8 ON THE ANATOMY OF THE FLY. 



them, but I think to add to them as regards the development of the 

 proboscis. I believe that those segments of the fly immediately 

 connected with the functions of vegetative life, those surrounding 

 the anterior portion of the alimentary canal, that is, the 

 proboscis, as well as the abdominal segments, are immediately 

 dependent upon the corresponding larval segments for their 

 development and form ; whilst the head and thorax with all their 

 appendages, the chief centres of animal life, are developed from 

 Weismann's Imaginal discs, which do not coalesce with each 

 other until the third day of the pupa state, so that these re- 

 place the larval segments from the fourth to the eighth inclusive. 



The brain seems to form a centre around which these changes 

 take place ; even this organ is entirely altered, but by a process 

 similar to ordinary development. The other structures are formed 

 at the expense of a granular fluid which is chiefly derived from 

 the so-called fatty bodies ; these extend along the entire length 

 of the larva on either side of the alimentary canal. 



The skin of the larva of the fly is not shed when it 

 reaches maturity, but dies and becomes very hard, turning 

 yellow at the first, then red, and afterwards almost black. 

 It forms the pupa case. This is not the representative of 

 the skin of the pupa in most insects, for the corresponding 

 larval skin is usually shed, and the pupa case is either altoge- 

 ther wanting, as in many hawk-moths and butterflies, or it is 

 replaced by a cocoon of silk spun by the larva, as in the silk 

 moth, or by some other adventitious covering. 



The respiratory function ceases, and all the organs proper to 

 the maggot undergo degeneration soon after the formation of 

 the pupa case. The nerve centres grow rapidly, and the Imaginal 

 discs unfold into delicate cellular expansions, which coalesce 

 with each other, and with the new layers of cells formed within 

 the three anterior and nine posterior larval segments, and ul- 

 timately form an exceedingly delicate membrane corresponding 

 to the pupa skin of butterflies. In both, this marks out the position 



