BREED: METAMORPHOSIS OF THE MUSCLES OF A BEETLE. 339 
J. APPENDAGES. 
The imaginal appendicular muscles of Thymalus are apparently all 
metamorphosed larval muscles. No evidence of the degeneration of 
larval muscles nor of the new formation of imaginal muscles in the pupa 
has been observed. The changes of these muscles in some beetles are quite 
different from those of Thymalus. This is especially true of the forms 
with legless grubs. In these, the imaginal leg muscles are of new 
formation in the pupa. 
4, Discussion of Results. 
Summing up the anatomical changes which the muscles of Thymalus 
undergo during pupal life, we find that : 
1. The only larval muscles which remain unchanged in both position 
and histological structure are found in the abdominal region, this being 
the region of least change in external form during pupal life. This 
persistence of the larval muscles might have been inferred from the fact 
that the pupa retains throughout life the power to roll itself about by 
means of the movements of the abdominal somites on each other. 
2. However, only about half of the larval muscles of the abdomen 
remain unchanged, those of the more peripheral layers undergoing a 
metamorphosis into imaginal muscles. Most of the muscles of the lar- 
val thorax and all of the muscles of the head and appendages metamor- 
phose into imaginal muscles. 
3. The larval muscles which degenerate are found in the thorax and the 
first and last abdominal somites. They occupy in nearly every case 
positions similar to the positions of the muscles of the abdomen which 
persist unaltered by the metamorphosis. Exceptions to this statement 
have been noted in the mesothorax, where there is a degeneration of 
dorso-ventral muscles other than intersegmental ones. 
4. Probably two new metathoracic muscles are formed during pupal 
life, one being a flexor of the metathoracic coxa and the other, the 
muscle which operates the fold of the episternum into which the elytra 
catch when closed. 
The most radical changes in the musculature are found in the thoracic 
region. This is to be expected as the imaginal thorax differs greatly 
from the larval in both form and function. The least radical changes 
are found in those somites of the abdomen whose larval condition most 
resembles the imaginal. The serial homology between the degenerating 
muscles of the thoracic region and the persistent larval muscles of the 
