TRACHEATA. 423 



discs. The hypodermis of the larva passes directly into that of 

 the imago. 



The pupa stage of Corethra is relatively very short, and the 

 changes in the internal parts which take place during it are not 

 considerable. The larval abdominal muscles pass for the most 

 part unchanged into those of the imago, while the special 

 thoracic muscles connected with the wings, etc., develop directly 

 during the latest larval period from cords of cells already formed 

 in the embryo. 



In the Lepidoptera the changes in the passage from the 

 larval to the adult state are not very much more considerable 

 than those in Corethra. Similar imaginal discs give rise during 

 the later larval periods to the wings, etc. The internal changes 

 during the longer pupa period are somewhat more considerable. 

 Important modifications and new formations arise in connec- 

 tion with the alimentary tract, the nervous and muscular 

 systems. 



The changes which take place in the true flies (Muscidse) are 

 far more complicated than either those in Corethra or in the 

 Lepidoptera. The abdomen of the larva of Musca becomes 

 bodily converted into the abdomen of the imago as in the above 

 types, but the whole epidermis and appendages of the head and 

 thorax are derived from imaginal discs which are formed within 

 and (so far as is known) independently of the epidermis of the 

 larva or embryo. These imaginal discs are simple masses of 

 apparently indifferent cells, which for the most part appear at 

 the close of embryonic life, and are attached to nerves or 

 tracheae. They grow in size during larval life, but during the 

 relatively long pupa stage they unite together to give rise to a 

 continuous epidermis, from which the appendages grow out as 

 processes. The epidermis of the anterior part of the larva is 

 simply thrown off, and has no share in forming the epidermis of 

 the adult. 



There are a pair of cephalic imaginal discs and six pairs of 

 thoracic discs. Two pairs, a dorsal and a ventral, give rise to 

 each thoracic ring, and the appendages attached to it. 



Though, as mentioned above, no evidence has yet been 

 produced to shew that the imaginal discs of Musca are derived 

 from the embryonic epiblast, yet their mode of growth and 



