Key to Subgenera, species groups, and species of Neotropical Ephydra

1. Cruciate interfrontal setae weak or lacking; crossvein dm-cu forming acute angle with vein CuA1; palpus small; coloration of mesonotum strongly whitish gray (subgenus Halephydra Wirth).................................. E. gracilis Packard

- One pair of well-developed, cruciate, interfrontal setae; crossvein dm-cu forming nearly right angle with vein CuA1; palpus well developed; coloration of mesonotum shiny to moderately grayish (subgenus Ephydra Fallén)..................... 2

2. Anterior acrostichal setae in 2 distinct rows; coloration generally grayish, microtomentose; tarsal claws generally nearly as long as tarsomere 5; male aedeagus nearly straight, lacking recurved, basal process (The glauca Group)...................................................................................................... E. mexicana Cresson

- Anterior acrostichal setae unseriated or in 4–5 irregular rows; coloration generally olive green, microtomentose; tarsal claws generally not more than half as long as tarsomere 5; male aedeagus with strongly recurved basal process (The riparia Group).................................................................................................... 3

3. Aedeagal apex distinctly pointed, curved, hook-like, becoming much stouter toward base; sternal plate with strong transverse ridges on strongly convex midportion; surstylus stout, thumb-like, rounded distally.................... E. millbrae Jones Aedeagal apex rounded, not hook-like, tapered to apex; sternal plate not strongly convex; surstylus moderately expanded apicolaterally, yellowish..................................................................... E. packardi Wirth