Species nigrescens Calvert, 1899 [Dythemis]
This is a dark Mexican species similar to Swift Setwing (D. velox). It is found in the southwestern part of the region. The top of the frons is metallic purple and older individuals become entirely pruinose dark blue. The face and thorax of tenerals and females are olivaceous. There is a broad, dark middorsal stripe on the thorax that covers nearly the entire front of the thorax. Diffuse lateral stripes form a HII or HIY pattern on the side of the thorax. The entire thorax becomes obscured by a dark blue pruinosity in older individuals. The wings are clear with at most a slight spot of dark brown at the extreme base of both wings and generally at the extreme tips. The wings become amber throughout with age. Generally there are 4 rows of cells between vein A2 and the hindwing margin. The legs are brown basally, becoming black on the tarsi and tibiae. The abdomen in the male is slender beyond the swollen, pale basal segments. There are paired, pale, middorsal spots on segments 4-7, with those on 7 conspicuously enlarged. The remaining segments and caudal appendages are dark.
Total length: 42-50 mm; abdomen: 26-32 mm; hindwing: 31-34 mm.
The vertex in Swift Setwing is not metallic and there are no pale lateral spots on segments 4-7. The lateral thoracic stripes appear as YIY. Marl Pennant (Macrodiplax balteata ) is more robust with a distinct dark basal spot in the hindwing. Slaty Skimmer (Libellula incesta ) is much larger.
Creeks, streams and rivers with moderate current.
Individuals perch atop twigs near the water, but generally in open areas. Exhibits similar behavior to Swift Setwing.
Arizona, New Mexico and Texas south through Mexico.