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**Female Baetis bicaudatus (BWO) Mayfly Nymph Pictures**

Mayflies

Caddisflies

Stoneflies

Classification

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)

I 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.

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.) rules out Acentrella turbida, the lack of a fringe of long setae (Seta: Little hairs on insects.) on the tibiae (

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.

This mayfly was collected from Holder Creek on April 12th, 2021 and added to Troutnut.com on April 13th, 2021.

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Female Baetis bicaudatus (BWO) Mayfly Nymph from Holder Creek in Washington

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The tibiae lack a thick fringe of setae, pointing away from Acentrella and toward Baetis instead.

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This one actually shows the dark-bilobed markings on the pronotum characteristic of the Baetis rhodandi group (to which Baetis tricaudatus belongs). Most specimens I've seen don't have it.

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Closeup of a tarsal claw

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Ventral closeup of the mouthparts highlighting the labial palps. If you view each one as an arm with a hand in a mitten, the "thumb" of the mitten is the part that points clearly toward Baetis rather than Acentrella.

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The hind wingpad is clearly visible in the bottom center but not yet darkened in this specimen.

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Jargon Defined:

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.

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.

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.