BREED: METAMORPHOSIS OF THE MUSCLES OF A BEETLE. 363 
given for or against this view, but it seems to me that more probable 
explanations of the source of these leucocytes can be given. 
Transitional conditions between degenerating and metamorphosing 
muscles have been noticed, especially in the musculus lateralis meso- 
thoracis and other mesothoracic muscles whose counterparts in the meta- 
thorax metamorphose into imaginal muscles. Until a few days before 
pupation, there are few differences between the changes of these meso- 
thoracic muscles and those of their counterparts in the metathorax. That 
is, the changes of the mesothoracic muscles differ from those of the type 
of degenerating muscles just described in the following particulars : they 
begin their changes in the early resting larva, instead of at the time of 
pupation ; they split into a definite number of longitudinal strands ; 
their nuclei divide amitotically, though not as abundantly as in most 
of the metamorphosing muscles; the muscle substance stains with 
thionin ; and the tracheal cells are present in considerable numbers. 
All these features so resemble those of the metamorphosing muscles that 
for a long time I supposed that these muscles likewise metamorphosed. 
It was only by tracing the history of each muscle individually that I was 
able to establish their final and total disappearance. Their final disin- 
tegration takes place in the old pupa at the same time, and in the same 
manner, as that of the other degenerating muscles. The fate of the 
tracheal cells connected with them is not certain, but eventually they 
must become free in the blood plasma, where they presumably form 
tracheae or leucocytes. 
The probable explanation of the similarity of these degenerating muscles 
to the metamorphosing muscles is, that in some ancestral form not far re- 
moved, the former also metamorphose to become imaginal muscles. That 
such a condition (i.e. a metamorphosis of the 1. m?thx. and the other 
degenerating mesothoracic muscles) will be found in some of the hemimet- 
abolic insects, is very probable. A similar relation between the fibrillar 
wing muscles of certain beetles is almost certain. In Thymalus these 
fibrillar muscles are metamorphosed larval muscles, but in the imagines 
of certain wingless beetles they are not found (Aubert, ’53). It is prob- 
able, therefore, that investigation would show their presence in the larvae 
of these forms and that they degenerate in the pupa. 
d, HISTOGENESIS OF THE IMAGINAL MUSCLES. 
Nothing has been determined with certainty about the origin of the 
two metathoracic muscles of Thymalus which were absent in the larva. 
They probably are derived in the same manner as the muscles of new 
VOL. XL. — NO. 7 4 
