BREED: METAMORPHOSIS OF THE MUSCLES OF A BEETLE. 357 
of the extensor of the wing. Group IT. includes those muscles which be- 
gin their metamorphosis soon after the muscles of Group I. have begun 
theirs, but which retain their cross striation until the time of pupation. 
Examples of metathoracic muscles of this group are: the first and second 
flexors of the wing and the third extensor of the coxa. The remaining 
group (III.) includes the muscles which show little evidence of metamor- 
phosis even at the time of pupation. Among these may be mentioned 
the dorsal muscle of the mesofurea, the lateral muscle of the inferior 
process of the mesophragma, the lateral muscle of the mesofurca, the 
depressor of the tergum, and the flexor of the postero-lateral process 
of the metafurca. It will be noticed that the examples of Group ITI. 
include all of the intersegmental muscles which lie between the meso- and 
metathorax, and also all of those between the metathorax and the first 
abdominal somite. Why these muscles should all belong to the group 
which is the most retarded in beginning its metamorphosis, is not 
evident. : 
a. Larval Period. In the muscles of this type the larval existence 
does not include the entire period of destructive changes, these extend- 
ing into the pupal stage. In the destructive alterations, the differences 
between those larval muscles which metamorphose into muscles of the 
wing type and those which assume the leg type are not great; these 
differences alone need be mentioned. Figure 49 (Plate 7) shows a 
cross section of the second flexor of the wing drawn from an older larva 
than the one from which Figure 14 (Plate 6), of the wing-muscle series, 
was drawn. These muscles are at nearly the same stage of development 
and will serve to illustrate the differences in the metamorphoses of the 
two types. These differences are chiefly, that the muscles of the leg 
type divide into a greater number of smaller longitudinal strands (19-22 
in the particular muscle figured), and that the fibrillae of most of the 
leg-type muscles do not disappear as quickly as those of the wing type. 
8B. Pupal Period. Eventually the substance of these muscles reaches 
a structureless condition, the same as is shown in Figures 25, 28 (Plate 
6) for the wing muscles, though this stage in some cases is not attained 
until the middle of pupal life. In fact, the structureless condition has 
not been observed in all of the muscles of Group III. mentioned above. 
It is even possible that in some cases the fibrillae of the larval muscles’ 
of this group may persist as fibrillae in the imaginal muscles. If 
so, these muscles would form a transition, so far as the contractile 
elements are concerned, to those which remain entirely unchanged from 
the larva to the imago. The structureless period is certainly of shorter 
