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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
Blue-winged Olives

Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

Lateral view of a Clostoeca disjuncta (Limnephilidae) (Northern Caddisfly) Caddisfly Larva from the Yakima River in Washington
This one was surprisingly straightforward to identify. The lack of a sclerite at the base of the lateral hump narrows the field quite a bit, and the other options followed fairly obvious characteristics to Clostoeca, which only has one species, Clostoeca disjuncta.
27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
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Mayfly Species Heterocloeon frivolum

Where & when

In 4 records from GBIF, adults of this species have been collected during July (50%), April (25%), and August (25%).

Species Range

Physical description

Most physical descriptions on Troutnut are direct or slightly edited quotes from the original scientific sources describing or updating the species, although there may be errors in copying them to this website. Such descriptions aren't always definitive, because species often turn out to be more variable than the original describers observed. In some cases, only a single specimen was described! However, they are useful starting points.

Male Spinner

Described in Needham et al (1935) as Baetis frivolus
Body length: 5 mm
Wing length: 6 mm

Abdominal tergites 2-6 of male imago deep brown; genitalia of the modified moffati type (now a synonym of Baetis tricaudatus), the fourth joint of the forceps very long; no costal projection on hind wing, which has two veins only.

Turbinate eyes very large, deep blackish brown, the edges paler. Head and thorax shiny blackish. Light brown markings on the postero-lateral edges of the prothorax, the antero-lateral edges of the mesothorax, the pleural sutures (these are tinged with reddish), the lateral edges and side projections of the mesosternum, and the posterolateral margins of the mesonotum near the scutellum. Fore legs smoky brown; middle and hind legs pale yellowish. Wings hyaline, venation pale. Cross veins in the stigmatic area of the fore wing 6-7 in number, anastomosed. First intercalary in the first interspace rudimentary or lacking; all others well developed. Hind wing long and narrow, third vein entirely lacking. Costal projection entirely wanting.

Abdominal tergites deep brown, opaque. Sternites dull yellowish, becoming whitish posteriorly. Traces of a broken black spiracular line. Second joint of the forceps cylindrical, the fourth unusually long, being fully three times as long as wide, its tip knob-shaped. Between the bases of the forceps is a small raised plate.

This species, related, by reason of the complete lack of costal projection of the hind wing, to propinquus (now a synonym of Labiobaetis propinquus), dardanus (now a synonym of Labiobaetis dardanus), insignificans (now a synonym of Acentrella insignificans) and amplus (now a synonym of Heterocloeon amplum), may be distinguished from each of these by the deep brown color of the abdomen.

Start a Discussion of Heterocloeon frivolum


  • Caucci, Al and Nastasi, Bob. 2004. Hatches II. The Lyons Press.
  • Needham, James G., Jay R. Traver, and Yin-Chi Hsu. 1935. The Biology of Mayflies. Comstock Publishing Company, Inc.

Mayfly Species Heterocloeon frivolum

Species Range
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