150 HYMENOPTEBA OF AMERICA. [PART I. 



face of metathorax qnite free, and with the wings removed side- 

 wise. If not prepared in this way, it is impossible to determine 

 them with certainty. Before all things, one must thoroughly 

 acquaint himself with the characters of the metathorax, which I 

 will briefly repeat for this purpose. 1 



In the most characteristic type, the metathorax is truncate, 

 together with the post-scutel ; its hinder face is excavated, and 

 the excavation or hinder plate is angulate, forming on each side 

 a lateral dentiform angle. 2 The concavity is also margined with 

 sharp ridges. — The ridges running from the lateral angles up to 

 the angles of the post-scutel are the superior edges ; the ridges 

 running from the lateral angles down to the articulation of the 

 abdomen, are the inferior edges. — Beside these, there is a third 

 sort of edge placed on the lateral faces of metathorax, running 

 from the lateral angles to the base of the posterior wing; these 

 are the lateral edges. There should be no confusion between 

 the superior ridges, which, seen from behind, may also be taken 

 for lateral ridges. The superior edges are often rather elevated, 

 and at their superior extremities are separated from the post- 

 scutel by a little fissure or by a notch. These characters become 

 more and more effaced in the series of species until the meta- 

 thorax becomes quite convex, rounded, destitute of ridges, except 

 on the lateral edges, which nearly always continue, more or less. 

 The superior ridges become less salient, no longer making a sort 

 of tooth behind the post-scutel; they then become blunted (0. 

 dorsalis); or quite effaced (0. sulfureus, pratensis) ; finally, 

 also, the lateral angles disappear. In another series the mar- 

 gins of the hinder plate of mesothorax are quite arcuated, and 

 there are no lateral angles, although the margins are very sharp 

 (0. Megaera, turpis, etc.) ; but, nevertheless, the same names can 

 be applied for the superior and inferior part of the marginal 

 ridges. These circular margins also become more and more 

 blunted (0. molestus), until the metathorax becomes quite con- 

 vex, rounded, as in the preceding case (0. leucomelas). In some 

 cases the lateral ridges are continued with the inferior ridges, 

 the superior ones being effaced, etc. (Numerous Stenodyneri.) 



1 I have noticed this in my Etndes sur la Famille des VespMes, vol. Ill, 

 p. 184; pi. xii, fig. 1, 2, 3. 



2 Compare Odyn. Hidalgi, Boscii, dorsalis, natulens, Cubensis, capra, etc. 



