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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
Baetis

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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Stonefly Species Skwala americana (Large Springflies)

This species represents the earliest and often most important hatch of Large Stoneflies on many western rivers. During the late winter and early spring, its hatches are a welcome relief for the angler tired of squinting at baetids and equally tiny Dark Winter Stones. Trout must feel the same way about them as they often feed ravenously when they appear. See the Skwala genus hatch page for more information and likely photos of this species.

Where & when

Time of year : April

The collection data for the original description of this species ranges from the beginning to the end of April across the West, with a couple collections in May.
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 Adult

Described in Hanson (1942) as Arcynopteryx americana
Body length: 12–15 mm
Wing length: 8–10 mm

Head color pattern as shown in Figure 7. All pale areas conspicuously yellow. Transverse pale M-line very conspicuous. Tentorial rugosities yellow. Large yellow crown patch joined to extensive yellow region of the occiput. No dark brown color behind the compound eyes. Forewing length 5–6 mm.

Ninth abdominal segment very slightly emarginate above and without spinulae. Tenth segment widely split above; dorsal lobes small, thumb-like, and bearing several spinulae. Supraanal process largely imbedded in membrane, and its lateral styles not visible when not extruded. When extruded, the supra-anal process shows as a well sclerotized, nearly cylindrical process, membranous and somewhat bilobed at the tip. Lateral styles small but well sclerotized; the free part tapers rapidly and curves slightly inward to its sharply pointed apex. The membranous region between the lateral stylets and the base of the supra-anal process is thickly beset with a mat of very fine hairs. See Frison, 1936, for accurate figures of the male of this species.

Female Adult

Described in Hanson (1942) as Arcynopteryx americana
Body length: 15–20 mm
Wing length: 16–20 mm

Head color pattern as shown in Figure 7. All pale areas conspicuously yellow. Transverse pale M-line very conspicuous. Tentorial rugosities yellow. Large yellow crown patch joined to extensive yellow region of the occiput. No dark brown color behind the compound eyes. Forewing length 5–6 mm.

Forewing length 12–15 mm. Subgenital plate protrudes only slightly over segment nine; with a very broad apical emargination. See Frison, 1937, for figure of subgenital plate (shown under the defunct name Hydroperla parallela).


Start a Discussion of Skwala americana

References

Stonefly Species Skwala americana (Large Springflies)

Taxonomy
Species Range
Resources
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