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. 
