breed: metamorphosis of the muscles of a beetle. 371 



Summary. 



During the metamorphosis of the larvae of Coleoptera into the 

 imagines, some of the larval muscles remain unaltered during the meta- 

 morphosis, a few degenerate, while many metamorphose into imaginal 

 muscles. Imaginal muscles are formed in the pupa from cells of an 

 embryonic nature, but they are few in number. 



I. Anatomical. 



1. The muscles which remain unaltered by the metamorphosis are all 

 found in the abdominal region. They compose the inner layer of the 

 antero-posterior muscles, and the inner muscles of the dorso-ventral 

 intersegmental muscles. Exceptions to this statement are found in the 

 first and last abdominal somites, where muscles occupying these positions 

 are found to degenerate. This is explained by the greater changes of 

 external form which these somites undergo. 



2. The typical degenerating muscles are found in the thorax and 

 the abdominal somites just mentioned. They occupy positions in these 

 somites serially homologous to the positions of the persistent larval 

 muscles of the abdomen. There are some cases of the degeneration of 

 dorso-veutral muscles other than intersegmental muscles. These were 

 noticed especially in mesothoracic muscles whose counterparts in the 

 metathorax metamorphose into imaginal muscles. Their histological 

 changes show transitional stages between metamorphosing and degenerat- 

 ing muscles. The muscles which show these conditions are such as 

 would be functional in the adult, if the elytra were used as organs of 

 flight, as presumably was the case in the ancestors of beetles. 



3. Imaginal muscles of new formation in the pupa are not very com- 

 mon, only two somewhat questionable cases having been observed in 

 Thymalus. In Bruchus and other forms with legless larvae, the leg 

 muscles belong to this class. 



4r. The metamorphosing larval muscles are by far the most numerous, 

 and include all of the remaining larval muscles. In general, these are 

 the muscles of the head, the peripheral layers of the hypodermal muscles, 

 and the intestinal muscles. There is a metamorphosis of larval muscles 

 into imaginal muscles of both the wing and the leg types. 



II. Histological. 



1. The fibres of the larval muscles which pass unaltered from the 

 larva to the imago, present the usual structure of this type of muscle 



