THOEAX OF THE BLOW-FLY. 25 



the meso- is separated from the metathorax, it always comes away 

 with the former. In the mesial line it projects far backwards 

 into the posterior portion of the thoracic cavity so as to leave but 

 little space between it and the posterior wall, which, as I have 

 said, is formed by the dorsal plate of the fifth segment. Thus the 

 longitudinal muscles of the mesothorax, which are the only tho- 

 racic longitudinal ones developed, pass from it to the prsescutum, 

 across the minute groove of the metathoracic tergum and the 

 cavity of the mesothoracic scutellum, as may be seen in Plate II. 

 fig. 5, which represents a longitudinal section in the mesial line 

 of the thorax of the Humble-Bee. There is no fusion of the 

 muscles of the two segments. The mass of muscles which nearly 

 fills the united cavity of three segments belongs but to one of 

 them, viz. the mesothorax ; the metathoracic muscles, both longi- 

 tudinal and vertical, being no longer required, are altogether 

 obsolete. 



Now for the application, so far as the longitudinal muscles are 

 concerned. Plate I. fig. 8 shows the longitudinal muscles of the 

 Blow-fly. Tbey are seen to extend from the mesothoracic pra> 

 scutum in front to that vertical posterior surface which Bur- 

 meister and Lowne regard as the metathoracic tergum, but which, 

 I venture to submit, is again, as it has been shown to be in all 

 previous cases, the mesothoracic postscutellum, the mesophragma, 

 and not the metaphragma. If it be otherwise, w r e shall have what 

 I can find no other instance of, viz. a commingling of the prin- 

 cipal muscles of two segments into one homogeneous muscular 

 mass. 



Again, be it observed from Plate I. figs. 8 & 11, that although 

 the longitudinal muscles extend in the mesial line almost to 

 the verge of the cephalothoracic foramen, they have not suffi- 

 cient breadth to reach the anterior angles where the humeri are 

 situated. It will be obvious that if this were the case, I could 

 no longer hold the humeri to be prothoracic ; but as it is, the 

 avoidance by the mesothoracic muscles of these portions of the 

 integument is, I submit, significant of the correctness of my 

 view. 



Further, let us see what may be learnt from a study of the 

 transverse or vertical muscles of the thorax, proceeding on the 

 assumption, warranted by general observation, that none at least 

 of the principal of these can have their origin in one segment and 



LINN. JOLBN. — ZOOLOGY, VOL. XV. 3 



