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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.

Dorsal view of a Grammotaulius betteni (Limnephilidae) (Northern Caddisfly) Caddisfly Larva from the Yakima River in Washington
This is a striking caddis larva with an interesting color pattern on the head. Here are some characteristics I was able to see under the microscope, but could not easily expose for a picture:
- The prosternal horn is present.
- The mandible is clearly toothed, not formed into a uniform scraper blade.
- The seems to be only 2 major setae on the ventral edge of the hind femur.
- Chloride epithelia seem to be absent from the dorsal side of any abdominal segments.
Based on these characteristics and the ones more easily visible from the pictures, this seems to be Grammotaulius. The key's description of the case is spot-on: "Case cylindrical, made of longitudinally arranged sedge or similar leaves," as is the description of the markings on the head, "Dorsum of head light brownish yellow with numerous discrete, small, dark spots." The spot pattern on the head is a very good match to figure 19.312 of Merritt R.W., Cummins, K.W., and Berg, M.B. (2019). The species ID is based on Grammotaulius betteni being the only species of this genus known in Washington state.
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 Leucrocuta thetis

Where & when

In 5 records from GBIF, adults of this species have been collected during May (40%), June (40%), and July (20%).

In 6 records from GBIF, this species has been collected at elevations ranging from 881 to 2690 ft, with an average (median) of 2522 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 Heptagenia thetis
Body length: 8-9 mm
Wing length: 9 mm

A species of the maculipennis group (now a synonym of Leucrocuta maculipennis); very similar to H. juno (now a synonym of Leucrocuta juno), but with no dark marks on pleura.

Head pale yellowish white; a large smoky spot on each side of vertex between eye and lateral ocellus. Thorax yellowish; no dark longitudinal marks above leg bases, as in other species of this group. Legs pale yellowish; a short black streak apically on ventral margin of fore femur, not evident on other femora. Costa, subcosta and radius of fore wing pale amber; other longitudinal veins hyaline. Cross veins in anterior half of fore wing blackish; all others hyaline. Basal costal cross veins and a few in subcostal and third space slightly thickened; two cross veins at bulla in each of first three spaces; stigmatic cross veins finer and fainter than in H. juno, and more numerous ( 11 to 13 in thetis, 8 to 9 in juno). Abdomen pale whitish; segments 2 to 6 and basal portion of 7 semi-hyaline; apical segments opaque, light red-brown dorsally, somewhat paler ventrally. Posterior margins of tergites narrowly purplish black, most evident in dorsal portion. Tails whitish, unmarked. Genitalia much as in juno; penes with more distinct apical lobe, and longer spine on inner apical margin (see fig. 96).

Female very similar to male, aside from usual sex differences.


Described in Needham et al (1935) as Heptagenia thetis
Body length: 8-9 mm

No pale marks on frontal margin of head. Pale median marks on tergites 7 and 8 not coalesced. Lateral dark streaks on sternite 9 only; dark transverse marks at median line of anterior margin of sternites 8 and 9. Tracheae of gills very indistinct.

Start a Discussion of Leucrocuta thetis


Mayfly Species Leucrocuta thetis

Species Range
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