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**Mayfly Species Baetis bicaudatus (BWO)**

Mayflies

Caddisflies

Stoneflies

Taxonomic Navigation -?-

Kingdom Animalia (Animals)

» Phylum Arthropoda (Arthropods)

» Class Insecta (Insects)

» Order Ephemeroptera (Mayflies)

» Family Baetidae (Blue-Winged Olives)

» Genus Baetis (Blue-Winged Olives)

» Species bicaudatus (BWO)

Common Names

Match	Common Name
	BWO
	Pale Olive Dun
	Little Blue-Winged Olive
	Rusty Spinner
	Light Rusty Spinner

Pictures Below ↓

Baetis bicaudatus is a western taxon of some local importance.

Where & When

Region: West

Pictures of 3 Mayfly Specimens in the Species Baetis bicaudatus:

Baetis bicaudatus (BWO) Mayfly Nymph

View 5 PicturesHere I'm just copying and pasting, without cleaning up, my notes from spending a long time with this one under the microscope (and keying with Merritt & Cummins 5th Ed) only to end up confirming the most likely guess.

7. Baetis bicaudatus nymph
1. Hind wingpad present but small and hidden beneath forewing pad
2. Segment 2 of labial palp (

The palp on the maxilla of an Ephemerella nymph (detached and photographed under a microscope) is highlighted in red here.

Palp: A long, thin, often segmented appendage which can protrude from certain insect mouth parts such as the maxillae. Also known as the < />palpus.) with well-developed medially projecting corner —> Baetis (couple 44)…. BUT no sign of scale-like setae (Seta: Little hairs on insects.) on abdominal 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.). Conflicts at this couplet.
3. Gills on segments I-VII
4. Tarsal claws (Tarsal claw: The claws at the tip of the tarsus, on an insect's "foot.") with denticles (

View Full Size

The denticles on the tarsal claw of this Ephemerella nymph are highlighted in red.

Denticle: Small tooth-like projects, often appearing like serrations on the tarsal claws of certain mayfly nymphs.), seemingly 2 rows but very hard to tell… and the key options with 2 rows don’t make sense
5. Assuming no villipore, we land confidently at couplet 48
6. Leads to Fallceon, except antennal scape doesn’t have robust setae (Seta: Little hairs on insects.)
7. Treated as Baetis, leads to brunneicolor, but McDunnough et al 1932 (Can Ent 64) suggests middle tail should be 5/6 as long as outer ones
8. Keys VERY confidently to couplet 36 in M&C (villipore)
9. If assuming villipore present:
1. 37 —> Scape of anntenae has no distal (Distal: Far from the point of attachment or origin; near the tip.) lobe —> rules out Labiobaetis (100 % certain)
2. 38 —> Terminal filament much shorter than cerci (Cercus: The left and right "tails" of an insect are known as the cerci or caudal cerci. The middle tail of a three-tailed insect is not.) —> not Barbaetis benfieldi (100 % certain)
3. 39 —> Terminal filament reduced (100 % certain)
4. 40 —> Tarsal claw (Tarsal claw: The claws at the tip of the tarsus, on an insect's "foot.") dentical count couplet. If two rows of denticles (

View Full Size

The denticles on the tarsal claw of this Ephemerella nymph are highlighted in red.

Denticle: Small tooth-like projects, often appearing like serrations on the tarsal claws of certain mayfly nymphs.): Either Iswaeon or Heterocloeon. Can’t be Iswaeon because cerci (Cercus: The left and right "tails" of an insect are known as the cerci or caudal cerci. The middle tail of a three-tailed insect is not.) lack dark median band. Can’t be heterocloeon because it’s not in the Platte drainage or in Texas. Thus, it must be one row of denticles (

View Full Size

The denticles on the tarsal claw of this Ephemerella nymph are highlighted in red.

Denticle: Small tooth-like projects, often appearing like serrations on the tarsal claws of certain mayfly nymphs.). Moving on to 42.
5. 42 —> Hind wing pads (

The wing pads on this final instar Baetidae mayfly nymph are extremely dark.

