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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.

Mayfly Species Caudatella hystrix

This generally unimportant species sometimes produces fishable hatches.

Where & when

Time of year : July and August

Nymph biology

Current speed: Fast

Physical description

Most of these descriptions are direct 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.

Body length: 7.5–8.5 mm
Wing length: 11–12 mm

Eyes reddish brown. Thorax dark reddish brown, lateral sutures and base of forewings light yellowish brown with a black lateral line which runs from the base of the forewing to the coxae of the forelegs; prosternum dark brown, nearly black, light vellowish brown near bases of legs; mesosternum dark brown medially, becoming lighter laterally; anterior half of metasternum dark brown, nearly black, posterior half yellow-brown, tibiae and distal end of femora of prothoracic legs dark brown; wings hyaline, longitudinal veins brown, crossveins pale. Abdominal terga black; segments 1 and 2 uniformly black; segments 3-9 with light oblique yellowish streaks that originate near the anterolateral corner of the tergum and extend beyond the middle, streaks broader and more pronounced on posterior segments; terga 3-8 with yellow lateral border and yellow maculae at posterolateral corners; terga 8 and 9 yellow along posterior margins; tergum 10 with yellow lateral and posterior margins; sterna 1-9 each marked with a broad dark reddish brown chevron as in fig. 8; sternum 10 dark brown, the chevron partially or entirely obscured. Caudal filaments brown, lateral cerci 65%-75% as long as terminal filament. Penes with two submedian apical lobes forming a deep median depression as in fig. 2.

Body length: 7.5–8.5 mm
Wing length: 11–12 mm

Thorax olive-brown, lighter than in male but with same yellowish lateral sutures and dark lateral line. Abdominal segments darker than in male with color pattern less distinct. Other characters as in male except for usual sexual differences.

Body length: 9–11 mm

Mature Nymph—Length: body 9-11; cerci 6-8; terminal filament 9-11 mm. Head brown with pale markings; without occipital tubercles (fig. 17); antennae brown; maxillary palpi three-segmented. Pronotum and mesonotum brown with pale markings; legs brown, with scattered spines and hairs only, as in fig. 16; tarsal claws with 6-10 denticles. Abdominal terga brown with darker brown markings; terga 1-9 with median dark brown triangles; terga 2, 3, and 8 with dark lateral oblique dashes; tergum 9 dark brown, partially obscuring median and lateral markings: tergum 10 entirely dark brown; paired dorsal submedian tubercles on segments 2-9; tubercles short with bases close together on tergum 2, tubercles longer and bases more widely separated to tergum 7; tubercles on terga 2-7 uniformly divergent; tubercles on terga 8 and 9 are less divergent, shorter and more narrowly separated than those on tergum 7 (fig. 26); sterna light brown, each with a dark brown chevron (fig. 8); sternum 9 may be dark brown, partially obscuring chevron. Caudal filaments brown, darker at joinings, lateral cerci 65% to 75% as long as terminal filament.


Described by Allen, R.K., and Edmunds, George F. Jr. (1961) as Caudatella cascadia
Body length: 5–7 mm

Cerci 4-5 mm; terminal filament 6-7 mm. Head brown with pale markings; moderately developed paired submedian occipital tubercles (fig. 18); antennae light brown: maxillary palpi three-segmented. Pronotum brown, with low, rounded, paired submedian thoracic tubercles and paired, sublateral smaller tubercles near the hind margin; mesonotum brown, roughened, but without tubercles. Legs brown with scattered spines and hairs only, similar to those in fig. 16; tarsal claws with 7-10 denticles. Abdominal terga light brown with variable lateral darker markings; tergum 10 light brown; long, thin, paired dorsal submedian tubercles on segments 2-9 (fig. 25) ; base of tubercles on segment 2 close together, becoming more widely separated to segments 6 to 8, and closer together on segment 9; all tubercles divergent apically; tubercles on terga 2 to 4 arise at a nearly perpendicular angle with their tips directed posterolaterally; tubercles on terga 5-8 directed more laterally and less perpendicularly than any of the preceding tubercles; tubercles on tergum 9 nearly straight and directed posterolaterally; sterna dark brown with light brown lateral margins similar to those in fig. 9. Caudal filaments brown, lateral cerci 65% to 75% as long as terminal filament.

Specimens of the Mayfly Species Caudatella hystrix

1 Nymph
Caudatella hystrix  Mayfly Nymph
This is a widely distributed species in western Montana but never abundant. Note the length of the middle tail, a key to the identification of this genus.
1 Male Spinner

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  • Allen, R.K., and Edmunds, George F. Jr. 1961. A Revision of the Genus Ephemerella (Ephemeroptera: Ephemerellidae) II. The Subgenus Caudatella. Annals of the Entomological Society of America 54: 603-612.
Species Range
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