148 



ENTOMOLOGY 



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, however, the internal metamorphosis 



FIG. 220. Diagram- 

 matic transverse section of 

 Corethra larva, to show 

 imaginal buds of wings (w) 

 and legs (/); h, hypoder- 

 mis; i, integument. Modi- 

 fied from Lang's Lehrbuch. 



FIG. 221. Diagrammatic transverse sections of muscid 

 larvae, to show imaginal buds, h, larval hypodermis; i, larval 

 integument; ih, imaginal hypodermis; /, imaginal bud of leg; 

 w, imaginal bud of wing. Modified from Lang's Lehrbuch. 



is nevertheless continuous and gradual, without the abruptness that 

 characterizes the external transformation. In the larval stage imaginal 

 organs arise and grow; in the pupa stage the purely larval organs grad- 

 ually disappear while the imaginal organs are continuing their develop- 

 ment. 



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

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



