the necessity of taking 

 from the blood supplied the material necessary for its 

 own growth. Each individual cell of the insect must, 

 like the one cell of the amoeba, absorb and assimilate food 

 and excrete worn-out material for itself. 



It is oxygen that puts the body material in shape for 

 excretion. The food of animals, being organic, is made 

 up of complex chemical compounds, of which carbon and 

 hydrogen are abundant elements. Oxygen, in combining 

 with these elements, forms simple compounds, and breaks 

 up the complex ones, liberating their potential energy in 

 the form of various physical and vital forces, heat, 

 electricity, nerve force, etc. 



The commonest products of oxidation are carbonic- 

 acid gas and water. These have to be removed from 

 the body. 



Excretion is the name of the process, and it is effected 

 by various organs. Such gaseous molecules as escape into 

 the minute air tubes may be exhaled through the tracheae. 



