THE LENGTH OF THE LARVAL PERIOD 47 



The larvse present the most diverse characteristics for the 

 different groups of insects, and vary extremely even for 

 closely related species. There has been a distinct adapta- 

 tion to conditions during this stage, and larvae with slightly 

 varying habits have doubtless been affected by natural 

 selection in the same manner as adults have been affected 

 by their particular environment. 



THE LENGTH OF THE LARVAL PERIOD. 



The length of the larval period is also in a wide degree 

 an adaptation of this kind, which is frequently of the utmost 

 importance in economic treatment of the species. 



The pupa stage is the connecting stage between the larva 

 and the adult, and may be similar to larval form or differ 

 markedly from it, according as the insect has incomplete 

 or complete metamorphosis. While in some forms it may 

 feed to some extent, the more common condition is that of a 

 quiescent non-feeding period. During this stage, however, 

 important internal changes occur which lead to the maturing 

 of the insect. For those forms which have a perfectly 

 quiescent pupa stage, various forms of cells are made within 

 which the pupation occurs, others secrete themselves in 

 rubbish, folds of leaves, crevices or cracks in bark, while 

 some construct a tough, silken cocoon as a permanent 

 protecting case. 



The adult stage or imago differs usually from the preced- 

 ing stage in the acquisition of well-developed wings, except- 

 ing, of course, in the wingless forms, and especially in the 

 maturity of the organs of reproduction. The period of 

 life varies in the adult also in quite a degree for species 

 living over winter and others for varying periods, although 

 more commonly the adult perishes soon after the comple- 

 tion of the reproductory process. To indicate the various 

 forms of adults would be to review all the different groups 

 of insects, and hence need not be attempted even in brief. 



A very distinct grouping of insects may be made with 

 reference to the definiteness of transformation. Those 

 which develop without marked changes between the different 



