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 iraaginal 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 

 raeso- 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 Inindred 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 autero-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 ah, 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). 



