SECTION XLV. HOW INSECTS GROW 



ONE difference between the structure of insects and 

 four-legged animals is that the insects have their skele- 

 ton or bones, as we may call them, on the outside of 

 their bodies. This is what makes an insect hard. For 

 this reason, insects cannot grow slowly and steadily, as 

 animals do that have their skeletons inside and covered 

 with the soft and easily stretched muscles and skin. 

 When insects grow they do so by sudden jumps, as it 

 were. When they have grown so that they fill their out- 

 side skins very tightly, a new skeleton is formed inside 

 of the old; the old skeleton bursts and is shed by the 

 insect. While the new skeleton is yet soft it allows a con- 

 siderable growth of the insect. This process is repeated 

 several times in the life of every insect before it becomes 

 fully grown. Having the skeleton on the outside is a great 

 protection to the insect. 



With some insects these changes of skeleton are ac- 

 companied only by a change in the size from the newly 

 hatched form to the adult; but with others there are 

 great changes in structure and appearance during the 

 last two changes of skin. We must know something of 

 these changes in order to recognize the different stages 

 in the life of the same insect. 



Immature and adult forms of insects. Among the in- 

 sects that change but little with their development are 



249 



