304 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 number in the whole muscle, 
however, as the volume of the muscle has increased without a corres- 
ponding increase in the number of tracheal cells. Nearly every tracheal 
cell in Figure 21 shows its future plainly. Some (cl. ¢r.’) have formed 
tracheoles through their cytoplasm and show connections with tracheae. 
Most of the others are connected with tracheae, but their connections are 
severed by the plane of the section (cl. ¢v.”). There are a few, however, 
Misitabnghelces. ve - 
