THE BODY WALLS 15 



in the case of caterpillars that are reaching full growth and getting 

 ready to transform. 



Fig. 14. — Nervous system of an insect. (Diagrammatic.) Original. 



The Body Walls 



The bodies of most insects are covered with a more or less horny 

 or hard coating, to which the muscles are attached, and which takes 

 the place of the bony, internal skeleton of higher animals. The basis 

 of this coating is a fluid substance called chitin, which hardens on 

 exposure to air. In order to permit of movement, this outer shell is 

 made up of distinct plates, joined to one another by flexible skin. 



