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 Thymalus 
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 fibrillae show clearly in both cross and longitudinal 
sections, and are nearly all of one size. In Thymalus they are about 1 pu 
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 (¢r/.) 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 
