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 (
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 (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 wing pads on this final instar Baetidae
mayfly nymph are extremely dark.
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 palp on the maxilla of an Ephemerella
nymph (detached and photographed under a microscope) is highlighted in red here.
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.
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.
Closeup of a tarsal claw
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.
The hind wingpad is clearly visible in the bottom center but not yet darkened in this specimen.
The tibiae lack a thick fringe of setae, pointing away from Acentrella and toward Baetis instead.
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