30 
Journal of Agricultural Research 
Vol. XXVIII, No. 1 
liberated and grow into elongate migratory cells, the myocytes , which gather 
at the points where new muscles are to be, or group themselves with the remain¬ 
ing parts of old muscles, to build up the muscular system of the imago. Anglas 
(1) described the imaginal muscles of the wasp as originating from myoblasts , 
or fragments of the larval muscle nuclei in pieces of the contractile tissue. 
Ganin (11) and Van Rees (47), however, had described the imaginal muscles 
of the blow-fly as formed in part or entirely from cells proliferated from the 
mesodermic tissue of the imaginal buds of the body wall. P6rez, in a more 
recent study of the metamorphosis of the blow-fly (39), reasserts this same view 
concerning the origin of the new muscles or of new parts of old one^, and claims 
that the larval muscle nuclei finally perish by phagocytosis. 
One of the most convincing papers on the metamorphosis of the muscles is 
that by Breed (10) in which are described the changes that occur in the muscles 
of certain beetles. In the species studied, Breed found no evidence of phagocy¬ 
tosis, and, according to his observations, muscles that metamorphose are re¬ 
constructed from their own elements, not from migratory “myocytes,” while 
muscles of new formation are probably developed from the mesoderm of the 
imaginal buds. Cells of external origin do penetrate the growing muscles, but 
these, he points out, are derived from neighboring tracheole cells and form the 
imaginal tracheal branches of the muscles. Such cells, Breed suggests, are the 
ones observed by Berlese and described as migrating “myocytes” on the mis¬ 
taken belief that they formed the new elements of the muscles. Breed’s account 
agrees entirely with that of Rengel (43) on the metamorphosis of the muscles of the 
alimentary canal. Needham (36) says that some of the nuclei of the degenerat¬ 
ing fat cells of the flag weevil become associated with the developing 
muscles, apparently themselves becoming nuclei of the new muscle fibers. This, 
however, seems no more reasonable than Berlese’s idea, later discarded by himself, 
that some of the muscle nuclei form the imaginal fat cells. 
The muscles of any pupa, then, may be classed in not more than four groups, 
as given by Breed (10). The first includes such muscles as pass unaltered 
from larva to imago, but most writers agree that there are no muscles of this 
sort in the higher Diptera. The second group includes such larval muscles as 
undergo a more or less complete metamorphosis by a reorganization of their 
own elements. Van Rees (47) described three pairs of larval muscles in the 
thorax of the blow-fly that persist with alterations, and this is confirmed by Breed. 
The third group consists of those larval muscles that disappear by complete 
histolysis. This includes most of the larval muscles of the Muscoidea. The 
fourth group consists of muscles that are newly formed in the pupa from pre¬ 
existing imaginal buds, and includes most of the muscles of the adult fly. 
The muscles of new formation are to be regarded as present in rudimentary 
form in the imaginal buds. These muscles are those that the larva has found 
unnecessary or unsuited to its purposes, and, as stated by Breed, “such muscles 
would tend naturally to be retarded in their development until they came to 
be muscles newly formed in the pupa; but in their final development they would 
arise from the cells which had originally formed them.” It has not been shown 
yet just what elements in a metamorphosing muscle generate the new parts, but 
the evidence is against the idea that regenerating elements enter from without, 
and Breed sees no evidence that the new elements in such cases are derived from 
the imaginal discs. Perhaps each muscle, then, contains cells with persistent 
vitality that regenerate it, just as the hypoderm and the walls of the alimentary 
canal are regenerated by cells in their own tissues that have remained dormant 
through the larval stage or that have retained their vitality into the pupal period. 
