426 



hand, the cells elongate remarkably, producing long threads, 

 as much as 75/x in length, and consisting each of two or 

 three cells joined one behind the other (fig. 136). These 

 threads have become inserted into the syncytial columns of 

 the developing wing muscles, and as they lengthen, the muscle 

 bands contracting a little, become suspended in the upper 

 part of the thorax. 



Meanwhile in the newly formed pupa, the ectodermal 

 cells of the mid-dorsal region of the propodeal segments 

 elongate and grow forwards. Passing underneath the more 

 anterior muscle insertions of the metathoracic integument 

 they extend forwards and penetrate the muscle column. At 

 the anterior end of the muscle columns more distant integu- 

 mental cells likewise communicate with the developing mass 

 of myoblasts, so that we get a condition not unlike what has 

 been observed in the head and leg muscles; the insertion cells 

 from a considerable area all converge upon the great muscle 

 band. The result when the insertion cells contract is the 

 same as what occurs in these other muscles. The two bands 

 are pulled apart into their five constituent columns, and these 

 on differentiating form the longitudinal thoracic muscles of 

 the adult. 



Contractions of the ''suspension threads" is preceded by 

 a longitudinal splitting of them, and each thread is now a 

 unicellular structure (fig. 137). 



The sarcostyles, as they develop within these minute 

 columns, communicate several with one thread. The threads in 

 the twenty-four hour pupa then begin to contract. The muscles 

 become stretched and at the same time the walls of the pro- 

 podeum and mesothorax become drawn closer together: in 

 this way the arched thorax of the imago is formed. 



The insertion cells meanwhile have secreted at their 

 exterior, the cuticle of the integument, but they do not 

 undergo complete chitinisation (fig. 137a). On the contrary, 

 their more internal parts remain protoplasmic, and split up 

 into a number of fibrils, each communicating now with a 

 single sarcostyle. In this condition they are seen in the 

 imago (fig. 138). 



The Structure of the Adult Muscles. 



From the above description it follows that the adult 

 muscles are of several types. The simplest are the longi- 

 tudinal abdominal muscles, formed by the fusion of succeed- 

 ing myoblasts, in one line. 



The dilators of the pharynx are more complex and 

 correspond in reality to a number of these longitudinal 



