Sunday, January 2, 2011

The "Small Minnow" Mayfly (family: Baetidae)

The "Blue-winged Olive" hatch is anticipated by fly fishermen all winter long since it's one of the earliest we see each spring (April-May, sometimes late March).  It is also a hatch that happens again in the fall (normally, September-October).   These hatches may be those of different genera -- but not necessarily.  This is an insect that typically has multiple broods in a year.  So, the mayflies that hatch in April may lay eggs that hatch and develop as nymphs over the summer, hatching into adults in the fall.

The "Blue-winged Olive" is the adult, terrestrial form of the nymph known as the "Small Minnow" mayfly (family: Baetidae).   Baetid nymphs are very small , rarely exceeding 1/4" in length even when they're mature.  They are also thin and "streamlined," and very good swimmers.  When put into trays by stream samplers, they often dart about so quickly that picking them up again with tweezers can be a real challenge.

Small Minnow nymphs are found in many streams throughout our watershed, though StreamWatch statistics seem to indicate higher numbers are found in our northern and northwestern streams (i.e. Buck Mt. Creek, the Doyles River, Lickinghole Creek, the Lynch River, North Fork of the Rivanna, and Swift Run).   I have just started to find them -- they're still very small -- at Buck Mt. Creek, the Doyles River, and the Lynch.

We have at least three genera of Baetidae in our streams, and the genus of the nymphs I've been finding right now is Heterocloeon (photo above).   Heterocloeon nymphs have only two tails (most mayflies have three), and their gills are distinctive: they are clear in color, lack veins, and have a strip of pigmentation in the center (all of this can be seen in the photo above).

 Another Baetidae genus that has only two tails is Acentrella.   Acentrella nymphs tend to be very "robust," or maybe "broad-shouldered," i.e. wider through the thorax than their fellow family members.
I have only seen this genus in the fall -- but that may be a matter of chance.  I've seen them in the Doyles River, the Lynch River, and the Moormans River.

The other genus I've seen in our streams is known as Baetis.  Baetis nymphs can have two or three tails, but when they have three (as the one in this photo), the one in the middle is always shorter than those on the sides.  Two-tailed Baetis nymphs can be distinguished from Heterocloeon and Acentrella nymphs by looking under their wingpads.  A second set of wingpads -- albeit very tiny, finger-like wingpads -- can be seen underneath the primary wingpads: this is not true for the two other genera.  The nymph in the photo above was found this summer in Meadowcreek.   This is a poor stream that flows through the city of Charlottesville, and it's filled with common netspinning caddisflies, aquatic earthworms and midges -- and very little else.   In most cases, this is the only mayfly we find in this stream and, at least this summer, this is the only genus I saw.

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