26 APPLIED ENTOMOLOGY 



The remaining insects, from this standpoint, form two groups: those 

 which on hatching show some resemblance to the adults and reach matu- 

 rity by a certain series of changes; and those which on hatching are 

 totally unlike the adults and attain that condition in a different way. 

 These groups are known as the Hemimetabola or Heterometabola, and 

 the Holometabola respectively, these names suggesting the amount of 

 metamorphosis required for members of each group to become adult. 



A member of the group Ametabola, upon hatching, will begin to 

 feed and grow. Growth, however, is restricted because the insect is 

 enclosed by chitin which, while elastic to some extent, at least at its 

 thinner portions, has its limitations in this regard. In some cases the 

 insect is able to reach its adult size within the chitin, but in other cases 

 this proves impossible, and a process called molting takes place. This is 

 begun by pouring out of fluid by the outside layer of living cells, the 

 hypodermis, between it and the chitin, separating the two. Next a 

 split in the chitin appears somewhere, usually along the back, and the 

 insect crawls out of its skin, i.e., molts. It is now soft and unrestricted by 

 an outer shell and grows rapidly. A new chitinous shell begins to appear 

 and is completed in a short time (within a day or so) and thereafter only 

 such growth is possible as the elasticity of the new shell will permit. 



In most of the Ametabola, molting as thus described is not usual, 

 the shell being sufficiently thin to stretch the amount needed for growth 

 to adult size, though sometimes two or even three molts may occur. 

 In both cases, however, the reproductive organs appear not to be mature 

 at the time of hatching, and only gradually become so during the period 

 following. In a few cases molting seems to occur at intervals throughout 

 life. 



In the Hemimetabola (or Heterometabola) the young insect on es- 

 caping from the egg, though resembling its parent to some extent, 

 must nevertheless undergo many changes in structure and a considerable 

 increase in size as well, before reaching maturity. Thus a young short- 

 horned grasshopper, on hatching, will need to grow to be about ten times 

 as long before becoming adult; it is without wings, which will need to be 

 developed; its reproductive organs are not mature and must become so, 

 and other differences occur. All of these must be transformed into their 

 condition in the adult, and to accomplish this, energy is necessary. In 

 the egg the energy for development had been provided by the yolk: 

 after hatching the young insect must provide it by gathering food. 



The young insect therefore, soon after hatching seeks for food, and 

 having found it begins feeding. The nourishment thus obtained results 

 in growth so far as this is possible within a shell which is tightly fitting 

 and only to some degree elastic. When no further growth in this way 

 can occur and the body has stored within it all the materials needed for a 

 greater increase in size, it proceeds to molt in the manner already de- 



