Troutnut.com Fly Fishing for Trout Home
User Password
or register.
Scientific name search:

Mayfly Genus Isonychia (Slate Drakes)

Taxonomic Navigation -?-
Species in IsonychiaNumber of SpecimensNumber of Pictures
Isonychia bicolorMahogany Dun19102
Isonychia campestrisSlate Drake00
Isonychia siccaSlate Drake00
Isonychia velmaSlate Drake00

12 species aren't included.
Common Names
Pictures Below
Sporadic hatches are rarely as outstanding as those of Isonychia. On streams with good populations, they are reliably hatching in light numbers, here and there, for most of the evening through most of the mid- to late season.

The spinners, and occasionally the duns, produce more concentrated action, but the real value of the Isonychia hatch is its duration and the size of the flies; large trout become ever watchful for them, even when they aren't emerging.

All the species of Isonychia are similar in appearance and behavior.
  

Where & When


Regions: East, Midwest, West

Preferred Waters: Best in freestone streams

Isonychia is mostly important in the East and Midwest, where all the action is provided by Isonychia bicolor. The former species Isonychia sadleri and Isonychia harperi were recently discovered to be synonyms (Synonym: A former name of a taxon, usually a species. Entomologists frequently discover that two insects originally described as different species are one in the same, and they drop one of the names. The dropped name is said to be a synonym of the remaining name. These changes take a while to trickle into the common knowledge of anglers; for example, Baetis vagans is now a synonym of Baetis tricaudatus.) of bicolor. In the West, Isonychia hatches are not very important, and they may be attributed to several minor species, especially Isonychia velma.

Some people say the Isonychia species are multibrooded (Multibrooded: Producing more than one generation in a single year. Baetis mayflies are a classic example. Insects which produce a single generation with two distinct peaks (like the June and September hatches of Isonychia bicolor mayflies) are not multibrooded, because the fall insects are offspring from the previous fall instead of the current year's spring.), but this is not technically correct, even though their pattern of emergence is similar. In true multibrooded (Multibrooded: Producing more than one generation in a single year. Baetis mayflies are a classic example. Insects which produce a single generation with two distinct peaks (like the June and September hatches of Isonychia bicolor mayflies) are not multibrooded, because the fall insects are offspring from the previous fall instead of the current year's spring.) mayflies like the Baetidae, the flies emerging in midsummer or Fall are the offspring of the earlier hatch from the same year. In Isonychia, the Fall emergers are offspring from the previous Fall. They are present as half-grown nymphs when the first of their generation emerge.

Hatching Behavior


Isonychia duns may emerge on the surface or by crawling out onto shore. This behavior can vary within a single species, and it seems to depend on geographic location and the weather conditions.

Spinner Behavior


Time Of Day: Dusk

Habitat: Riffles
Isonychia duns molt into spinners within a couple days of hatching, and when they return they provide much more concentrated action than the emergence. They mate in swarms twenty to thirty feet in the air, and the females drop their eggs from high above the water before they join the males in falling spent (Spent: The wing position of many aquatic insects when they fall on the water after mating. The wings of both sides lay flat on the water. The word may be used to describe insects with their wings in that position, as well as the position itself.). Some conflicting accounts say the females oviposit by descending to dip the tips of their abdomens into the water over and over.

I experienced a good Isonychia bicolor year on one northern Wisconsin river during which there were many duns for up to a month, but I never saw any spinners despite fishing appropriate riffles dozens of times at dusk. I have not solved this mystery, but there is more to timing these events than has been so far discovered.

Nymph Biology


Diet: Mostly plankton; sometimes other aquatic insects

Current Speed: Medium to Fast

Substrate: Boulders and gravel

Environmental Tolerance: Quite tolerant of pollution and marginal temperature

Isonychia nymphs are among of the fastest-swimming mayflies in the world. They can power their way through fast riffles with ease, and their imitations should be fished with fast twitches.

They are very unusual mayfly nymphs for three reasons:

  • They have tufts of setae (Seta: Little hairs on insects.) on the insides of their forelegs which they hold up in the current as nets to filter out plankon for food.
  • Despite their superb plankton-feeding capabilities, they are also among the few types of predatory mayflies. They may feed on midge and caddis larvae and smaller mayfly nymphs.

  • They are unique among mayflies in that they have extra tuft-shaped gills at the base of their fore legs, a structure normally found in stoneflies.

