THORAX OF THE BLOW-FLY. 21 



given portion of the tegumentary structures must belong to one 

 or other of those between which it seems to occur ; and it will, I 

 think, be found that the spiracles are in every case more nearly 

 approximated to the segment in front of them than to that behind. 

 Indeed I have noticed that the largest tracheal branch of the spi- 

 racle between the pro- and mesothorax of Acrida viridissima pro- 

 ceeds immediately down the fore leg to that peculiar organ in the 

 fore tibia which has been supposed to be connected with the 

 sense of hearing. 



Prom this and similar indications I think that the spiracle is 

 always the property of the posterior surface of the segment in 

 front of it. And this is an additional reason for thinking that 

 the posterior spiracles of the Diptera are mesothoracic, viz. that 

 they are thus made to occupy the posterior or postscutellar region 

 of the segment to which they belong. That they should in the 

 Blow-fly be surrounded by well-developed corneous plates instead 

 of membranous integument, is only an indication of the general 

 fact that the postscutellar region has participated fully as much 

 as the other portions of the mesothorax in the exceptional 

 development which the segment has received in this order. 



Evidence from Developmental Change. — In a paper read three 

 years ago before the Quekett Microscopical Society, " On the 

 Metamorphosis of the Crane-fly and of the Blow-fly," I took 

 occasion to notice the dorsal appendages on the thorax of the 

 pupa of these insects. I believe that these processes, which are 

 indicated in my figures (PL II. fig. 1 and PL I. fig. 13), are 

 the proper dorsal appendages of the prothorax, corresponding on 

 that segment to the wings on the following one. The purport 

 of their being seems to terminate with the pupa state ; and in 

 the imago their development as appendages seems to be arrested. 

 As I endeavoured to show on that occasion, the fact of their 

 being the serial homologues of the wing is not only attested by 

 their position, but by the manner of their development, arising, 

 as they do in either case, from a special imaginal disk, which, in 

 the Blow-fly at ]east, had hitherto escaped notice from its 

 minuteness. 



This disk is shown in the case of the Crane-fly (PL II. 

 fig. 12), where it will be seen to correspond exactly in position 

 to those of the wings and halteres which follow it, viz. a little 

 outside of, and posterior to, that of the corresponding leg. The 

 corresponding disk of the Blow-fly is situated just behind the 



