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Lateral view of a Female Hexagenia limbata (Ephemeridae) (Hex) Mayfly Dun from the Namekagon River in Wisconsin
Hex Mayflies
Hexagenia limbata

The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.

27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
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Tiny Sulphur Duns

Like most common names,"Tiny Sulphur Dun" can refer to more than one taxon. They're previewed below, along with 4 specimens. For more detail click through to the scientific names.

Mayfly Genus Anafroptilum

These are pretty much always called Tiny Sulphur Duns.
These species were previously in Centroptilum, a genus no longer represented in North America.

They are closely related to the important European "Spur Wings", a name given them due to the hook on the leading edge of their hind wings. These very small mayflies are usually reported in this country as inhabiting water too warm and/or stagnant for trout but this is not always the case. Significant numbers have been reported in many cold water environments throughout the West. These tiny sulphurish mayflies are easy to confuse with small Pale Morning Duns which is perhaps the reason for their under-reporting by anglers. Their having only two tails is the easiest to way to tell the difference.

From an angling perspective, a downside of many taxonomic advances is the loss of what was thought to be the ability to use easily identifiable morphological differences for determining between genera. This is especially problematic in sorting out baetids. The truth is the old rule used by anglers involving the hindwings (i.e., spurwing - prev. Centroptilum, no wing - prev. Pseudocloeon, the rest - Baetis) has been dubious for years. They can no longer be applied as more has been learned about new genera and species properly reassigned. It is now recognized that some species of Anafroptilum and Baetis lack hindwings altogether.
Female Anafroptilum album (Baetidae) (Tiny Sulphur Dun) Mayfly Dun from the Fall River in California
Approx. 5 mm. This is a very significant hatch. It is mostly seen at the height of Summer and I was very surprised to see it emerging this late in the season and in such numbers. When this one is emerging, the fish can become quite selective.
Anafroptilum conturbatum (Baetidae) (Tiny Sulphur Dun) Mayfly Nymph from Bowman Lake in Montana
A moderately rare mayfly perhaps because the nymphs are fragile that nymphs are usually damaged in collection making identification more difficult.

Mayfly Genus Cloeon

These are pretty much always called Tiny Sulphur Duns.
Many relevant (although not outstanding) species were once classified in this genus, but entomologists recently found that they belong in the Procloeon genus instead. Only one insignficant species remains in the genus Cloeon in North America.

Mayfly Genus Procloeon

These are pretty much always called Tiny Sulphur Duns.
This genus is not well-known to fly fishers because most of its species were, until very recently, classified as either Centroptilum or Cloeon.

These mayflies may be abundant enough to cause fishable hatches in spite of their tiny size.

It's difficult to discern from previous angling literature exactly which statements now apply to Procloeon, because its species come from other little-known genera and the accounts for them conflict. They were mentioned in combination with other genera, adding to the confusion. So the information below is highly tentative and, where accounts seem to conflict, both have been given.
Lateral view of a Female Procloeon (Baetidae) (Tiny Sulphur Dun) Mayfly Dun from Enfield Creek in New York
This dun of a fairly large Baetidae species was one of only a couple I saw all evening.
Procloeon pennulatum (Baetidae) (Tiny Sulphur Dun) Mayfly Nymph from Murphy Lake in Montana
This fragile nymph is easily damaged during sampling making identification more difficult.

Mayfly Species Procloeon venosum

These are pretty much always called Tiny Sulphur Duns.
This taxon can be found in large western rivers.

Tiny Sulphur Duns

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