Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
The tiny mayflies of this family are usually found in warm, slow, marginal trout water, although some trout streams hold good populations too.
The only genus ever known to produce fishable hatches is Caenis. It turns up frequently for anglers who sample nymphs, but it is rarely of any practical importance for fly fishing because its emergence traits and tiny size (even smaller than Tricorythodes) make it relatively unimportant.
This interesting little Caenis nymph is way darker than any of the others I collected in January 2004, probably a different species. It also has shorter tails.