372 bulletin: museum of comparative zoology. 



fibre. Each muscle is composed of a few fibres whose nuclei are 

 placed at the surface of the fibre in an abundant sarcoplasm. They show 

 a well-marked sarcolemma and evident cross and longitudinal striations. 

 The intracellular tracheoles which supply the muscles apparently pene- 

 trate the sarcolemma and ramify in the superficial layer of the sarcoplasm. 



2. The larval muscles which metamorphose into muscles of the iving 

 type begin their metamorphosis at an early stage of the resting larva. 

 The metamorphosis consists of (1) a longitudinal division of the original 

 fibre into from four to ten fibres, (2) the destruction of the fibrillae of the 

 larval muscles, and the formation of the larger separate fibrillae of the 

 imaginal muscles in the remaining structureless sarcoplasm, and (3) a 

 great increase in the number of the nuclei, which become redistributed 

 throughout the substance of the muscle. All of the muscles of this type 

 increase in size during these changes. At an early stage in the meta- 

 morphosis, mesenchymatous cells derived from the intracellular tracheoles 

 make their appearance between the newly divided fibres. These cells 

 increase rapidly by mitotic division, and, in a late stage of the pupa, form 

 the abundant new tracheoles which supply these muscles in the imago. 

 Possibly some of these mesenchymatous cells become imaginal leucocytes. 



3. The metamorphosis of the larval muscles into muscles of the leg 

 type does not differ essentially from that of muscles of the wing type. 

 The principal difference is that the muscles of the leg type divide into 

 smaller fibres, and a greater number of them, fifteen to twenty fibres 

 being frequently formed by this division. The nuclei divide frequently 

 by amitosis, and in the redistribution may take either of two positions in 

 the new fibres. They may come to lie at the periphery, as in Thymalus, 

 or in a row along the axis of each fibre, as in Bruchus. There is in 

 different muscles a great variation in the time of the beginning of this 

 metamorphosis. Some begin their changes as early as those which meta- 

 morphose into imaginal muscles of the wing type ; others begin their 

 changes at various periods during the resting larva; while a few show 

 scarcely any evidence of metamorphosis, even at the time of pupation. 

 It is barely possible that in the muscles last mentioned some of the 

 fibrillae of the larval muscles may persist as fibrillae of the imaginal 

 muscles. This cannot be commonly the case, however. In the region 

 of the leg muscles the mesenchymatous tracheal cells are not as nu- 

 merous as in the wing muscles, and the tracheae developed from them 

 do not penetrate the substance of the muscle fibres. 



4. The metamorphosis of the intestinal muscles is later in starting 

 than tliat of any of the other muscles. Not until well along in pupal 



