OF THE CRANE FLY AND THE BLOW FLY. 145 



much as I can discover only two pairs of thoracic discs attached to 

 the nerve centres and certainly three, if not four, attached to 

 the trachese ; and if so, it is a circumstance which appears to re- 

 quire some explanation : you will understand my meaning when 

 I say that these discs seem to have stepped out of their places to 

 consort with those with which they have no lawful business. 



Thus much for the anatomical peculiarities of the parts con- 

 cerned in the metamorphosis of these two insects. I will now 

 give a comparison of the manner in which this change is re- 

 spectively brought about. Mr. Lowne says — " Every degree of 

 metamorphosis exists among insects, from that in which the larva 

 nymph and imago closely resemble each other, where the successive 

 changes are merely those of ordinary development, as in the cock- 

 roach; to that in which the change is so complete that it might 

 almost be doubted whether the larva and imago should be con- 

 sidered the same individual at all, so closely does the process 

 resemble an alternation of generations ;" and he adds, " This is the 

 case in the fly." I must confess I find considerable difficulty in 

 regarding the processes herein concerned as at all resembling the 

 production of a sexual zooid by gemmation from an a-sexual 

 larval one; but I quite concur in the estimate these words convey 

 as to the totality of the change involved. Mr. Lowne further 

 says — " All the tissues of the larva undergo degeneration, and the 

 imaginal tissues are re-developed from cells which originate from 

 the disintegrated parts of the larva under conditions similar to 

 those appertaining to the formation of the embryonic tissues from 

 the yolk." With the exception of the brain and nervous system, 

 the whole substance of the maggot is converted into a soft cellular 

 mass, deprived of every organ of sense or motion, and the chain of 

 being seems, indeed, -almost snapped. Not so, however, with the 

 Crane Fly. The intermediate cellular stage does not occur. The 

 larval tissues, itis true, waste wholly away, and are replaced by those 

 proper to the fly, but in a manner gradual and more consonant with 

 our ideas of continuity of existence. Before the assumption of the 

 pupa state the larval muscles of the thoracic segments have, to a 

 very great extent, been supplanted by the newly forming muscles of 

 the imago. The abdominal segments, on the contrary, present an 

 opposite phenomenon, for the larval muscles here persist in their 

 full integrity to a very late period of pupa life, Pong after the 

 metamorphosis of the thoracic segments is complete ; they then 



