They may be considered, together with their respective physiological functions, under the following heads: 1. Nutrition; 2. Sensation; 3. Muscular System; 4. Reproduction.
Nutrition.—As nutrition consists in the renewal of the molecules which constitute an animal body, and as that is accomplished by means of the food, we have only to consider the principal processes which the latter undergoes, in order to obtain an indication of the different heads under which this department of the subject may most naturally be treated. The first of the most important changes to which the food is subjected internally, is its conversion into chyle, previous to which it is incapable of affording any nutriment; this process is called digestion, and takes place in a special organ named the alimentary canal. But the chyle is not perfectly adapted to its nutritive functions till it has been brought into contact with the atmosphere, in order to receive a supply of its vital principle, viz. oxygen. There being no lungs in insects to which it can be conveyed, as to a reservoir, to receive this supply, the necessity is provided for by bringing the air into contact with it by means of small tubes ramifying through the body; this process is called respiration. In consequence of the blood being aërated at so many places, instead of a particular point, often remote from the extremities, there is no necessity for a rapid or general circulation of it; still, however, it is by no means stagnant, and its movement is called, although the term is rather too strong, the circulation of the blood.
