All the species of Isonychia are similar in appearance and behavior
They may be abundant on one river, and be absent from a neighboring river.[1]
In Pennsylvania mountain Freestones look for isonychia in early to mid-June, the Catskills in mid-June through early July and the Midwest and Appalachians from late June through mid-July.
This is an evening hatch occurring in light numbers. It is important because of the hatches duration and the size of the flies.
The second peak usually comes in September, and those flies are more likely to emerge at midday and be smaller than their Summer brethren. They are offspring of the previous autumn. During the Spring emergence they appear as half grown nymphs.[1] [2]
A few Isonychia may be found in light numbers between the Spring and Autumn emergences.
NYMPHS
Nymphs feed on plankton and sometimes other aquatic insects. They swim easily through medium to fast currents over a gravel and boulder bottom.
Isonychia are tolerant of pollution and marginal temperatures
Point to remember..
Fish them with fast twitches.
DUNS
Depending on the weather or location isonychia may emerge on the surface in riffles or crawl out of slower water.
SPINNERS
The duns molt into spinners wihin a few days of emergence. They swarm twenty to thirty feet above the riffles where females jettison their eggs then join the males, in death, on the surface film.
The swarm provides more action than the emergence.
NOTES FOR THE TIER [2]
Dun average body Length-13-17mm
Standard Equivalent, Standard Hook-8, 10
Tails, 2
Wings, Dark Gray
Underbody, Medium Gray Brown
Top of Body, Med. Gray Brown (Spinners are reddish Brown)
Legs, Cream Front Legs Dark, Light colored feet
For some wonderful isonychia patterns please go to Don Bastian’s web site.
Notes
[1] http://www.troutnut.com/hatch/649/Mayfly-Isonychia-bicolor-Mahogany-Dun
[2] Don Douple, March 2003