NORTH AMERICAN DIPTERA. 



metathorax is not seen at all from above; the scutellum, 

 cut off by an impressed line, usually a semi-oval body, 

 really belongs to the mesothorax, the dor sum of which 

 is often called the mesonotum. 



Transverse suture, an impressed line usually running 

 straight across the mesonotum and terminating a little in 

 front of the root of the wings. It is more or less incom- 

 plete in the middle. 



Humerus or humeral callus, each of the anterior superior 

 angles of the mesothorax, usually a more or less rounded 

 callus. 



Post-alar callus, a more or less distinct, rounded swell- 

 ing, situated between the root of the wing and the scu- 

 tellum. 



Pre-alar callus, a similar, but usually less prominent, 

 projection situated before the root of the wings on the 

 sides of the mesonotum, just back of the outer ends of 

 the transverse suture. 



Scutellar bridge, a small ridge on either side of the scu- 

 tellum, connecting it with the mesonotum. 



Presutural depression, a depression, usually triangular 

 in shape, at the outer ends of the transverse suture, near 

 the dorsopleural suture. 



Supra-alar groove, a groove on the mesothorax imme- 

 diately above the root of the wings, along the inner mar- 

 gin of which there are, usually, characteristic bristles. 



Fig. 8. Pleural divisions of 

 Syrphus. 



Pro, propleura. 

 Afeso, mesopleura. 

 Sterno, sternopleura. 

 Ptero, pteropleura. 

 Hypo, hypopleura. 

 Mela, rnetapleura. 



