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Lateral view of a Female Hexagenia limbata (Ephemeridae) (Hex) Mayfly Dun from the Namekagon River in Wisconsin
Hex Mayflies
Hexagenia limbata

The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.

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.

Stonefly Species Skwala curvata (Large Springflies)

This species is virtually identical in appearance to Skwala americana. Telling them apart is usually only possible by comparing genitalia. See the Skwala genus page for detailed information.

Where & when

Specimens noted in the original species description were all collected in April.
Species Range

Identification

According to Records And Descriptions of North American Plecoptera. Part Ii. Notes On North American Perlodidae:
This species may be distinguished easily from Skwala americana in the male sex by the long recurved dorsal lobes, the larger lateral stylets (seen only when supraanal process is extruded), and the longer supraanal process. The subgenital plate of the female is indistinguishable from that of Skwala americana, but this sex can be distinguished by color pattern difference as indicated above. It is easily conceivable, however, that further collecting in various regions will show intergradations of color pattern characters between these two species.

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)
Body length: 15–17 mm
Wing length: 11–12 mm

Head color pattern similar to that of Skwala americana but with the yellow crown patch often larger in extent and the tentorial rugosities not yellow. Also in this species the dark brown color extends behind the compound eyes.

Length to tip of wings 15–17 mm. Ninth segment of abdomen very slightly emarginate above and without spinulae. Tenth segment widely split above; dorsal lobes long, recurved, and bearing many spinulae. Supraanal process elongate, cylindrical, well sclerotized, membranous and finely setiferous at tip. Lateral stylets flat, straight or slightly curved near apex, and tapering to a sharp point; conspicuous when the supraanal process is extruded but not visible when that process is retracted within its membranous sheath. The membranous region between the lateral stylets and the base of the supraanal process is thickly beset with a mat of very fine hairs.

Female Adult

Described in Hanson (1942)
Body length: 14–19 mm
Wing length: 14–17 mm

Head color pattern similar to that of Skwala americana but with the yellow crown patch often larger in extent and the tentorial rugosities not yellow. Also in this species the dark brown color extends behind the compound eyes.

Length to tip of wings 18–22 mm. Subgenital plate very similar to that of A. americana, but probably on the average slightly more protrusile and slightly more emarginate than in A. americana.

Specimens of the Stonefly Species Skwala curvata

1 Female Adult

Start a Discussion of Skwala curvata

References

Stonefly Species Skwala curvata (Large Springflies)

Taxonomy
Species Range
Resources
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