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.
This genus is represented by several important species across the country. In the East, Attenella attenuata (Little Blue-winged Olive) has garnered much attention as an important hatch in past angling texts. However, possible confusion with more abundant Drunella species of Blue-winged Olives has perhaps led to overstatement regarding its importance.
The species Attenella margarita (Little Western Blue-winged Olive) is disributed nationally. Though its eastern presence is relatively minor, in the West this mayfly can produce exceptional hatches.
Two other western species, Attenella delantala and to a lesser extent Attenella soquele often show up quite prominently in stream samples taken in the coastal states. The dramatically marked delantala nymph cannot be easily confused with any other western ephemerellid. Information regarding their winged appearance seems to be unavailable for now. Why they have gone unreported in angling texts is a mystery perhaps due to emergence behavior or misidentification by anglers of the dun and spinner stages. There is much to learn about these species that can be fairly abundant in some locales.
Where & when
In 18 records from GBIF, adults of this genus have been collected during June (56%), May (17%), July (17%), April (6%), and March (6%).
In 6 records from GBIF, this genus has been collected at elevations ranging from 194 to 8350 ft, with an average (median) of 5420 ft.
A significant difference between this genus and other ephemerellids is it's reported propensity to emerge from its nymphal shuck from the stream bottom in heptagenid fashion. While other ephemerellids can and do emerge underwater, this usually takes place within a foot at most from the surface. Attenella's availability as a submerged dun in the entire water column means use of the winged wetfly or soft hackles fished deeply can be an important tactic.
This dun was badly damaged in transport and all its legs fell off, but I photographed it anyway because I wasn't sure I would fine more and it's an interesting species. Luckily I collected better specimens a few days later.
Notes from the microscope on the ID: Maxillary palp is present, distinctly 2-segmented, but very small. Gills on segment 4-7. This specimen has some unfortunate damage to the abdomen, but it's the only one I found in my sample.