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Ephemeroptera: Baetidae: Baetis nymphs of Gunnison County, Colorado

Nymph key to Species

Updated 29 Oct 2008

Please see Wiersema, Nelson and Kuehnl (2004) for wierdos that don't fit in this key. This key is still under construction!

1a. Two - tailed. Median caudal filament (middle tail) absent or reduced to short stub; occasionally up to 20 percent of the length of the cerci; middle tail stub distinctly tapering and without fringe of fine setae.

Acentrella are also two-tailed.

1b. Three - tailed. Or more scientifically, they have 3 caudal filaments or cerci with the median caudal filament at least 40 percent the length of the cerci; lateral margins of median caudal filament more parallel and with a fringe of fine setae in at least the apical half.

The setae on the end of the middle tail may be very fine, try a black colored stage and adjust the light. Also immature mayflies often have less setae than the older instars.

2a. Antennal scape and pedicel with robust setae; On male larvae, paraproct surface with robust setae.

2b. Antennal scape and pedicel without robust setae; On male larvae, paraproct surface without robust setae.

3a. Gill margins with large robust setae, margins serrate or not serrate.

3b. Gill margins without large robust setae, gill margins serrate.

4a. Patterned abdomen, tails plain colored, without a median stripe.

4b. Less distinctly patterned abdomen, tails with median stripe.

Good Links

Key to Baetidae nymphs


Morihara,DK; McCafferty,WP 1979 The Baetis larvae of North America (Ephemeroptera:Baetidae). Transactions of American Entomological Society 105, 139-221.
     Has pleasing illustrations, some of which were redrawn for this key. The "whole dorsal views" are some of my favorite drawings in the key. Good information for many Baetidae, however many species have been renamed or moved to other genera since this key was written. Table of name changes.

Wiersema,NA; Nelson,CR; Kuehnl,KF 2004 A new small minnow mayfly (Ephemeroptera: Baetidae) from Utah, USA. Entomological News 115 3, 139-145.
     This is the main source for the above key, drawings made comparing illustrations from this, Morihara and McCafferty and from my collection. Abstract: "Baetis moqui, new species, is described from larvae collected from the Escalante Canyon Region of Garfield County in south-central Utah. The new species is unique among North American Baetis in having gill number one highly reduced or absent. Labial morphology and overall setation characteristics indicate a close relationship between B. moqui and the northern California species Baetis alius and the eastern North American species Baetis pluto. An updated key to the North American Baetis larvae is provided. "

Brown, Wendy S. 2005 Ephemeroptera or Mayflies of Gunnison County, Colorado, USA