BREED: METAMORPHOSIS OF THE MUSCLES OF A BEETLE. 337 
c. That thirteen of the larval muscles degenerate, these being in 
general dorso-ventral intersegmental muscles and the inner layer of the 
antero-posterior muscles. Two of the imaginal muscles (musculus 
episternalis and flexor coxae metathoracis quattuor) are muscles of new 
formation in the pupa. 
b. MrsoTHoRAX. 
In the mesothorax the muscles are arranged similarly to those ot the 
metathorax. For the dorsal group of antero-posterior muscles, the figures 
of the similar group of the metathorax (Plate 1, Figures 1, 2) would 
serve with only minor changes. It is very interesting to find that the 
serial homology is practically complete even to the changes which take 
place during pupal life. The three muscles which in the metathorax 
metamorphose into musculus metanoti have counterparts in this somite 
which metamorphose into musculus mesonoti. The same relations hold 
true between musculus lateralis metanoti and musculus lateralis mesonoti 
(retracteur de Vaile of Straus-Diirckheim). The remaining mesothoracic 
muscles of this group degenerate during pupal life, as do their counter- 
parts of the metathorax. 
The close similarity of the muscles of the lateral dorso-ventral groups 
in the two somites is likewise remarkable. A careful comparison be- 
tween these muscles in a series of frontal sections of a resting larva 
showed only the following slight anatomical differences. The muscle in 
the mesothorax corresponding to the third extensor coxae metathoracis 
(Plate 1, Figure 4, ext. cox. mt’thx. 3) was composed of three fibres in- 
stead of two, and the muscle corresponding to the oblique muscle v 
(Figure 4) was divided dorsally into two parts. The changes of the 
mesothoracic muscles of this group do not correspond exactly to the 
changes of their counterparts in the metathorax. A greater number of 
muscles degenerate in the mesothorax than in the metathorax. The 
additional muscles of this somite which have been noticed to degenerate 
are the musculus lateralis mesothoracis and the second flexor of the coxa. 
It is evident from the muscles which are present in the imago that a few 
others degenerate also, but their identity has not been established. 
These additional degenerating muscles are such as would function in the 
imago as muscles of flight, if the elytra were used as organs of flight. 
In the ventral antero-posterior group, only seven muscles are found 
in the larva ; three of these degenerate, while the remaining four meta- 
morphose to form the retractor prothoracis inferior. The only difference 
between the metathorax and the mesothorax in this case is, that in the 
latter there are only four metamorphosing muscles, whereas, in the 
