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Mayfly Species Siphloplecton basale (Pseudo-Gray Drake)

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

Regions: East, Midwest, West

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.

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 (
The tarsus of this Isonychia bicolor mayfly spinner is highlighted in red.
The tarsus of this Isonychia bicolor mayfly spinner is highlighted in red.
Tarsus: The often multi-segmented outer leg section of an insect, which attached to the tibia.

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.

Recent Discussions of Siphloplecton basale

S. basale on Clarks Creek, PA 1 Reply »
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

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