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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
Blue-winged Olives

Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

Dorsal view of a Holocentropus (Polycentropodidae) Caddisfly Larva from the Yakima River in Washington
This one seems to tentatively key to Holocentropus, although I can't make out the anal spines in Couplet 7 of the Key to Genera of Polycentropodidae Larvae nor the dark bands in Couplet 4 of the Key to Genera of Polycentropodidae Larvae, making me wonder if I went wrong somewhere in keying it out. I don't see where that could have happened, though. It might also be that it's a very immature larva and doesn't possess all the identifying characteristics in the key yet. If Holocentropus is correct, then Holocentropus flavus and Holocentropus interruptus are the two likely possibilities based on range, but I was not able to find a description of their larvae.
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.
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This topic is about the Mayfly Genus Isonychia

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.

Example specimens

williamsport pa

Posts: 1
Jsell925 on Jul 17, 2007July 17th, 2007, 2:07 pm EDT
Penns is one of the few places where a #10 iso will nail em' all year long
Site Editor
"Bear Swamp," PA

Posts: 1681
GONZO on Jul 18, 2007July 18th, 2007, 4:43 am EDT

Welcome to the site! Many find that the Delaware is also a great place to use the slate drake as a searching fly. The rainbows of the Big D seem to be especially fond of a big Iso imitation fished anytime from late spring into the fall. Personally, I'm a big fan of imitating the nymphs.
Northumberland, PA

Posts: 1
Jimphoto on Sep 20, 2007September 20th, 2007, 1:08 pm EDT
I've found an emerger pattern to be successful on Penns. Apparently not all the nymphs crawl out on stones or the bank to hatch.
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Site Editor

Posts: 498
Konchu on Sep 22, 2007September 22nd, 2007, 12:24 pm EDT
Curious. Could it be that some get splashed back into the stream? Just a thought for discussion.
Pleasant Gap, PA

Posts: 1197
Shawnny3 on Sep 23, 2007September 23rd, 2007, 2:25 am EDT
...or could it be that the technical details of our flies don't matter all that much to the fish?

(I say this as much to draw Gonzo out of hiding as to make any serious point.)

Jewelry-Quality Artistic Salmon Flies, by Shawn Davis

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