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-ventral 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. 
4, 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. HiIstoLoGica.. 
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 
