3i8 THE DEVELOPMENT OF THE NYMPH. 



depressed upon the ventral surface of the pupa-case in a dry 

 and brittle condition. The formation of the anterior abdominal 

 fold is also probably due to muscular contractions, but the 

 reduction in the size of the abdominal region, which gives rise 

 to the evolution of the head and thorax, is certainly not 

 muscular. It occurs after the muscles are all completely 

 detached from the body wall, and when most of them are in a 

 far advanced stage of histolysis. These changes are clearly due 

 to contraction of the paraderm, the cells of which increase in 

 thickness as they diminish in the extent of their surface. 

 Moreover, the character of the contraction differs entirely 

 from the result of a muscular act. The reduction of the magni- 

 tude of the abdomen occurs very slowly, and gives it the form 

 characteristic of that of the nymph. The anterior abdominal 

 fold is obliterated and the thorax exposed, whilst the latter is 

 subsequently distended, and the head is evolved by the flow of 

 pseudo-yelk from the abdomen forwards. 



b. The Development of the Integument of the Head and Thorax. 



As has been already stated, the integument of the thorax is 

 developed from the epiblast of six pairs of imaginal discs. The 

 manner in which these overgrow and replace the paraderm has 

 been described by Van Rees [147], and, except that he terms 

 the paraderm ' the larval hypoderm,' my own observations 

 confirm his statement that the edge of each disc overlaps the 

 paraderm and extends by the development of new cells, whilst 



DESCRIPTION OF PLATE XX. 



The exterior of the pronymph and nymph, and the anterior spiracles of the nymph. 

 FIG. I. The thorax of the nymph at the end of the third day of the pupa, seen from 



its ventral surface (after Weismann). 

 FIG. 2. The same seen from its dorsal surface. 

 FIG. 3. The anterior spiracular apparatus of the nymph on the ninth or tenth 



day of the pupa stage : s c, stigmatic cornu ; s, intersegmental spiracle ; 



t r, tracheal vessel. 

 FIG. 4. The pronymph at the end of the third day, showing the position of the 



abdominal imaginal discs. 

 FIG. 5. The nymph on the sixth day of the pupa stage. 



