Isonychia bicolor
Mahogany Dun/Slate Drake
NYMPH 4
Size: 12- 16 mm
Body: Light to dark reddish brown with distinct whitish dorsal
band over most or all of head and body
Legs: Light reddish brown with darker bands, tufts of hair on
inner margins of forelegs
Tails: Three tails, banded
Note: Only Isonychia bicolor and I. obscura are known to occur in
Massachusetts. I. bicolor nymphs lack the paired dark brown spots
on the underside of each abdominal segment that are present on I.
obscura.
Nymph Look Alikes 4
Owing to the distinct white/yellow stripe running the length of the body, I. bicolor
nymphs are not readily confused with other mayfly nymphs. I. bicolor nymphs can be distinguished from those of I. obscura as noted above.
DUN (Subimago) 4
Size: 12 to 16
Body: Dark red-brown to dark grey
Wings: Dark gray, blackish venation
Tails: Two pale tails
Legs: Front legs dark with cream tips; rear legs cream
SPINNER (Imago) 4
Size: 12 to 16 mm
Body: Dark reddish brown
Wings: Clear wings (hyaline)
Tails: Two grey tails
Legs: Front legs dark brown, rear
legs cream
Dun/Spinner Look Alikes 5
Slate Drake duns and spinners are easily recognizable by the light/white
middle and hind legs and dark fore legs.
HABITAT & DISTRIBUTION 4
Isonychia bicolor are said to be one of the fastest swimming mayflies; and as such are at home moderate- to fast-flowing sections of well oxygenated small to medium-sized rivers. While both I. bicolor and I. obscura are known to occur in Massachusetts, I. bicolor appears to be much more common.
Photo of I. bicolor typical habitat: http://riverscalendar.drupalgardens.com/media-gallery/detail/37/2096
LIFE CYCLE - EMERGENCE 4
The Slate Drake has two primary emergence periods: one in early to mid summer and a second in late summer/early fall. Mature nymphs are known to hatch into adults often by climbing out of the water onto rocks, boulders, and vegetation, where they work to free themselves of their nymphal shucks. This process is said to take as long as five minutes or longer. They are also known to sometimes swim to and hatch on the water’s surface midstream. Stream size and river flow are thought to at least partially determine which of the strategies is employed.
Duns mature to the spinner stage within two days of hatching. Mating flights occur in
the evenings, and after a short rest, the females fly upstream to deposit their eggs.
Once eggs are laid, the females join the males in falling spent on the water’s surface.
This can occur in concentrated fashion on eastern rivers, providing excellent spinner
fishing opportunities at last light.
Massachusetts: Late June through July and again in September, depending on weather and water temperatures.
Degree of Difficulty 6
This table is meant to help users get an idea of how easy/hard it is to ID this species to any taxonomic level, and to give an idea of the characters (or features) that are important to accurately identifying a specimen. As a general guideline, these degree of difficulty levels translate to the type of effort needed to reliably ID a specimen to that taxonomic level, as long as the associated features are confirmed. Of coarse there will/may be exceptions to these general rules.
Isonychia bicolor
Taxonmoic level
Taxonomic Name
Feature
Degree of difficulty
Order
Ephemeroptera
Wing Shape
1
Family
Isonychiidae
Dark forlegs and others light
3
Genus
Isonychia
Dark forlegs and others light
2.5
Species
bicolor
male imagos genetalia needs to be checked under the scope.
4
Degree of Difficulty
Degree of Difficulty
Description
1
Can be ID’d on the wing or water.
2
A specimen in the hand can be ID’d.
3
Can be ID’d by studying a decent photo of the specimen.
4
Can be ID’d by studying a captured & preserved specimen, usually under some magnification.
5
Proper ID requires study of preserved specimen under magnification, usually looking for some feature that can be difficult to confirm. Usually requires training or confirmation from a professional.
Sources and Credits
- (c) Jerry Schoen, all rights reserved, uploaded by Jerry Schoen
- (c) Jerry Schoen, some rights reserved (CC BY-SA), https://www.flickr.com/photos/79718069@N00/14467349083/
- (c) Jerry Schoen, some rights reserved (CC BY-SA), https://www.flickr.com/photos/79718069@N00/20793748588/
- Adapted by Jay Aylward from a work by (c) Jerry Schoen, some rights reserved (CC BY-SA)
- (c) Jerry Schoen, some rights reserved (CC BY-SA)
- (c) Jay Aylward, some rights reserved (CC BY-SA)
