I0 6 ELEMENTARY ZOOLOGY. 



other groups of animals. (See Figs. 94 and 118.) The 

 immature young is called a larva. Very often its habits 

 and general mode and place of life is wholly different 

 from that of the adult. For example, the larva of the 

 mosquito is a water form and lives on the matter in stag- 

 nant water. The adult has a wholly different range of 

 life and sucks the blood of higher animals as its food. 



The change from the larva to the adult may be gradual 

 (as from the tadpole to the frog) or may seem sudden 

 because of the fact that the process takes place in a cocoon 

 or something of the kind, with a distinct resting stage, 

 as in the butterfly. This resting stage is called the pupa. 

 The changes in the* organs of the caterpillar are so pro- 

 found that it could not adjust itself to an active life while 

 they are being made. 



The larval stage is usually one of voracious eating. 

 During this time much fat is stored up for the change 

 which is to come. 



113. Practical Exercises for Library and Field. Enumerate all the 

 animals you are familiar with that undergo a metamorphosis. What 

 are the chief differences in structure between the larva and adult? 

 Illustrate the differences between their mode of life in the larval 

 and the adult stages. What is the possible value of such an indirect 

 development to an animal? What are its possible disadvantages? 

 What is the special advantage of going into a pupa stage for the 

 transformation from larva to adult, as compared with the way of the 

 frog? 



114. Specialization and Differentiation. We have seen 

 that the earliest stages of development consist largely of 

 cell multiplication. The later stages may be said to 

 consist of growth and differentiation. Sometimes there 

 is differentiation during cell divisions as in those em- 

 bryos where the first cells are unequal (Fig. 20, B, C). 



