Troutnut.com Fly Fishing for Trout Home
User Password
or register.
Scientific name search:

Light Blue Duns

Scientific Name
MatchScientific Name
*Baetis tricaudatus


This common name refers to only one species.

Mayfly Species Baetis tricaudatus

These are very rarely called Light Blue Duns.
Baetis tricaudatus is undeniably the most widespread and abundant baetid on the continent and arguably the most important mayfly species to trout and anglers alike. Eastern anglers used to know these important mayflies by the storied name of Baetis vagans. Conversely, the usually much larger and late Fall hatching brood of Baetis tricaudatus was considered an important Western species with its own tradition. But, entomologists recently determined that they are both in fact the same species. The nomenclature conventions guiding entomologists do not account for a name's regional fame among fishermen, and new or obscure species names may replace their old favorites. Sometimes taxa with disparate traditions are combined. Baetis vagans is one such casualty. Fortunately, trout think like Shakespeare: A rose by any other name would smell as sweet. The rose that was vagans has lost none of its charm. This species is multibrooded (Multibrooded: Producing more than one generation in a single year. Baetis mayflies are a classic example. Insects which produce a single generation with two distinct peaks (like the June and September hatches of Isonychia bicolor mayflies) are not multibrooded, because the fall insects are offspring from the previous fall instead of the current year's spring.) with the hatches of Spring being larger flies. As the weather warms the following broods are composed of progressively smaller flies. In the East, they range in size from 16 to 20. In the West, they may run a size larger.
Baetis tricaudatus (Blue-Winged Olive) Mayfly NymphBaetis tricaudatus (Blue-Winged Olive) Mayfly Nymph View 20 PicturesA nymph of the same species as this one emerged into a dun in my studio so I got photos of both stages.

NOTE: I missed an important key characteristic the first time I tried to identify this one (robust setae (Seta: Little hairs on insects.) on the abdominal sternites (
One sternite of this Isonychia bicolor mayfly spinner is highlighted in red.
One sternite of this Isonychia bicolor mayfly spinner is highlighted in red.
Sternite: The bottom (ventral) part of a single segment on an insect's abdomen.
)
, which were harder to see than I expected but are clearly present), so I went on a bit of a wild goose chase and landed at a dead end. After spotting that characteristic, this one keys more straightforwardly to either Baetis tricaudatus or the Baetis piscatoris complex. It doesn't seem to be a perfect fit for either one in the key, but I'm going with tricaudatus based on range and abundance. It's not certain.

However, I'm leaving the flawed analysis below with this disclaimer, because some aspects of how I approached that dead end might be informative in the future.

----Incorrect analysis below----

After spending a lot of time with this one under my shiny new microscope, I'm still not quite sure what it is. I botched my attempt to expose the mouth parts that might make the ID more definitive. Based on the key in Webb et al 2018's "Baetis Larvae of North America," here's my reasoning at each key couplet.

Couplet 1. The pronotum (Pronotum: The top of the insect prothorax.) lacks dark, submedian U-shaped markings. Also, if I were to follow through to couplet 2, there seem to be characteristics that rule out each of the options: the intercalaris complex is ruled out by the abdominal markings, and the caudal (Caudal: Toward the posterior tip of the body.) filaments have neither a dark median band (ruling out the flavistriga complex) nor uniform pale coloration (ruling out Baetis notos). This sends me with decent confidence to couplet 4.

Couplet 4. I cannot find robust setae (Seta: Little hairs on insects.) in my microscope on the scapes, pedicels, paraprocts, or sterna. I also do not see a pair of dark, bilobed markings on the pronotum (Pronotum: The top of the insect prothorax.). Unless I overlooked these characteristics, proceed to couplet 9.

Couplet 9. Abdominal tergum (Tergum: the dorsal part of an abdominal segment or segments (terga). Also used to describe the entire abdominal dorsum or the thoracic dorsal segments of Odonata.) 5 is a bit paler than adjacent terga (Tergum: the dorsal part of an abdominal segment or segments (terga). Also used to describe the entire abdominal dorsum or the thoracic dorsal segments of Odonata.), but "distinctly paler"? The figure for Baetis alius in the paper, as well as a very nice picture posted by Millcreek in the forum here, shows that Baetis alius would have darker tergites (
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
Tergite: The top (dorsal) part of a single segment on an insect's abdomen when it consists of a single chitinous plate (sclerite), or an individual sclerite if the segment has more than one.
)
surrounding #5. So proceed to couplet 11.

Couplet 11. The length of the gills is obviously less than 2X their width. This leads to the Baetis vernus complex, which could include that species or Baetis brunneicolor. This key doesn't say how to tell those species apart.

Switching over to Burien et al 2018 as the source, the characteristics used to distinguish vernus from brunneicolor seem to rule out either one. Brunneicolor should have more uniformly brown abdominal tergites (
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
Tergite: The top (dorsal) part of a single segment on an insect's abdomen when it consists of a single chitinous plate (sclerite), or an individual sclerite if the segment has more than one.
)
, whereas vernus should have a lack of visible tracheation in most of the gills.

The fore femur (
The femur of this Isonychia bicolor mayfly spinner is highlighted in red.
The femur of this Isonychia bicolor mayfly spinner is highlighted in red.
Femur: The main segment of an insect's leg close to the body, in between the tibia and the trochanter.
)
length is about 3.8x its width.

Also worth noting: In the genus ID, I thought I could see the villipore in my microscope, but I'm not sure. If I back out of Baetis altogether and assume there's no villipore, I end up at Fallceon, but this specimen doesn't seem to have the frontal keel on the head that's supposed to be present on Fallceon quilleri. So that seems like a dead end as well.
Collected September 12, 2020 from in
Added to Troutnut.com by on September 19, 2020
Male Baetis tricaudatus (Blue-Winged Olive) Mayfly DunMale Baetis tricaudatus (Blue-Winged Olive) Mayfly Dun View 8 PicturesThis male was associated with a female of the same species.
Collected April 3, 2007 from in
Added to Troutnut.com by on April 3, 2007
Male Baetis tricaudatus (Blue-Winged Olive) Mayfly SpinnerMale Baetis tricaudatus (Blue-Winged Olive) Mayfly Spinner View 1 Pictures
Collected April 15, 2010 from in
Added to Troutnut.com by on June 27, 2011
Male Baetis tricaudatus (Blue-Winged Olive) Mayfly AdultMale Baetis tricaudatus (Blue-Winged Olive) Mayfly Adult View 2 Pictures
Collected May 16, 2012 from in
Added to Troutnut.com by on May 18, 2012
Top 10 Fly Hatches
Top Gift Shop Designs
Top Insect Specimens
Miscellaneous Sites