650 TEXT-BOOK OF ENTOMOLOGY 



"The optic and anteunal nerves have nearly attained their full development, 

 and those numerous and most intricate plexus of nerves in the three thoracic 

 segments of the larva form only a few trunks, which can hardly be recognized 

 as the same structures. The arrangement of the whole nervous system is now 

 nearly as it exists in the perfect insect. The whole of these important changes 

 are thus seen to take place within the first three days after the insect has under- 

 gone its metamorphosis ; and they precede those of the alimentary canal, 

 generative system, and other organs, which are still very far from being com- 

 pleted, and indeed, as compared with the nervous system, have made but little 

 progress." (Art. Insecta, pp. 962-965.) 



The initial steps and many of the subsequent internal changes 

 escaped the notice of Newport and others of his time, and it was 

 not until the epoch-making work of Weismann on the ultimate 

 processes of transformation of Corethra and of Musca, that we had 

 an adequate knowledge of the subject. 



Weismann (1864) was the first to show for the Muscidse and 

 Corethra that the appendages, wings, and other parts of the imago 

 originate in separate, minute, cellular masses called imaginal disks, 

 buds, or folds (histoblasts of Kilnckel). These imaginal buds, 

 which arise from the hypodermis, being masses of indifferent cells, 

 are usually present in the very young larva, and even in the later 

 embryonic stages. It has been shown that such imaginal buds exist 

 for each part of the body, not only for the appendages and wings 

 (p. 126), but for the different sections of the digestive canal. Dur- 

 ing the semipupal stage these buds enlarge, grow, and at the same 

 time there is a corresponding destruction of the larval organs. 

 The process of destruction is due to the activity of the blood cor- 

 puscles or leucocytes (phagocytes), the larval organs thus broken 

 up forming a creamy mass, the buds from which the new organs 

 are to arise resisting the attacks of the virulent leucocytes, which 

 attach themselves to the weakened tissue and engulf the pigments 

 (see p. 422). The two processes of destruction of the larval organs 

 (histolysis) and the building up of the imaginal organs (histogenesis) 

 go hand in hand, so that the connection of the organs in question in 

 most cases remains entirely continuous ; while the last steps in the 

 destruction of the larval organs only take place after the organs of 

 the imago have assumed their definite shape and size. Other ob- 

 servers have corroborated and confirmed his statements and obser- 

 vations, Gonin extending them to the Lepidoptera and Bugnion to 

 the Hymenoptera. 



It is a pity that the observations, such as were set on foot by 

 Weismann, were not first made on the Trichoptera and Lepidoptera* 

 which are much more primitive and unmodified forms than the 



