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.
These mayflies are a sleeper hatch which has barely received a nod in the fly fishing literature. Their abundance never matches that of the classic superhatches, but they can be important on some waters. The taxonomic common name for this family and genus is Armored Mayfly but angler reports often refer to them as Humpback Nymphs and the adults as Speckled Duns or Batflies.
Their primary claim to fame is their peculiar shape, both as nymphs and as adults. On most rivers they are a rare oddity, but there are places where their spinner falls elicit greedy rises and staunch selectivity from the trout. These spinners never even come close to blanketing the water, but trout have such an affinity for them that even their sparsest falls should excite the angler.
I favor two explanations for their importance. First, they have a very stout profile, and late-season beetle action suggests that trout like this trait. Second, the Baetisca action lasts for up to a month on a single stretch of water. This caters to the selective trout's love of the familiar.
BRAT on Oct 25, 2009October 25th, 2009, 8:46 am EDT
Hi there; I just gathered a sample from our local Black River here in SE Vermont, and found a thriving population of armored mayflies. Wasn't sure what they were at first, with their bubble-humped backs and short little tails, but once I used my hand lens I realized they have tiny little "thorns" behind their last legs. Their mottled coloring resembles everything I've seen here, confirming my ID. I'll be sending off a sample to a friendly bug lab to let the pros look at them.