THE INTEGUMENT. 



of the new organs about to be developed, and in both, this is shed 

 when the insect emerges from the pupa. These changes complete 

 what Dr. Weismann calls the first stage of the development of 

 the fly the formation of the pupa. 



It may be confidently asserted that not one structure exists 

 in the fly as it exists in the maggot. Every portion of the larva, 

 except the brain, and perhaps the basement membrane of the 

 alimentary canal, undergoes rapid degeneration, and the fly is 

 formed within the pupa skin by a process of redevelopment. 



Section III. The Integument. 



The integumental system of the perfect fly originates upon the 

 inner surface of the pupa skin, but as its parts become fully de- 

 veloped they shrink away from the pupa skin, so that it forms a 

 loose investing membrane around the new formed fly, and may be 

 looked upon as one of the integuments shed in the process of 

 development; for, although it is never thickened and indurated 

 in the fly, it is precisely analogous to the pupa case in the 

 lepidoptera, which is always more or less hardened. The reason 

 of its not being hardened in the fly seems to be due to the fact 

 that it is invested in the pupa shell or last larval integument. 



The integuments of insects are usually said to .consist of three 

 layers, and these may be easily traced in the fly. The outer- 

 most is transparent and continuous over the whole surface of the 

 insect, investing all the appendages and processes of the skin, 

 even the hairs, and covering the surface of the eyes. It appears 

 to be continuous with the lining membrane of the tracheal system, 

 and to extend throughout the digestive cavity, although it is 

 somewhat modified in the latter. 



