424 



expense of the disintegrated muscle, nourishing themselves, 

 perhaps, also upon the blood. At any rate, the syncytium 

 formed from the myoblast grows, and then undergoes -longi- 

 tudinal fibrillation, the "fibrils" corresponding in no way 

 with the individual myoblasts. 



The fact that in Nasonia the number of fibres is 

 approximately equal to that of the myoblasts, and that these 

 ■can, though with difficulty, be observed to form each a fibril, 

 renders the conclusions of Perez in regard to Calliphora 

 doubtful ; it is also possible that the five pairs of larval muscles 

 which persist and into which the myoblasts migrate, are in 

 reality the syncytial columns observed in Nasonia^ and that 

 Perez failed to observe the earlier state in which larval muscles 

 were being overwhelmed. It is, of course, unsafe to argae 

 by analogy, but the fact that Van Rees observed only three 

 pairs of persisting muscles is significant. 



During pupal life there is a considerable thickening of 

 the five pairs of longitudinal wing muscles. In the late larva 

 they represent a very narrow strip measuring 55/x in breadth, 

 14/x to 15/x in thickness. In the fresh pupa, when five columns 

 of muscles have developed within these, they have become 

 more prominem} ; but they do not yet form the predominating 

 organ of the thorax. But in the twenty-one hour pupa, when 

 they have broken up into five pairs of muscle columns, they 

 begin to replace the fat-body, which has till now filled the 

 greater part of the thorax, and growing larger and larger, 

 displace this more and more, and with the vertical (dorso- 

 sternal) muscles which have meanwhile been developing, 

 occupy almost the whole of the cavity of the thorax. 



The development of the dorso-sternal muscles is very 

 similar to that of the longitudinal thoracic muscles, and need 

 only be briefly referred to here. However, they serve to 

 illustrate that the myoblasts may be quite independent of 

 the degenerate muscles over which they are extending. In 

 the imago five pairs of sterno-dorsales are present; three of 

 these are formed by the extension of the myoblasts over the 

 three pairs of degenerate vertical (oblique) muscles of the 

 three thoracic segments of the larva; but the absence of 

 sufficient larval muscles does not prevent the other two pairs 

 fiom developing (the muscles of the propodeum play no part 

 in the process, but degenerate in the same curious way as 

 do the other vertical abdominal muscles). They simply grow 

 as two pairs of vertical cell columns, quite independently of 

 any larval muscle. Within each, as also in the other three 

 developing sterno-dorsales after the larval muscle has finally 

 been absorbed, a single columnar syncytium (not five, as occurs 



