BREED : METAMORPHOSIS OF THE MUSCLES OF A BEETLE. 369 



imaginal muscles. At no stage do these metamorphosing muscles lose 

 their identity, so that a dissolution of these muscles and a survival of 

 their nuclei only, is impossible. 



Berlese's mistake may be easily explained, however. He has neglected 

 entirely the study of the anatomical changes ; these would have immedi- 

 ately revealed the falsity of his view. Moreover, he is unfortunate in 

 his choice of the adductor of the mandible, as a muscle in which to study 

 these changes. This muscle is composed of numerous fibres (50 in the 

 larva, 250 in the imago of Thymalus), so that it is impossible to follow 

 any particular one of them in its development. When the destructive 

 changes in the metamorphosis of this muscle are completed, there re- 

 mains simply a confused mass of these fibres still retaining their nuclei, 

 with numerous spindle-shaped cells scattered between the fibres, pre- 

 cisely as Berlese describes and figures (:02% p. 65, Fig. 253). His 

 mistake arises from his imagining that spindle cells are derived from the 

 muscle nuclei, a mistake very easily made. In some of the beetles 

 which I have examined, the diS"erence between these cells and the 

 muscle nuclei is not obvious at first sight. In Thymalus, however, there 

 can be no doubt of a difference between them at all stages. As already 

 shown, the spindle cells develop from tracheae and into tracheae, while 

 the muscle nuclei persist as they are in the nndi£ferentiated sarcoplasm 

 and form the imaginal muscles. The conditions which Berlese shows in 

 his second figure (Fig. 254) are different from anything observed in 

 Thymalus. That all the cells pictured in this figure are of the same 

 nature, is open to question. It has also been shown that there is no 

 need of supposing a derivation of complete cells from nuclei alone, as 

 Berlese has done. This assumption itself is enough to shake one's 

 confidence in his views. 



He also lays great stress on the simplicity of his idea, and the fact that 

 he has been able to make it apply in every case which he has studied. 

 But there may be a fault in too great simplicity, as well as in too great 

 complexity. The reasonableness of the ideas of the present paper, as 

 contrasted with those of Berlese, may best be shown by tracing what 

 may have been the phylogenetic development of these muscular changes. 



It is fair to assume that in primitive insects the muscles were the 

 same in number, function, and position, when the larva escaped from the 

 egg, as they were when the imaginal form was attained, since there 

 doubtless was little difference between the two stages except in size. 

 Now, in the development of such primitive insects into hemimetabolic 

 forms, and the development of these into holometabolic forms, it has 



