6 american hymenoptera. 



External Anatomy. 



Head (PI. I, fig. 4). — The hypognathous head is large, 

 viewed from in front usually as broad or slightly broader than 

 high, rarely higher than broad ; when viewed from above obli- 

 que in outline and somewhat hollowed out behind. The com- 

 pound eyes are usually large, extending almost to the base 

 of the mandibles ; rarely small, distant from the base of the 

 mandibles and more narrow ; the anterior margin is usually 

 emarginate opposite the antennal fossae, but is sometimes 

 only slightly so, being almost straight. Each eye is some- 

 times surrounded by a more or less distinct crenulated carina. 



The three usually prominent ocelli are located on the ver- 

 tex between the tops of the eyes, in the form of a triangle, 

 which varies somewhat in shape and size. The size and dis- 

 tance of the ocelli from the tops of the eyes vary in differ- 

 ent genera — and somewhat in different species — the latter 

 from twice the diameter of the ocellus in Thyreodon, to little 

 or practically nothing in Athyreodofi, with the other genera in- 

 termediate but nearer Athyreodon. A few species of Ophion 

 have, however, small ocelli and some species of Thyreodon, 

 especially Thyreodon spectabilis, indicate that the size is not 

 well fixed. I have seen specimens of this species which ap- 

 parently bridge the distance between the large and small 

 ocelli of Athyreodon and Thyreodon. 



The vertex is not distinctly separated from the occiput, 

 except in Ophion costale, where a distinct carina runs from 

 the tops of the eyes to the posterior ocelli, though similar 

 but indistinct carinas sometimes occur in other members of 

 the tribe. 



The vertex is usually smooth, sometimes rugose, aud often 

 with a median carina running from the anterior ocellus to 

 or below the antennal fossae ; these, located slightly above 

 the middle of and not far from the eyes, vary somewhat in 

 size and depth and are sometimes inclosed by a distinct 

 carina. The long filiform antennae are divided into a scape, 

 pedicel and flagellum of about 61 segments, the number 

 varying somewhat. The frons, which is continuous with the 

 vertex, extends to or below the base of the eyes. It usually 



