146 A. HAMMOND ON A COMPARISON OF THE METAMORPHOSIS 



quickly disappear, a few traces of them only remaining in the 

 newly emerged imago. There is, therefore, no such period of 

 almost total disorganization as occurs in the Blow Fly ; and the 

 pupa wriggles actively, if touched, during almost the whole of its 

 existence. 



But in addition to this, there are other striking contrasts in the 

 manner of the development of the two insects. In both the 

 transformation takes place within a new cellular investment formed 

 by the deposition of new layers of cells within the old larval skin. 

 This forms the pupa skin, and upon it are marked out the position 

 of the organs about to be developed, and of those whose develop- 

 ment seems here to be arrested. Here, however, the parallel ends, 

 and diversity again shews itself. In the Crane Fly, the old larval 

 skin is discarded on the assumption of the pupa form. In the 

 Blow Fly it is retained, forming around the pupa a hard 

 horny protective covering, called the pupa case ; the enclosed 

 insect thus has two wrappings — the delicate pupa skin and the 

 hard pupa case. Now, why this difference ? I cannot pretend to 

 speak with certainty on the subject, but I submit that it appears to 

 be intimately connected with certain manifest differences in the 

 mode of the formation of the thoracic segments to which I have 

 already partly alluded. You will remember the expression of my 

 opinion that in contradistinction to Mr. Lowne's description of the 

 imaginal discs of the Blow Fly, I regard those of the Crane Fly 

 as mere invaginations of the newly forming pupa skin ; and, in 

 fact, as such 1 question whether they are rightly to be called discs 

 at all, in the sense in which Mr. Lowne uses the term. He 

 describes the discs as enveloped in separate capsules which they 

 eventually rupture by their rapid growth, and by their subsequent 

 coalescence form the thoracic portions of the pupa skin. With a 

 view to ascertain personally, if possible, the accuracy of this 

 description, I made an examination of such larvae and pupa3 as I 

 could then find. These were too few to enable me to form a very 

 decided opinion. I could not satisfy myself as to the rupture of 

 the capsules or the coalescence of the discs, but in confirmation of 

 Mr. Lowne's views it appears that the discs instead of being 

 situated immediately beneath the integument, as in the Crane Fly 

 larvae, occupy the perivisceral space between the oesophagus and 

 nerve centres, and the surrounding muscular tissues in process of 

 degeneration. Under these circumstances a little consideration 



