THE DEVELOPMENT OF INSECTS 27 



scribed for the Ametabola. On escaping from its old skin or shell, how- 

 ever, besides a rapid increase in size, changes of structure also occur, 

 so that a difference in appearance now becomes evident. These changes 

 must be produced quickly, as the hypodermal cells of these parts, as 

 well as of all the surface, are producing a new chitinous skin, and when 

 this has once hardened, no further changes and little further growth are 

 possible. Molting then, marks the beginning of a brief period — a day, 

 more or less — of increase in size and of changes in appearance, these last 

 all being in the direction of making the young insect more nearly like the 

 adult it is to become. When the new shell has become hardened the 

 insect resumes its feeding. 



After another feeding period the young insect is again confronted 

 with the same difficulties as before, and it meets them in the same way, 

 by molting, and immediately thereafter, before its new shell has hardened 

 it seizes the opportunity to grow and change its appearance further. 

 Finally, after some molt, full adult size for the insect is attained and all 

 its organs have also fully developed and matured, producing the adult 

 insect itself. 



Thus the young insect becomes an adult by alternating periods of 

 feeding, with brief periods of molting, following which growth and change 

 take place, the total of which produces the adult. 



The number of molts and consequent opportunities for change which 

 occur, varies in different Hemimetabola. There may be only two or 

 three in some kinds: five is perhaps the average number though more 

 are not uncommon, and 21 are known to occur in one species. 



Certain names for these different conditions are convenient for use. 



The feeding periods between the molts (or ecdyses) are called instars, 

 so that the progress of an insect from hatching to adult is by an 

 alternation of instars and molts. The insect itself, from hatching until 

 maturity is generally called a nymph. Figure 32 shows the changes in 

 size and appearance of a grasshopper after each molt. 



With the remaining group of insects, the Holometabola, while there 

 is a little similarity in the metamorphosis to that in the Hemimetabola, 

 there are also many differences. 



When a young Holometabolous insect hatches, it in no way resembles 

 its adult. A caterpillar is totally different in appearance from the butter- 

 fly it finally becomes : the white grub in the earth is in no way suggestive 

 of the June bug (May beetle) into which it transforms. Nevertheless 

 it has to meet the same problems of growth and transformation to the 

 adult condition as do the Hemimetabola, and uses the same means for 

 accomplishing the needed results, viz., the utilization of the energy 

 derived from its food. 



Accordingly, upon hatching, in the Holometabola, a feeding period or 

 instar comes first, followed by a molt and growth. At this point the 



