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. 



6. Mksothorax. 



In the mesothorax the muscles are arranged similarly to those ot the 

 metathorax. For tlie 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. mfthx. 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 