Wing pad: A protrusion from the thorax of an insect nymph which holds the developing wings. Black wing pads usually indicate that the nymph is nearly ready to emerge into an adult.) present (100 % certain)
6. 44 —> Segment 2 of labial palpi with well-developed medially projecting corner (80 % certain), scale-like setae (Seta: Little hairs on insects.) not evident on 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 maybe limitation of my scope —> Baetis (alternative would be Acentrella, but pronotum (Pronotum: The top of the insect prothorax.) shape is all wrong for those, although not an official characteristic)
7. CONFIDENT in Baetis bicaudatus after distinctive leg markings (J-shaped light mark on first femur (

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.), L-shaped on second and third) matches original species description to a tee.

Collected August 4, 2020 from Green Lake Outlet in Idaho
Added to Troutnut.com by Troutnut on August 20, 2020

Male Baetis bicaudatus (BWO) Mayfly Nymph

View 7 PicturesThis specimen was collected along with a female which was quite a bit larger, different in color, but otherwise seemed the same morphologically.

Collected April 12, 2021 from Holder Creek in Washington
Added to Troutnut.com by Troutnut on April 13, 2021

Female Baetis bicaudatus (BWO) Mayfly Nymph

View 12 PicturesI collected this one along with a male that was quite a bit smaller but equally ready to emerge in mid April.

I spent (Spent: The wing position of many aquatic insects when they fall on the water after mating. The wings of both sides lay flat on the water. The word may be used to describe insects with their wings in that position, as well as the position itself.) quite a while on the identifications, because they really don't look very much like the Baetis bicaudatus nymph I caught last year in Idaho. However, the presence of hind wing pads (

The wing pads on this final instar Baetidae mayfly nymph are extremely dark.

The tibia of this Isonychia bicolor mayfly spinner is highlighted in red.

Tibia: A middle segments in the leg of an insect, located between the femur and the tarsus.) rules out Acentrella insignificans, range rules out Heterocloeon and Iswaeon, and the thumb-like projection on the labial palp (

The palp on the maxilla of an Ephemerella nymph (detached and photographed under a microscope) is highlighted in red here.

Palp: A long, thin, often segmented appendage which can protrude from certain insect mouth parts such as the maxillae. Also known as the < />palpus.) points to Baetis. Thus, Baetis bicaudatus is a fairly confident ID, and it's not too surprising that it looked different from my previous specimen because bicaudatus is thought to be a species complex with multiple types that haven't been fully sorted out yet.

The microscope pictures for this specimen aren't from the same exact nymph, but a mixture of a few others of the same kind that I didn't mind dissecting.

Collected April 12, 2021 from Holder Creek in Washington
Added to Troutnut.com by Troutnut on April 13, 2021

Your Thoughts On Baetis bicaudatus:

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Jargon Defined:

Cerci: The left and right "tails" of an insect are known as the cerci or caudal cerci. The middle tail of a three-tailed insect is not.

View Full Size

The denticles on the tarsal claw of this Ephemerella nymph are highlighted in red.

Denticles: Small tooth-like projects, often appearing like serrations on the tarsal claws of certain mayfly nymphs.

Distal: Far from the point of attachment or origin; near the tip.

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.

Nymph: The juvenile, underwater stages of mayflies, stoneflies, dragonflies, and damselflies and other aquatic insects whose juvenile stages are covered by hard exoskeletons. The word can also refer to the fishing flies which imitate these creatures, in which case it is used as a blanket term for flies imitating any underwater stage of an invertebrate (except for crayfish and leeches).

The palp on the maxilla of an Ephemerella nymph (detached and photographed under a microscope) is highlighted in red here.

Palp: A long, thin, often segmented appendage which can protrude from certain insect mouth parts such as the maxillae. Also known as the < />palpus.

Pronotum: The top of the insect prothorax.

Setae: Little hairs on insects.

Spent: The wing position of many aquatic insects when they fall on the water after mating. The wings of both sides lay flat on the water. The word may be used to describe insects with their wings in that position, as well as the position itself.

Tarsal claws: The claws at the tip of the tarsus, on an insect's "foot."

Terga: 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.

The tibia of this Isonychia bicolor mayfly spinner is highlighted in red.

Tibiae: A middle segments in the leg of an insect, located between the femur and the tarsus.

The wing pads on this final instar Baetidae mayfly nymph are extremely dark.

Wing pads: A protrusion from the thorax of an insect nymph which holds the developing wings. Black wing pads usually indicate that the nymph is nearly ready to emerge into an adult.

Useful References:

Caucci, Al and Nastasi, Bob. 2004. Hatches II. The Lyons Press.