No. 1774. THORAX OF HYMEN0PTERA—8N0DGRASS. 71 



more complicated structure ? The reader may take his choice. The 

 writer adopts the second view because it is the simpler. 



The mesotergum of Tremex (1) consists of a notum {N^) and a 

 postnotum (PN^). The first is the large plate of the back, consisting 

 of the mesoscutum (/Sc^j) ^i^d mesoscutellum (Scl^). The second 

 (PiVj) is, in the normal condition, mostly hidden beneath the pos- 

 terior edge of the scutellum, but when the metathorax is removed 

 from the mesothorax the postnotum of the latter is found to be 

 mostly invaginated into the groove between the two segments, for 

 it is now seen to be a distinct plate (3, 4, PiVj) carrying a large 

 two-lobed postphragma (Pph^ projecting far backward tlirough the 

 metathoracic cavity. 



On the anterior edge of the mesonotum is a prephragma (3, 4, 

 Aph^, while the lateral margins form the anterior and the posterior 

 notal wing processes (ANP and PNP). On the under surface is a 

 well-developed V-shaped entodorsal ridge (4, VNR) which forms the 

 line (3, vnr) on the surface separating the scutum (Sct2) from the 

 scutellum (Sclo) . There is no prescutal division of the mesonotum in 

 Tremex, though in many of the sawflies there is a distmct mesopre- 

 scutum (10,16-19, Psc^) defined by a V-shaped suture (h). A small 

 lobe on the posterior margin of the scutellum (3, o) might reasonably 

 be termed the postscutellum if this name did not already belong to 

 the postnotal plate. 



The mesopectus of Tremex (5) consists of three principal plates, the 

 combined sternum and episterna, and the two epimera. In many 

 of the Tenthredinoidea, however, there are distinct sterno-pleural 

 sutures, ventrad to the articulations of the mesocoxa? (10, 14, 16, 

 19, q), which separate the ventral sternum (S2) from the lateral 

 episterna {Eps^. On the interior surface of the mesopleurum (9) 

 is seen the heavy pleural ridge {PR) following the line of the pleural 

 suture (1, P-Sj), forming the wing process (9, WP) above and the 

 coxal process (CxP) below. Just above the latter it gives oft" the 

 small pleural arm (PA). In this view the parapterum (P) is seen 

 to support a disk (PD). Upon this disk is inserted the upper end 

 of the pronator muscle of the wing, the parapterum being connected 

 with the head of the costal vein. In most of the sawflies there are 

 two episternal paraptera in the mesothorax (10, 16, 18, 19, IP 

 and 2P), but in other Hymenoptera the first is lacking. For this 

 reason the single one present will be designated the second parap- 

 terum (2P). Fig. 5 on plate 1 gives a dorsal view of the interior of 

 the mesopectus, showing the large furca (Fu) of the mesosternum (S^). 



The metatergum of Tremex consists of a narrow notum (1, 6, N^ 

 carrying the hind wings, and of two small postnotal plates (PN^) 

 attached to the first abdominal plates. The metanotum is very 

 simple in all the Hymenoptera. In the Siricoidea and Tenthredi- 



