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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.

Ventral view of a Hydropsyche (Hydropsychidae) (Spotted Sedge) Caddisfly Larva from the Yakima River in Washington
With a bit of help from the microscope, this specimen keys clearly and unsurprisingly to Hydropsyche.
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 Siphloplecton basale (Pseudo-Gray Drakes)

Sadly, the largest mayflies of the early season come in small quantities. Siphloplecton basale matches the impressive size of the Ephemera drakes which follow two months later, but it does not match their numbers. It is common on one of my favorite rivers to see ten to fifteen of these elegant drakes dancing over a riffle in the early Spring. They are difficult to miss in the air, but on the water that quantity cannot get the trout or the angler excited.

There are rumors of fishable Siphloplecton days, but I know none of the details. They might be locally important in places angler-entomologists have yet to visit in the prime months, an unsung local treasure like Baetisca laurentina.

Where & when

Time of year : Late April through May

The Pseudo-Gray Drakes hatch around the time of the more important Hendricksons (Ephemeralla subvaria) in late April and early May. I've seen them in large and small streams in northwest Wisconsin. I've also seen some large drakes flying in the Delaware system of the Catskills which I suspect were of this species, but I could not collect a specimen in the deep water.

The Leonards write in Mayflies of Michigan Trout Streams that Siphloplecton basale inhabits strong currents in medium to large rivers, but I have found the nymphs in good numbers in a silty pool in the slow-flowing headwaters of a small stream.

The Leonards also say the species may emerge through mid-July, which conflicts with other authors and my own observations.

In 30 records from GBIF, adults of this species have mostly been collected during June (50%), March (23%), April (17%), and May (7%).

In 2 records from GBIF, this species has been collected at elevations of 30 and 614 ft.

Species Range

Hatching behavior

Time of day : Afternoon

This species emerges by crawling out onto shore, so the emerging duns are not important.

I once caught a dun drifting along on the surface, but it may have been blown in by the wind.

Spinner behavior

Time of day: Evening

Habitat: Riffles

The females may drop their eggs from high above the water or land and rest of the surface to oviposit. They are a common sight in small numbers for a couple weeks on any given stretch.

The spinners are unusualy elegant gray and black mayflies.

Nymph biology

Current speed: Slow

Substrate: Gravel, weeds

Siphloplecton nymphs are extremely fast swimmers, every bit the equals of Isonychia. They may be mistaken for Siphlonurus nymphs at first glance, but they are larger, slightly thicker, and they have two claws on the front tarsus.

I have not found a reference regarding their environmental tolerance, but in my studio they were much more sensitive to warm water than any other species, dying almost instantly in little photo stage tanks warmed by incandescent lights. That's one of many reasons why I switched to using a flash.

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

Body length: 13-14 mm
Wing length: 12-13 mm

A dark species; cross veins of costal area and disc of the fore wing heavily infuscated; a brown cloud at the base of each wing, and at the bulla of the fore wing. Frons pale; remainder of head blackish brown. Thorax deep black-brown, with a slight ruddy tinge. Fore legs reddish brown; a dark subapical femoral band, black knees, and darker tarsal joinings and tips of tibiae. Femora of middle and hind legs similar but paler; tibiae and tarsi paler, yellowish. A dark cloud at the base of each wing; cross veins of fore wing dark brown, those of the costal margin and the wing disc rather heavily margined; a brown cloud at the bulla, often a faint cloud in the stigmatic area, also at fork of hind branch of the radial sector. Wing of female with no such basal dark clouds. First segment of abdomen wholly dark brown; tergites 7-10 largely dark brown, 7 and 8 with pale antero-lateral angles, 10 pale laterally. Tergites 2-6 semi-hyaline, rather extensively shaded with dark brown on the posterior and lateral margins and the dorsal area; anterior margins, antero-lateral angles and lateral area pale. A curved line of black pencilings on each side of each tergite, and a dark brown oval patch on each side near the median line. Sternites 2-8 largely pale, semi-hyaline, with a reddish brown dot on each side of the median line, on each; sternite 9 dark brown with a pale streak on each side; ganglionic areas dark brown, most prominent on the apical segments. Tails whitish, joinings ruddy brown. Genitalia largely pale reddish brown. Penes as in fig. 115.

Described as S. signatum

Body length 14 mm, wing length 11.5 mm

In this species the wing is very similar to that of S. basale; the body differs from the latter in the much paler thorax and abdomen.

Frons greyish white; remainder of head pale brown. Pronotum dark brown, its posterior margin whitish. Mesonotum yellowish brown; scutellum margined with dark reddish brown; metanotum dark brown. Pleura yellowish, sutures brown. Fore femur light brown; tibia and tarsus yellowish white. Middle and hind legs pale yellowish brown; dark bands apically on all femora; tarsal joinings and tips of tibiae slightly darker. Abdominal tergites 1 and 2 largely brown; tergites 3-6 much paler than in basale, largely whitish and rather opaque. Brown markings on these tergites are reduced to a median triangle and lateral extensions of this, which do not quite attain the anterior margins; posterior margins are narrowly brown, and near the pale pleural fold is a brownish area. At the posterior margin of each tergite, a purplish mark on the median line and a lateral streak on each side of this. Tergites 7-10 largely dark reddish brown; pale antero-lateral areas on tergites 7 and 8 are more extensive than in S. basale. Ventrally wholly pale, except for a minute dark dot on the median line, at the anterior margin, and indistinct tan bars running forward from the posterior margin, parallel to the median line. Tails pale, joinings reddish brown. Genitalia light brown; penes as in fig. 115.


The gills of the nymph are double on segments 1-3. On the venter of the abdomen are three uninterrupted longitudinal brown lines, one median and two lateral. The claws of the second and third legs are almost as long as the tarsi.

Specimens of the Mayfly Species Siphloplecton basale

1 Male Dun
1 Male Spinner
7 Nymphs

Discussions of Siphloplecton basale

S. basale on Clarks Creek, PA
1 replies
Posted by Wiconisco37 on Feb 4, 2009
Last reply on Jun 10, 2009 by Martinlf
they were discovered on Clarks Creek by a man from halifax, PA
i dont know what their numbers were like thgough

Start a Discussion of Siphloplecton basale


Mayfly Species Siphloplecton basale (Pseudo-Gray Drakes)

Species Range
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