Header image
Enter a name
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.

Dorsal view of a Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
This is an interesting one. Following the keys in Merritt R.W., Cummins, K.W., and Berg, M.B. (2019) and Jacobus et al. (2014), it keys clearly to Ephemerella. Jacobus et al provide a key to species, but some of the characteristics are tricky to interpret without illustrations. If I didn't make any mistakes, this one keys to Ephemerella mucronata, which has not previously been reported any closer to here than Montana and Alberta. The main character seems to fit well: "Abdominal terga with prominent, paired, subparallel, spiculate ridges." Several illustrations or descriptions of this holarctic species from the US and Europe seem to match, including the body length, tarsal claws and denticles, labial palp, and gill shapes. These sources include including Richard Allen's original description of this species in North America under the now-defunct name E. moffatae in Allen RK (1977) and the figures in this description of the species in Italy.
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.
Troutnut is a project started in 2003 by salmonid ecologist Jason "Troutnut" Neuswanger to help anglers and fly tyers unabashedly embrace the entomological side of the sport. Learn more about Troutnut or support the project for an enhanced experience here.

Mayfly Species Heterocloeon amplum

Where & when

In 3 records from GBIF, adults of this species have been collected during June (33%), August (33%), and July (33%).

In 3 records from GBIF, this species has been collected at elevations of 440, 480, and 981 ft.

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 amplus
Body length: 7 mm
Wing length: 7 mm

Abdominal tergites 2-6 of male imago yellowish brown, the posterior margins hyaline whitish; hind wing two-veined, costal projection wanting.

Turbinate eyes large, almost circular; orange in color. A black mark across the face. Antennae light brown. Thorax reddish brown. Longitudinal dark brown streaks on the mesonotum, median and submedian in position. Metanotum distinctly reddish. Sternites light reddish brown. A red spot on the coxa of the fore leg; fore femur distinctly tinged with red, a band of this color near its apex. Fore tibia yellowish, tarsus yellowish white. Middle and hind legs yellowish, the femora light yellowish brown ; tarsal joinings may be very narrowly reddish. Wings hyaline, venation yellowish brown. Cross veins of the stigmatic area of the fore wing 7-8 in number; only a slight tendency toward anastomosis; almost no granulations between them. Intercalaries absent from the first interspace, well developed in other spaces. Hind wing very small (1 mm. in length), long and narrow; two veins only, the third entirely absent. Costal projection entirely wanting.

Abdominal segments 2-6 yellowish brown, tinged with grey. The posterior margins rather widely opaque, giving this part of the abdomen an annulate appearance. Segments 7-10 light reddish brown, rather darker dorsally. Basal sternites slightly paler than their tergites. Genitalia of the modified moffati type (now a synonym of Baetis tricaudatus), the second joint of the forceps being cylindrical. The distal forceps joint is relatively very long, fully three times as long as wide; it is almost one-half the length of the preceding joint.

This species is very close to B. frivolus (now a synonym ofHeterocloeon frivolum), but may be distinguished from it by the larger size, the color of the eyes and thorax, the paler abdomen and the distinctly brownish venation.

Start a Discussion of Heterocloeon amplum


Mayfly Species Heterocloeon amplum

Species Range
Troutnut.com is copyright © 2004-2024 (email Jason). privacy policy