172 U. S. NATIONAL MUSEUM BULLETIN 216 



difficult to define with the precision expected for such categories. The 

 less formal divisions of species groups seem more appropriate in this 

 case. 



An unusual characteristic of the genus is that many of the species 

 give off a pungent odor when disturbed (as when caught in a net). 

 This odor seems identical with that of Coccygomimus, Ephialtes, and 

 Echthromorpha, and in relation to their smaller size it is just as potent. 

 We have noticed the odor on several species in the field and have field 

 notes of it for the species russeus, albifrons, atriceps, decoratus, and 

 dor salts. For some species we failed to make a written record and for 

 others (e. g., E. tedulum) we could not detect an odor. 



Key to the Nearctic species of Exochus 



{Exochus albiceps Walsh is not included. It is discussed at the end of 

 the genus, on p. 266.) 



1. Epipleurum of third tergite wedge-shaped, the basal 0.75 of the mesal edge 



straight or slightly concave 2 



Epipleurum of third tergite semicircular or subrectangular, the basal 0.75 of 

 the mesal edge strongly convex 11 



2. Frons laterally yellow, the yellow area continuous to top of eye or rarely 



with a narrow interruption , apical transverse carina of propodeum usually 

 absent or incomplete between the lateral longitudinal carinae; front spur 



middle tibia about 0.75 as long as hind spur, pictus group 3 



Frons laterally black, or if partly yellow or white, this color is not continuous 

 to top of eye; apical transverse carina of propodeum complete; front spur 

 of middle tibia 0.40 to 0.85 as long as hind spur 5 



3. Thorax black, the upper margin of pronotum and the subtegular ridge yellow; 



apical transverse carina of propodeum present medially, at least as vestiges. 



4. pictus Holmgren 

 Thorax black in ground color but extensively marked with yellow and ferru- 

 ginous; apical transverse carina of propodeum entirely absent medially . 4 



4. Apical transverse carina of propodeum present mesad of lateral longitudinal 



carina as a short projecting stub; lower front edge of metapleurum produced 

 ventrad as a lamella that is about 0.55 as deep as long; first lateral area of 

 propodeum usually more or less ferruginous ... 5. russeus, new species 

 Apical transverse carina of propodeum entirely absent mesad of lateral 

 longitudinal carina; lower front edge of metapleurum produced ventrad 

 as a lamella which is about 0.3 as deep as long; first lateral area of propodeum 

 entirely black 6. enodis, new species 



5. Nervulus interstitial with basal vein, or distad of basal vein by less than 



0.28 of its length; front spur of middle tibia about 0.85 as long as hind spur. 



GRAVIPES GROUP 6 



Nervulus distad of basal vein by more than 0.30 of its length; front spur of 

 middle tibia 0.45 to 0.60 as long as hind spur, pullatus group ... 9 



6. Notaulus an ovoid impression, the long axis of which parallels edge of meso- 



cutum; combined areola and basal area of propodeum 1.5 to 1.7 as long as 

 wide; metapleurum with a few discal hairs, gravipes subgroup. 



10. gravipes (Gravenhorst) 



