EMBRYONIC GROWTH OF THE WINGS 



127 



In the course of his embryological studies on the Muscidse (Musca 

 rnmitoria and Sarcophaga ci(rnarla) Weisinann (1864) in examining 

 the larvae of these flies just before pupation, found that the wings, 

 as well as the legs and mouth-appendages, developed from micro- 

 scopic masses of indifferent cells, which he called " imaginal discs." 

 From the six imaginal discs or buds in the lower part of the thorax 

 arise the legs, while from four dorsal discs, two in the meso- and two 

 in the metathoracic segment, arise the fore and hind wings (Fig. 141.) 

 These imaginal buds, as we prefer to call these germs, usually appear 

 at the close of embryonic life, being found in freshly hatched larvae. 



As first observed by Weismann, the buds are, like those of the 

 appendages, simply attached to tracheae and sometimes to nerves, 



Fro. 140. Imaginal buds in Musca, A, in Corethra, B, in Melophagus, <?, in embryo of 

 Melophag-us ; dorsal view of the head ; b, bud ; p, peripodal membrane ; c, cord ; hi/, hypodermis ; 

 of, cuticula ; st, stomoda-um ; r, ventral cephalic, behind are the two dorsal cephalic buds. After 

 Pratt. 



in the former case appearing as minute folds or swellings of 

 the peritoneal membrane of certain of the tracheae. In Volucella 

 the imaginal buds were, however, found by Kunckel d'Herculais to 

 be in union with the hypodermis. Dewitz detected a delicate thread- 

 like stalk connecting the peripodal membrane with the hypodermis, 

 and Van Rees has since proved in Musca, and Pratt in Melophagus. 

 the connection of the imaginal buds with the hypodermis (Fig. 140). 

 These tracheal enlargements increase in size, and become differen- 

 tiated into a solid mass which corresponds to the upper part of the 

 mesothorax, while a tongue-shaped continuation becomes the rudi- 

 ment of the wing. During larval life the rudiments of the wings 



