78 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.52. 



The general color of the body is black. The color of the macula- 

 tions varies from white or yellowish white to rich orange yellow. 

 In some species the extensive development of the maculations almost 

 suppresses the black ground color. On the species of this genus, 

 especially on the males, the pubescence is on the whole better de- 

 veloped than on those of closely related genera. 



In the determination of species in this genus we take into con- 

 sideration (1) the relative direction of the inner borders of the com- 

 pound eyes, (2) the width of the frons in comparison with the width 

 of the compound eye at a given level, (3) the relative length of the 

 segments of the antennae, (4) the secondary sexual modifications of 

 the segments of the flagellum of males, (5) the development of the 

 mandibles, (6) the development of the labrum, (7) the character of 

 the middle femora of the males, (8) the form of the ultimate tergite, 



(9) the character of the processes on sternites 2 and 6 of the male, 



(10) the character of the seventh sternite of the male, (11) the form 

 of the male genital armature, and (12) the character of the macula- 

 tions, size, and habitat. 



KEY TO SPECIES. 



Males. 



1. Intermediate femora distinctly serrate or dentate 2. 



1. Intermediate femora smooth ; not distinctly serate or dentate 16. 



2. Second and sixth sternites without processes, plain 3. 



2. Second and sixth sternites with more or less well developed processes 6. 



3. First segment of middle tarsus distinctly curved, its inner surface beset with 



several stout spines 4. 



3. First segment of middle tarsus not so formed, development normal 5. 



4. Mesosternum marked with black; sternites 2-4 for the most part black; genital 



stipes as in fig. 140 arcuata. 



4. Mesosternum yellow; sternites 2-4 almost entirely yellow; stipes as in fig. 141. 



U-scripta. 



5. Clypeus with lateral borders black; tarsi more or less heavily suffused with black 



above cinerea. 



5. Clypeus wholly yellow; tarsi entirely yellow hind. 



6. Process on sixth sternite prominent, its ventral surface flattened and bifurcate 



at apex; genital stipes as in figs. 149, 151 7. 



6. Process on sixth sternite a transverse ridge not sharply pointed but slightly curved 



on either side the midline - 8. 



6. Process of sixth sternite prominent, flattened and bluntly pointed; an additional 



pair of small processes or ridges on this same sternite nubilipennis. 



6. Process on sixth sternite not as above; relatively small, acutely pointed or if 



flattened the sixth sternite lacks the lateral processes or ridges 9. 



7. Spur on the middle tibia reaching to or beyond the middle point of the metatarsus. 



amoena. 



7. Spur on the middle tibia not reaching to or beyond the middle point of the meta- 



tarsus sayi. 



8. Fasciae on tergites interrupted; labrum with transverse impression; seventh 



sternite not conspicuously narrowed belfragei. 



