356 bulletin: museum of comparative zoology. 



here (see Heidenhain, '98, for a bibliography of papers on cross-striated 

 muscle). Cross and longitudinal sections of these muscles in Tliymalus 

 are given in Figures 15 and 36, respectively. The changes since the old 

 pupa are few. Cross striation is readily distinguishable, showing the 

 J and Q bands. The fibrillao show clearly in both cross and longitudinal 

 sections, and are nearly all of one size. In Thymalus they are about 1 /a 

 in diameter, which is smaller than in many other insects. No sarcolemma 

 could be demonstrated, though it has been described for this type of 

 muscle (see Cajal, '88, p. 268). 



The tracheoles {trl.) are fully developed and are often to be seen in 

 the muscle substance. It is, however, much more difficult to distinguish 

 them than it was earlier, since they have thinner walls and these do not 

 stain as deeply as in the earlier stage. 



(2) Muscles of the Leg Type. 



The figures already described as showing the structure of the larval 

 muscles (Plate 6, Figures 16, 22, and Plate 7, Figure 33) will serve as 

 a starting point for the description of this type also ; for, as already 

 stated, both the wing and the leg muscles are at first alike. In some of 

 the larval muscles which are destined to metamorphose into muscles 

 of the leg type, changes begin at the same time that they do in those of 

 the wing type, i.e., at about the time the larva ceases feeding; but in 

 others of the leg type metamorphosis does not begin until later. The 

 muscles which are to undergo the greatest changes in position at the 

 time of pupation begin to show alterations first. The others start their 

 changes during the resting larval period, though some of them are not 

 greatly changed even at the time of pupation. On account of this varia- 

 tion in the time of the beginning of the metamorphosis in different 

 muscles, it is of great importance to be able to identify these muscles at 

 every stage of development. The details of their metamorphosis are, how- 

 ever, apparently the same in all instances, there being in no case which 

 has been observed transitional conditions between these metamorphosing 

 muscles and the muscles which pass unaltered from the larva to the 



imago. 



These muscles may be somewhat artificially divided into three groups, 

 according to the period in which they begin their metamorphoses. 

 Those of Group I. begin their metamorphosis at the same time as the 

 muscles of the wing type. This group includes, among other muscles, 

 the adductor of the mandible, and the following metathoracic muscles : 

 the third flexor of the wing, the relaxator of the wing, and the relaxator 



