OF THE CRANE FLY AND THE BLOW FLY. 147 



will, I think, make it evident that some such process as Mr. Lowne 

 has described is the only way of accounting for the continuity of 

 the pupa skin in these segments. From this, too, it results that 

 as the thoracic larval muscles degenerate outside of and around the 

 growing imaginal discs, and before their coalescence is complete, 

 the removal of the pupa case while the insect is in this condition 

 would leave it without any integument at all — at least in the 

 thoracic segments : hence the necessity for its retention. 



The tendency of my observations is to disprove the distinctive 

 character of the development of the cephalic and thoracic segments 

 in the Crane Fly, but to retain it in the Blow Fly ; and, if this be 

 true, seeing that in either case the development originates in 

 structures which are distinctly homologous, the question arises 

 whether the two methods are separated by an impassable gulf, or 

 whether the study of other insects, by revealing the existence of 

 intermediate links between the form, disposition, and connections 

 of these structures in the one case and the other, may bridge over 

 the chasm which at present seems to separate the two methods, and 

 indicate a gradation of modes of development, as well as of 

 external form and internal structure. 



EXPLANATION OF PLATES X. AND XL 

 Plate X. 



Fig. 1. — Part of the anatomy of the larva of the Crane Fly : the insect ia 

 represented as pinned out for dissection, and the head and digestive 

 organs removed. In the centre is seen the nervous cord sur- 

 mounted by the optic and antennal imaginal discs. Below these, 

 in connection with the thoracic nerve centres, are seen the 

 inferior thoracic imaginal discs, viz., those of the legs ; these, 

 again, are flanked on either side by the corresponding superior 

 ones, viz., those of the prothoracic pupal appendages, the wings, 

 and the halteres. On the left hand of the figure is seen the sub- 

 cutaneous muscular coat, and on the right, one of the main tracheae, 

 with its branches. 



Fig. 2. — The head of the larva from below shewing the ventral cleft 

 through which the oesophagus passes. The salivary duct is seen 

 as a ringed tube, and below it the cerebrum and cerebellum. 



Fig. 3. — The superior prothoracic appendage of the pupa of the Crane 

 Fly, with the trachea passing up its centre. 



Fig. 4. — One of the leg discs — early stage. < 



Fig. 5. — The same subsequently ; capsule (?) removed. 



