breed: metamorphosis of the muscles of a beetle. 357 



of the extensor of the wing. Group IT. includes those muscles which be- 

 gin their metamorphosis soon after the muscles of Group I. have begun 

 theirs, but which retain their cross striation until the time of pupation. 

 Examples of metathoracic muscles of this group are : the first and second 

 flexors of the wing and the third extensor of the coxa. The remaining 

 group (III.) includes the muscles which show little evidence of metamor- 

 phosis even at the time of pupation. Among these may be mentioned 

 the dorsal muscle of the mesofurca, the lateral muscle of the inferior 

 process of the mesophragma, the lateral muscle of the mesofurca, the 

 depressor of the tergum, and the flexor of the postero-lateral process 

 of the metafurca. It will be noticed that the examples of Group III. 

 include all of the intersegmental muscles which lie between the meso- and 

 metathorax, and also all of those between the metathorax and the first 

 abdominal somite. Why these muscles should all belong to the group 

 which is the most retarded in beginning its metamorphosis, is not 

 evident. 



a. Larval Period. In the muscles of this type the larval existence 

 does not include the entire period of destructive changes, these extend- 

 ing into the pupal stage. In the destructive alterations, the differences 

 between those larval muscles which metamorphose into muscles of the 

 wing type and those which assume the leg type are not great; these 

 differences alone need be mentioned. Figure 49 (Plate 7) shows a 

 cross section of the second flexor of the wing drawn from an older larva 

 than the one from which Figure 14 (Plate 6), of the wing-muscle series, 

 was drawn. These muscles are at nearly the same stage of development 

 and will serve to illustrate the differences in the metamorphoses of the 

 two types. These differences are chiefly, that the muscles of the leg 

 type divide into a greater number of smaller longitudinal strands (19-22 

 in the particular muscle figured), and that the fibrillae of most of the 

 leg-type muscles do not disappear as quickly as those of the wing type. 



p. Pupal Period. Eventually the substance of these muscles reaches 

 a structureless condition, the same as is shown in Figures 25, 28 (Plate 

 6) for the wing muscles, though this stage in some cases is not attained 

 until the middle of pupal life. In fact, the structureless condition has 

 not been observed in all of the muscles of Group III. mentioned above. 

 It is even possible that in some cases the fibrillae of the larval muscles 

 of this group may persist as fibrillae in the imaginal muscles. If 

 so, these muscles would form a transition, so far as the contractile 

 elements are concerned, to those which remain entirely unchanged from 

 the larva to the imago. The structureless period is certainly of shorter 



