354 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



the wing muscles, destructive changes taking place for only a short time 

 after pupation. As we have seen, the so-called wing muscles are at the 

 time of pupation composed of a few cylindrical strands or fibres of undif- 

 ferentiated sarcoplasm which contain many nuclei undergoing rapid 

 amitotic division. For some time in the pupa no very evident changes 

 occur. Many of the elongated muscle nuclei and numerous chains of 

 nuclei (Plate 6, Figure 30) are present. The tracheal cells are still 

 increasing rapidly by mitosis, and in a two- to three-day pupa have be- 

 come numerous, occupying most of the space between the strands 

 (Figure 19, cl.tr.). 



At a stage when pupal life is nearly half over, the fibrillae of the adult 

 muscles begin to show. Figures 29 and 30, represent the appearance of 

 the muscles at this period. The cross section (Figure 29) shows scattered 

 through it the cross sections of newly formed fibrillae of various 

 sizes. The longitudinal section (Figure 30), taken from another muscle 

 of the same series of sections, shows longitudinal fibrillation. Sections 

 of stages a little younger than this, e.g., the stage shown in Figure 19, re- 

 veal only the faintest hint of these structures under high magnifications. 



During the last half of pupal life, a number of important changes take 

 place, the most noteworthy being growth in size. In some muscles the 

 area of cross section doubles or even quadruples during this period 

 (compare Figure 19 with Figure 21, the latter showing three fibres of 

 the former, the magnification being in each case 800 diameters). This 

 increase in area of cross section is accompanied by a lengthening of the 

 muscles, sometimes to even twice their former length, so that their 

 volume increases many fold. A rough estimate of the changes in 

 volume during metamorphosis of any metathoracic muscle can be made 

 from the series of anatomical drawings given on Plates 1-5, as these are 

 all drawn to the same scale. 



The tracheal cells in a stage a few days before the emergence of the 

 imago (Figure 21, cl.tr.) arrive at a condition in which there are no 

 more cell divisions. In cross sections of the muscles at this stage the 

 tracheal cells are not as numerous as in the earlier stages (Figure 19). 

 This does not mean that they are fewer in jiumber in the whole muscle, 

 however, as the volume of the muscle has increased witliout a corres- 

 ponding increase in the number of tracheal cells. Nearly every tracheal 

 cell in Figure 21 shows its future plainly. Some {cl. ir.^) have formed 

 tracheoles through their cytoplasm and show connections with tracheae. 

 Most of tlie others are connected with tracheae, but their connections are 

 severed by the plane of the section (d. tr.^. There are a few, however, 



