J. H. MERRILL 111 



optera, no division of the sub-family is attempted but the key 

 is similar to the two preceding ones in the generic characters used. 

 The genus Alegarhyssa is fairly constant in both color and 

 structural characteristics, while the genus Rhyssa exhibits con- 

 siderable variation in both. For that reason it would seem unsafe 

 to describe a new species from a single specimen in Rhyssa. 



External Anatomy 



The following description is taken mainly from Megarhyssa 

 lunator. Certain modifications have been made to describe 

 such differences as may occur in different species of the group 

 treated here. 



Head 



The head is hypognathous, broader than long. Its general 

 shape when viewed from in front is rounded-triangular. The 

 compound eyes are large, extending from the top of the head to 

 the clypeus. They are broad, together constituting one-half 

 the width of the head when viewed from in front and when viewed 

 from the side, at their widest part, forming a little more than 

 half the width of the head. The inner margins of the compound 

 eyes are slightly emarginated on its upper third, but nearly 

 form two parallel lines with each other. The posterior margins 

 are entire. The eyes are naked. 



The clypeus occupies the lower portion of the front of the head 

 and extends up to the lower borders of the compound eyes. 

 The clypeus is narrow and elongated transversely. Laterally 

 below the eyes it has fused with the cheeks. There is an impres- 

 sion reaching from the eyes to about the middle of the mandibles 

 which suggests that it was a suture marking the division between 

 the clypeus and the cheeks. If this is true, then the clypeus 

 and cheeks together form the basal support of the mandibles. 

 The lower border of the clypeus between the mandibles varies 

 in outline, in different genera, from concave to convex. This 

 fact is made use of in the determination of genera. 



The frons, vertex, occiput and upper ends of the genae are 

 fused, but the occiput and genae are fused dorsally, while later- 

 ally they are separated by a suture, and ventrally by a ridge. 

 The occiput may be considered as that portion of the back of 

 the head which is nearly vertical. It is concave from side to 

 side, and the prothorax attaches at about its center. 



TRANS. AM. ENT. SOC, XLI. 



