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Ephemeroptera: Baetidae of Gunnison County, Colorado

Baetis magnus
Small Minnow Mayflies, Blue-winged Olive

McCafferty and Waltz 1986
Updated 21 Apr 2021
TSN 568577


A large three-tailed nymph, B. magnus is likely to be confused with Baetis tricaudatus. Since it emerges early (May and before) and as an adult has red-brown longitudinal stripes down both sides of the abdomen, it can be distinguished from B. tricaudatus adults. The larvae are distinguished by examining the edges of the gills. The gill margins have large robust setae in this species and no other Baetis species in our area so far. See the illustrations. They also have setae on the posterior margins of the terga of their abdomen.


This is one of the rhodani group of Baetis species.

Life History

B. magnus live in small, ususally spring-fed, often intermittant streams. In spring-fed Trout Creek in the the upper Arkansas River basin larvae were found living amid water cress (Rorippa nasturtium-aquaticum). They emerge as adults from January to May across their entire range. B. magnus may have a second generation later in the year. (Durfee and Kondratieff, 1999)

Locations Collected

Gunnison River near Gunnison.

Good Links

On this website:
Introduction to Baetis

Other Websites:
Photos, Map, Museum, DNA - Barcodinglife.org


Durfee,R and Kondratieff,BC 1993 Description of adults of Baetis magnus (Ephemeroptera:Baetidae). Entomological News 104(5) 227-232. PDF
    Describes all the taxonomic details for adult males and females. The authors examined B. magnus from the Gunnison River near Gunnison among other places. They discuss the details of telling this species apart from Baetis tricaudatus. B. magnus adults have red-brown longitudinal stripes down both sides of the abdomen. B. tricaudatus have no special coloring on the abdomen.

Durfee,RS and Kondratieff,BC 1999 Notes on North American Baetis (Ephemeroptera: Baetidae): Baetis moffatti new synonym of B. tricaudatus and range extension for B. bundyae. Entomological News 110(3) 177-180. PDF

McCafferty,WP; Durfee,RS and Kondratieff,BC 1993 Colorado mayflies (Ephemeroptera): an annotated inventory. Southwestern Naturalist 38(3) 252-274. PDF
     Quote from page 255: "This large-sized Baetis species is known from Arizona, New Mexico amd Nebraska (McCafferty and Waltz, 1986). The collecting dates above suggest that it is an early developing species with adults emerging in the early spring. The newly confirmed adult stage of this species will be described elsewhere."

McCafferty,WP and Provonsha, AV 2009 The Mayflies of North AmericaSpecies List (Version 12Jan2009)
     Here is the geographic range and synonyms:
# Baetis magnus McCafferty & Waltz, 1986 [MEX:FS,SW;USA:NW,SW]
    * Baetis libos Allen & Murvosh, 1987 (syn.)
* Baetis sp. B Morihara & McCafferty, 1979 (syn.)

McCafferty,WP and Waltz,RD 1986 Baetis magnus, new species, formal new name for Baetis sp. B of Morihara and McCafferty (Ephemeroptera: Baetidae). Proceedings of the Entomological Society of Washington 88:604.

McCafferty,WP; Waltz,RD 1990 Revisionary synopsis of the Baetidae (Ephemeroptera) of North and Central America. Transactions of American Entomological Society 116, 769-799. PDF
     Also notes Baetis sp. B is really Baetis magnus.

Morihara,DK and McCafferty,WP 1979 The Baetis larvae of North America (Ephemeroptera: Baetidae). Transactions of American Entomological Society 105, 139-221. PDF
    Original description of B. magnus as Baetis sp. B from specimens collected in Arizona and New Mexico.

Webb,JM; Jacobus,LM and Sullivan,SP 2018 The state of systematics of North American Baetis Leach, 1815 (Ephemeroptera: Baetidae), with recommendations for identification of larvae. Zootaxa, 4394(1), pp.105-127. html abstract and references


Notice the large setae on the gill edge. Other Baetis species lack this. The gill margins may be serrate (like a saw tooth) or not. Compare to Baetis tricaudatus

Brown, WS 2004 Mayflies (Ephemeroptera) of Gunnison County, Colorado, USA
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