OF THE CRANE FLY AND THE BLOW FLY. 141 



to which the name of " Imaginal Discs" has been given, and 

 within which the limbs of the future fly are developed. It is im- 

 possible to doubt that these are the homologues of similar structures 

 found in the same situation in the Blow Fly, which have been re- 

 garded by Dr. Weismann, who first observed them, and also by Mr. 

 Lowne, as indicative of a special mode of development of the seg- 

 ments where they are found. That this mode of development pre- 

 vailed also in the Crane Fly, I was led to believe from the follow- 

 ing quotation from Dr. Weismann's memoir on the subject, which 

 appears in Mr. Lowne's book as follows, p. 7, viz., " I believe in 

 all those insects in which the anterior larval segments are unpro- 

 vided with legs, the head and thorax of the imago are entirely 

 re developed, while in those in which the larva is furnished with 

 legs, these parts depend for their formation upon the anterior 

 larval segments." 



I have stated that the limbs of the insect are developed within 

 the discs ; it will be well, perhaps, to consider what these limbs 

 are. Mr. Lowne says, " Each segment in the lowest Articulata is 

 normally furnished with two pairs of lateral appendages, or rudi- 

 mentary limbs, one pair placed above the other, the superior being 

 dorsal, and the inferior ventral, at least such is their arrangement 

 in annelides. Both pairs are much modified in the higher forms, 

 and are often entirely suppressed." I believe I have recognised 

 eight pairs of discs in the Crane Fly, two pairs in the head, and 

 six in the thorax ; those in the head are the antennal and optic 

 discs, and are concerned in the formation of the cephalic ap- 

 pendages — viz., the antennas and the eyes; those in the thorax 

 are concerned in that of the superior and inferior thoracic appen- 

 dages — viz., the wings, legs, and halteres. Now, it is a curious 

 fact that, though in the imago these appendages are mostly well 

 represented by the limbs referred to, the upper prothoracic ones 

 seem to be wanting. On the contrary, in the pupa state, they are 

 as conspicuous by their presence as they are subsequently by their 

 absence ; the former being indicated by the existence of two 

 club-shaped processes on the dorsal region of the prothorax. 

 These processes are not peculiar to the insect in question — 

 they are very conspicuous in the pupa of the gnat, and I 

 believe I also recognise them in that of the Blow Fly, in two 

 button-like bosses in the same situation. A large trachea, con- 

 tinuous with the main trachea of the pupa, passes up the centre of 



