122 Development ivithin the Larva 



and mount in Farrant's medium. Several larvae, of 

 different degrees of maturity, should be prepared in this 

 way. The new rudiments will be found arranged in two 

 rows, dorsal and ventral, and there is a dorsal and ventral 

 set to each thoracic segment. The ventral rudiments 

 ultimately yield the legs of the fly. They can be 

 followed from the first simple buds, enclosed in trans- 

 parent sheaths (the outer walls of the imaginal folds), 

 until they become long and convoluted, divided into 

 joints, and covered with hairs. Sections taken at 



g 



w - 



Tig. 92. — Thoracic appendages of papa and fly, as seen in larva about 

 to pupate. The larval skin has been removed. Left hand, side view : 

 right hand, ventral view, i, I', I", first, second, and third legs, w, wing. 

 h, haltcre. g, prothoracic tracheal gill (of pni)a). 



different times reveal all the stages of tissue-development. 

 As the legs increase in lengtli they become folded 

 beneath the larval skin in the manner represented in 

 fig. 92. 



The two hinder pairs of dorsal appendages give rise to 

 the wings and halteres. The case of the prothoracic 

 dorsal appendage presents some perplexing features. 

 It develops into a short tube, from which three main 



