l6o ENTOMOLOGY 



cent period of reconstruction becomes inevitable (though this statement 

 does not, of course, explain anything) . 



As was said, the cruciform type of larva has been derived from th( 

 thysanuriform type, the strongest evidence of this being the fact that 

 among hypermetamorphic insects, the change from the one to the oth( 

 takes place during the lifetime of the individual. Furthermore, th< 

 cruciform condition is plainly an adaptive one, brought about by 

 abundant and easily obtainable supply of food. The lack of a thysanuri- 

 form stage in the development of the most specialized cruciform larvae, 

 as those of flies and bees, is regarded by Hyatt and Arms as an illustra- 

 tion of the general principle known as " acceleration of development," 

 according to which newer and useful adaptive characters tend to appear 

 earlier and earlier in the development, gradually crowding upon and 

 forcing out older and useless characters. In connection with this sub- 

 ject, the appearance of temporary abdominal legs in embryo bees is 

 significant, as indicating an ancestral active condition. In accounting 

 for the evolution of metamorphosis, the theory of natural selection finds 

 one of its most important applications. 



3. INTERNAL METAMORPHOSES 



In Heterometabola, the internal post-embryonic changes are as di- 

 rect as the external changes of form; in Holometabola, on the contrary, 

 not all the larval organs pass directly into imaginal organs, for certain 

 larval tissues are demolished and their substance reconstructed into 

 imaginal tissues. When indirect, the internal metamorphosis is 

 nevertheless continuous and gradual, without the abruptness that 

 characterizes the external transformation. In the larval stage imaginal 

 organs arise and grow; in the pupal stage the purely larval organs 

 gradually disappear while the imaginal organs are continuing their 

 development. 



Phagocytes. The destruction of larval tissues, or histolysis, is due 

 often to the amoeboid blood corpuscles, known as leucocytes or phago- 

 cytes, which attack some tissues and absorb their material, but later 

 are themselves food for the developing imaginal tissues. The construc- 

 tion of tissues is termed histogenesis. 



In Coleoptera the degeneration of the larval muscles is entirely 

 chemical, there being no evidence of phagocytosis, according to Dr. 

 R. S. Breed. Berlese, indeed, goes so far as to deny in general the 

 destructive action of leucocytes on larval tissues. 



