148 



acciiniulation of dark material around Krause's nienibrane, which is thus hidden 

 from view. 



The Structure, Development, and Action of Wing-moving 

 Muscles of Insects. 



In general appearance the structure of these remarkable tissues is similar 

 to that of other striated muscle fibres. There is usually, except in weak flyers, 

 a great preponderance of sarcoplasm ; the sarcostyles can be separated very 

 easily from one another, as Schafer observed, but in other respects they appear 

 fairly identical with other striated fibres. Double spiral striations, Krause's 

 membranes, and a sarcolemma are all present ; nuclei occur in very large numbers 

 on the outside of the muscles, but the muscles show no tendency to split trans- 

 versely, as do other striated fibres. Nevertheless, in spite of the wonderful 

 similarity between these two types, they develop in entirely different ways, and 

 are to be regarded as having evolved, therefore, from entirely different sources. 

 Their similarity, indeed, is to be looked upon as the result of a wonderful process 

 of convergent evolution, rivalling anything that one finds among the organs of 

 vision of the various groups of animals. 



The following account is the result of an investigation of the development 

 of the wing-moving muscles of a chalcid wasp (Nasonia). In the meta- 

 morphosing larva there is formed on either side of the thorax a longitudinal 

 column of embryonic myoblasts, which develop at the expense of certain of 

 the larval thoracic muscles. In the larva shortly before its pupation these 

 myoblasts have consolidated to form each a narrow band inserted behind and 

 in front upon the thoracic walls, and the larval muscles have been entirely 

 absorbed (fig. 16, pi. xiii.). Several hours later certain of these myoblasts 

 cohere to form five long syncytial columns, passing within the band from one 

 end to the other (fig. 17, pi. xiii.). Later these syncytia grow in thickness by 

 the incorporation of more cells (fig. 18, pi. xiii.). Shortly after pupation a 

 remarkable thing is to be observed. The myoblasts surrounding each syncytial 

 column develop a process at either end, and these processes grow into the 

 columns, and develop eventually into the sarcostyles of the adult insect, each 

 becoming inserted on to the thoracic wall by its terminations. Fig. 19, pi. xiii., 

 which is a longitudinal section along the edge of a syncytial column, shows this 

 process going on. 



As development continues, more and more of the myoblasts develop into 

 bipolar cells, and send their processes into the sarcoplasm, and when in the 

 adult insect the number of sarcostyles comprising a fibre is estimated, these 

 are found to be approxmately equal in number to the nuclei which surround the 

 fibre (between 800 and 900). Eventually the five complex columns so formed 

 become drawn apart by the pull exerted upon them by the thoracic walls, and 

 the five longitudinal wing-moving muscles are to be recognized, on either side 

 of the body. (Each of the five pairs of muscles in this insect is a single multi- 

 cellular fibre; in some insects more than one such fibre may be present.) 



By this extraordinary process, then, there is developed a true muscle fibre : 

 the outer walls of the cells represent the sarcolemma, the sarcostyles represent 

 the fibrillae, and the protoplasm of the syncytial columns becomes the sarcoplasm. 

 Eventually the sarcostyles develop alternate dark and light discs, adjacent ones 

 being so disposed as to form a double spiral (helicoid) within the fibre, and 

 the formation of minute Krause's membranes, produced apparently from the 

 sarcostyles, completes the extraordinary development. 



Perez (1910) has examined the development of the wing muscles of the 

 blow fly. He comes to entirely different conclusions from those stated above, 

 but his figures show clearly that the process is no different in that insect from 



