90 HOW ANIMALS DEVELOP 



Larvae 



In many animals the process of development is 

 complicated by the fact that the animal passes 

 through one or more larval stages before it becomes 

 adult. The best known illustration of this is found 

 in the life-history of insects. Everybody knows that a 

 butterfly's tgg develops first into a caterpillar and 

 then into a pupa or chrysalis, and only turns into a 

 butterfly after all this preparation. Some insects have 

 a much simpler development ; earwigs, for instance, 

 hatch out of the ^gg as little earwigs, which look 

 quite like the adult, although there are still a few 

 changes to be made. But a caterpillar is extremely 

 different from an adult butterfly: so different, in 

 fact, that nearly all its organs have to be broken 

 down and absorbed in the chrysalis stage, when they 

 provide material which nourishes the new organs out 

 of which the butterfly is built up. For instance, only 

 the three pairs of legs nearest the head persist into 

 the adult, and all the others disappear. The wings and 

 most of the adult organs develop from little lumps 

 of tissue called imaginal discs which are formed quite 

 early in the caterpillar and wait till the pupa stage 

 before they start growing. Fig. 24 shows the front 

 part of a caterpillar of a swallow-tail butterfly, with 

 part of the skin lifted up to show the imaginal discs 

 from which the wings will grow. 



Very many invertebrate animals and a few verte- 

 brates go through larval stages before they become 

 adult, and there may be a whole series of different 



