28 



BIOLOGY AND HUMAN LIFE 



size, for its outer covering, the exoskeleton, is of rather hard 

 material that can neither grow nor stretch. After some days, 

 however, this exoskeleton splits lengthwise on the dorsal side 

 (the back, or side away from the ground), and the animal 

 crawls out, head first (see Fig. ii). Now, with no stiff exo- 

 skeleton to interfere, the insect grows rapidly for a short time, 

 until a new exoskeleton is formed by the hardening of a sub- 

 stance w^hich is secreted, or given off, by the skin. The process 

 of casting off the exoskeleton is called molting. From time to 

 time the insect molts again, gaining in size after each molt ; and 



with each molt there are 

 other visible changes, such 

 as the development of the 

 wings and the altered pro- 

 portions of the parts. After 

 the fifth molt the animal 

 is full grown. 



23. Development of the 

 butterfly. The mother but- 

 terfly usually lays her eggs 

 upon the leaves of some 

 plant. When the young 

 hatch out they resemble 

 the parent so Httle that 

 many people never discover the relation between the wormlike 

 caterpillars and the beautiful adult moths and butterflies which 

 are their parents, and into which caterpillars in time develop. 



The larva, or young butterfly in this caterpillar stage, begins 

 at once to feed upon the leaves. The anim.al now has a biting 

 mouth : the jaws work right and left, very much like those of 

 grasshoppers. A young larva eats several times its own weight 

 of leaves in the course of a day, and it grov/s very rapidly. 

 Although the caterpillar looks more like a worm than like any 

 familiar insect, a closer examination shows many resemblances 

 to insects. The head is fairly distinct, and the rest of the body 

 is segmented. The first three segments behind the head, corre- 



Fig. II. Molting cicada 



In many jointed-legged animals (Arthropoda) 

 the growth takes place at intervals between 

 molts. The hard outer skeleton breaks open 

 and the soft-skinned animal crawls out. After 

 a while the shell hardens and the growth of 

 the animal stops again 



