338 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
former, there are five. The outline of the retractor of the prothorax is 
shown by the dotted lines in Figure 11, rtr. prothx. if. (Plate 5). This 
shows the imaginal position of the muscle, its origin being on the 
mesofurca and its insertion on the antefurca. 
c. PROTHORAX. 
The serial homology between the muscles of this somite and those of 
meso- and metathorax is not so marked as between those just compared. 
Yet, in general, muscles in similar positions undergo similar changes. 
The great majority of the larval muscles of the prothorax metamorphose 
into imaginal muscles, but a number degenerate. None of the larval 
muscles pass unchanged into the adult. 
d. HEAD. 
The muscles of the head of the larva are probably all metamorphosed 
into imaginal muscles, for there is no evidence that muscles degenerate, 
nor do any of the muscles remain unchanged. One point in regard to 
the adductor of the mandible may be of interest. In the larva this 
muscle is composed of about fifty fibres, whereas in the imago the same 
muscle has from two to three hundred fibres of smaller calibre, which 
have been formed by the longitudinal splitting of the larval fibres. 
e. ABDOMEN. 
The abdomen is the only region of the body where any muscle remains 
unaltered from the larva to the imago. The abdominal muscles which 
have this fate occupy in general positions homologous with those of the. 
muscles of the thoracic region which undergo degeneration. They are 
the inner muscles of the dorso-ventral intersegmental muscles and the 
inner layer of the antero-posterior muscles. Most of the remaining 
larval muscles in the abdomen metamorphose into imaginal muscles ; 
there are a few, however, which degenerate. The latter are found in the 
somites in which the greatest changes in external form take place during 
pupal life, i. e., the first and last abdominal somites. No muscles newly 
formed in the pupa have been observed, though some may be present. 
Such are quite probably to be found in connection with the sexual 
organs, — ovipositors, etc. 
Two of the metamorphosed muscles of the first abdominal somite are 
shown at ab, in Figure 9 (Plate 4). The metamorphosis of extensor 
coxae metathoracis secundus from muscles of the first abdominal somite 
has already been described (page 334). 
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