PROC. ENTOMOL. SOC. WASH. 81(3), 1979, pp. 486^98 A KEY TO THE SPECIES OF ECTEMNIUS IN AMERICA NORTH OF MEXICO WITH NOTES AND DESCRIPTION OF A NEW SPECIES (HYMENOPTERA: SPHECIDAE) R. M. BOHART AND L. S. KiMSEY Department of Entomology, University of California, Davis, California 95616. Abstract. — The 27 species of Ectemnius known from America north of Mexico are keyed and many of the structural characters are figured. Taxo-nomic notes on several of the species are presented along with one new species, nigellus Bohart from the western U.S. New synonyms are prole-tarius, placed under borealis; yosemite placed under sexcinctus, and cor-rugatiis, pauper, operus and drymocallidus, placed under atriceps. A neo-type is established for trifasciatus (Say). Ectemnius is the second largest genus in the Crabroninae after Crosso-cerus. Some of its approximately 160 species are found in each of the zoo-logical regions. In America north of Mexico 27 species are known. Discus-sions of the generic characters, subgenera and/or species groups, and species synonyms were given by LeClercq (1954) and Bohart (1976). Mor-phological features which when taken together separate Ectemnius from related genera are: Low ocellar triangle, well developed verticaulus, trough shaped (usually) female pygidial plate, indistinct orbital foveae, evenly punctate upper frons, and recurrent vein joining submarginal cell beyond its distal third (rare exotic exceptions). In North America the only genera with which it might be confused are Crabro, which has the recurrent vein ending before the distal third of the submarginal cell; Crossocerus which has the ocellar triangle nearly equilateral; and Lestica, in which the orbital foveae are distinct and punctation of the upper frons is irregularly coarse. Ectemnius species vary considerably in markings, and a number of sub-species have been named as a result. The value of such names is question-able, even when there is some correlation with geography. Criteria for the more definitive varieties are given in the notes following the key. For brevity we have used F for flagellomere (F-I, F-II, etc.) and T for tergum (T-I, T-II, etc.). Drawings were designed to supplement the key and were made by the junior author.