Pictures of 20 Mayfly Specimens in the Genus Isonychia:

Specimen Page:123
Male Isonychia bicolor (Mahogany Dun) Mayfly SpinnerMale Isonychia bicolor (Mahogany Dun) Mayfly Spinner View 15 PicturesI got several really nice pictures of this spinner. I also collected a female on the same trip.
Collected August 9, 2006 from the West Branch of Owego Creek in New York
Added to Troutnut.com by Troutnut on August 11, 2006
Isonychia bicolor (Mahogany Dun) Mayfly NymphIsonychia bicolor (Mahogany Dun) Mayfly Nymph View 7 PicturesThis Isonychia bicolor nymph from the Catskills displays the prominent white stripe sometimes characteristic of its species. This is the first such specimen I've photographed, because members of the same species in the Upper Midwest have a more subdued stripe (and were once thought to be a different species, Isonychia sadleri). The striking coloration on this eastern nymph is more appealing.
Collected April 19, 2006 from the Beaverkill River in New York
Added to Troutnut.com by Troutnut on April 21, 2006
Specimen Page:123

5 Streamside Pictures of Isonychia Mayflies:

Streamside Photo Page:12
Closeup of some recently emerged Isonychia bicolor nymphs from a small stream.  In this picture: Mayfly Species Isonychia bicolor (Mahogany Dun). From the West Branch of Owego Creek in New York.
Closeup of some recently emerged Isonychia bicolor nymphs from a small stream.

In this picture: Mayfly Species Isonychia bicolor (Mahogany Dun).
Date TakenAug 9, 2006
Date AddedAug 10, 2006
AuthorTroutnut
CameraPENTAX Optio WPi
Several Isonychia bicolor nymphs had recently crawled out onto these rocks to emerge, leaving behind their telltale shucks.  In this picture: Mayfly Species Isonychia bicolor (Mahogany Dun). From the West Branch of Owego Creek in New York.
Several Isonychia bicolor nymphs had recently crawled out onto these rocks to emerge, leaving behind their telltale shucks (
Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Shuck: The shed exoskeleton left over when an insect molts into its next stage or instar. Most often it describes the last nymphal or pupal skin exited during emergence into a winged adult.
)
.

In this picture: Mayfly Species Isonychia bicolor (Mahogany Dun).
Date TakenAug 9, 2006
Date AddedAug 10, 2006
AuthorTroutnut
CameraPENTAX Optio WPi
In this picture: Mayfly Species Isonychia bicolor (Mahogany Dun). From Schoharie Creek in New York.
Date TakenSep 7, 2006
Date AddedOct 4, 2006
AuthorTroutnut
CameraPENTAX Optio WPi
Streamside Photo Page:12

Recent Discussions of Isonychia

Are Isonychia mayflies technically multibrooded? 4 Replies »
Posted by Troutnut on Jul 22, 2006
Last reply on Apr 17, 2009 by GONZO
Here's what I've written in my article on Isonychia about their hatching:

Some Isonychia species are multibrooded, but not in the same way as most other multibrooded mayflies like the Baetidae. In those species, the flies emerging in midsummer or Fall are the offspring of the earlier hatch from the same year. In Isonychia, the Fall emergers are offspring from the previous Fall. They are present as half-grown nymphs when the first of their generation emerge. Although Isonychia broods have distinct peaks, some may be found on the water at any time in between.

I'm curious if they can really be called multibrooded or not, since they don't produce more than one generation per year (as far as I know). They simply have distinct populations within the same generation which emerge at different times during the year. Does that count?

All my books are packed up in boxes right now so I don't have a technical definition of the term handy.
ReplyPenns Creek Slate Draker's 4 Replies »
Posted by Jsell925 on Jul 17, 2007
Last reply on Sep 23, 2007 by Shawnny3
Penns is one of the few places where a #10 iso will nail em' all year long
ReplyIso 1 Reply »
Posted by JMV on Sep 21, 2006 in the species Isonychia bicolor
Last reply on Sep 21, 2006 by Troutnut
Great site, I'm an Iso. fanatic... JM
Reply
There are 2 more topics.

Your Thoughts On Isonychia:

You must log in at the top of the page to post. If you haven't registered yet, it's this easy:

Username:          Email:

Password:    Confirm Password:

I am at least 13 years old and agree to the rules.
Top 10 Fly Hatches
Top Gift Shop Designs
Top Insect Specimens
Miscellaneous Sites