breed: metamorphosis of the muscles of a beetle. 363 



given for or against this view, but it seems to me that more probable 

 explanations of the source of these leucocytes can be given. 



Transitional conditions between degenerating and metamorphosing 

 muscles have been noticed, especially in the musculus lateralis meso- 

 thoracis and other mesothoracic muscles whose counterparts in the meta- 

 thorax metamorphose into imaginal muscles. Until a few days before 

 pupation, there are few differences between the changes of these meso- 

 thoracic muscles and those of their counterparts in the metathorax. That 

 is, the changes of the mesothoracic muscles differ from those of the type 

 of degenerating muscles just described in the following particulars : they 

 begin their changes in the early resting larva, instead of at the time of 

 pupation ; they split into a definite number of longitudinal strands ; 

 their nuclei divide amitotically, though not as abundantly as in most 

 of the metamorphosing muscles ; the muscle substance stains with 

 thionin ; and the tracheal cells are present in considerable numbers. 

 All these features so resemble those of the metamorphosing muscles that 

 for a long time I supposed that these muscles likewise metamorphosed. 

 It was only by tracing the history of each muscle individually that I was 

 able to establish their final and total disappearance. Their final disin- 

 tegration takes place in the old pupa at tlie same time, and in the same 

 manner, as that of the other degenerating muscles. The fate of the 

 tracheal cells connected with them is not certain, but eventually they 

 must become free in the blood plasma, where they presumably form 

 tracheae or leucocytes. 



The probable explanation of the similarity of these degenerating muscles 

 to the metamorphosing muscles is, that in some ancestral form not far re- 

 moved, the former also metamorphose to become imaginal muscles. That 

 such a condition (i. e. a metamorpliosis^o^ the l.mfthx. and the other 

 degenerating mesothoracic muscles) will be found in some of the hemimet- 

 abolic insects, is very probable. A similar relation between the fibrillar 

 ■wing muscles of certain beetles is almost certain. In Thymalus these 

 fibrillar muscles are metamorphosed larval muscles, but in the imagines 

 of certain wingless beetles they are not found (Aubert, '53). It is prob- 

 able, therefore, that investigation would show their presence in the larvae 

 of these forms and that they degenerate in the pupa. 



d. Histogenesis of the Imaginal Muscles. 



Nothing has been determined with certainty about the origin of the 

 two metathoracic muscles of Thymalus which were absent in the larva. 

 They probably are derived in the same manner as the muscles of new 



VOL. XL. — NO. 7 4 



