420 



sternodorsales, attached to the more anterior portion of the 

 mesothorax above, while below they are inserted close to the 

 origin of the legs. The muscles which move the wings .have, 

 therefore, no direct communication with these ; their action 

 merely causes changes in the shape of the thorax, changes 

 which alter the disposition of certain prominences and depres- 

 sions on the thorax at the wing insertion, into which fit other 

 depressions and projections from the base of the wing. A dis- 

 cussion of the mechanism of flight is beyond the scope of this 

 paper. 



The development of these muscles begins in the larva 

 at about the time of defaecation. An examination cf even 

 the earliest larvae will reveal scattered cells, the myoblasts of 

 Perez lying near the muscles, but remaining during larval 

 life in an embryonic condition. They are not unlike leuco- 

 cytes in appearance; but they are smaller, and do not show 

 protoplasmic vacuolation, which is so frequently seen in the 

 former. 



But at the time of defaecation the longitudinal thoracic 

 muscles show distinct indication of degeneration. Sometimes 

 the striations merely , become ill-defined. At other times the 

 muscle substance undergoes a total disintegration, breaking 

 down into a granular fluid which remains within the un- 

 ruptured sarcolemma. This condition is especially well seen 

 in the larva four hours later. 



But while the majority of larval muscles disintegrate at 

 a later stage, under the phagocytic action, apparently, of the 

 leucocytes, a certain number of dorsally situated thoracic 

 muscles — three, or sometimes four pairs — become enveloped 

 by the myoblasts, which shortly before this have become very 

 active. 



Sometimes the myoblasts begin to spread over the muscles 

 while these are almost normal in appearance (fig. 113); but 

 at other times advanced degeneration is very apparent. 



The proliferation of myoblasts, always by mitosis, is most 

 marked at the metathoracic and the rear of the mesothoracic 

 segment; from here the myoblasts extend forwards in a 

 pair of great columns partly upon, partly independently of 

 the larval muscles, drawing the neighbouring longitudinal 

 muscles together as they advance (fig. 128); in the larva 

 at the end of the period of defaecation they have extended 

 right along the mesothoracic muscles (fig. 112), while the 

 myoblasts at this stage are beginning to extend along the 

 prothoracic muscles, which have almost retained their normal 

 appearance (figs. 113, 129). Three or four of the dorsal pairs 

 of longitudinal muscles of the prothorax and mesothorax are 



