bkeed: metamorphosis of the muscles of a beetle. 339 



f. 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 iiexor 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 



