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WiflyfisherAugust 2nd, 2007, 9:13 pm

Posts: 604
While on the river tonight with lots of caddis I saw a few mayfly spinners that I don't believe I have seen before. It is a dark reddish-brown, and 9/16" long body with 2 tails. It looks a lot like Isonychia bicolor that I have caught before but this is definitely smaller in size. although my guess is it is Isonychia bicolor. See photo for a digital shot I took tonight.
John S.
TaxonAugust 2nd, 2007, 9:44 pm
Site Editor
Plano, TX

Posts: 1299

I believe it to be an Isonychia bicolor male imago, probably the one previously classified as I. harperi, which has brown forelegs with whitish tarsi. It is listed in The Biology Of Mayflies as having a body length of 12 mm., which is slight less than 9/16".
Roger Rohrbeck
WiflyfisherAugust 3rd, 2007, 6:16 am

Posts: 604
Roger, thanks for the reply. I am still surprised because of the size. I realize flies may vary in size. But in the past on this Midwest river the Isonychia bicolor spinners I have collected were a good, solid 1" long and I have never seen them in August. Although I know sparse, sporatic hatches can occur later on for most aquatic insects.
John S.
KonchuAugust 3rd, 2007, 7:33 am
Site Editor

Posts: 496
Isonychia probably is correct, but it could a species other than bicolor. However, bicolor has a lot of variation in its appearance.
WiflyfisherAugust 3rd, 2007, 11:23 am

Posts: 604
By the way, what ever happened to Isonychia sadleri? I always thought I. bicolor had the white stripe down the middle of the back of the nymph and I. sadleri did not.
John S.
TroutnutAugust 3rd, 2007, 11:43 am
Bellevue, WA

Posts: 2549
I. bicolor has a wide spectrum of stripey-ness (to use the technical term) in the nymph. I even found one that kept the stripe as a dun in Penn's Creek.

This one is surely Isonychia, and probably it's just a small bicolor. (I would say "definitely," but Konchu's "probably correct" phrasing leads me to think there's some very obscure and improbable family he can't quite rule out. Or maybe he's just displaying a scientist's habitual caution.)

I've caught I. bicolor duns in a variety of sizes. This might be the first one I've seen that's actually as small as all the imitations sold in fly shops.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist
WiflyfisherAugust 3rd, 2007, 1:20 pm

Posts: 604
Jason, you can guess which river I caught it on, probably the same river you photographed yours on too. Just amazes me that it's such a smaller size. Maybe I discovered a NEW sub-species!! We could call it: Isonychia bicolor runta, or Isonychia bicolor namie. :)
John S.
TaxonAugust 3rd, 2007, 2:11 pm
Site Editor
Plano, TX

Posts: 1299
By the way, what ever happened to Isonychia sadleri?


It became one of the many synonyms of Isonychia bicolor. See below, which is cut and pasted (and in my opinion, properly italicized) from Mayfly Central's N. American Master Species List:

Isonychia bicolor (Walker), 1853 [CAN:NE;USA:NE,SE]
    Chirotonetes albomanicatus Needham, 1905 (syn.)
    Isonychia albomanicata (Needham), 1905 (syn.)
    Isonychia christina Traver, 1934 (syn.)
    Isonychia circe Traver, 1934 (syn.)
    Isonychia fattigi Traver, 1934 (syn.)
    Isonychia harperi Traver, 1934 (syn.)
    Isonychia matilda Traver, 1934 (syn.)
    Isonychia pacoleta Traver, 1932 (syn.)
    Isonychia sadleri Traver, 1934 (syn.)
    Palingenia bicolor Walker, 1853 (orig.)
    Siphlurus bicolor (Walker), 1853 (comb.)
Roger Rohrbeck
WiflyfisherAugust 3rd, 2007, 2:50 pm

Posts: 604
Roger, thanks. I saw that earlier today when I did a Google search. I am curious to know how they determine all those species are all the same insect?
John S.
TaxonAugust 3rd, 2007, 3:50 pm
Site Editor
Plano, TX

Posts: 1299

I could probably provide an answer that is adequate for your purposes, but I'm hoping Konchu will step to the plate, and give you something better.
Roger Rohrbeck
KonchuAugust 3rd, 2007, 4:41 pm
Site Editor

Posts: 496
Boris Kondratieff worked on this group in the 1980s. He reported his findings, with coauthor JR Voshell, in the following paper:

Kondratieff BC; Voshell JR Jr. 1984. The North and Central American species of Isonychia (Ephemeroptera: Oligoneuriidae). Transactions of the American Entomological Society 110:129-244.

You'd have to locate a copy at a university that carries that journal.

The old-fashioned way to determine that all the species are the same insect (and still the way most people do it now) is to look at lots and lots of specimens from as many different places and from as many different times as possible. Spinners, duns, nymphs, eggs. The works. Raise the larvae through to the adult stage to see what you get. Compare what you see to museum specimens (especially those that are the 'official' holders of names). Even better yet: get out in the streams and observe who lives where and how each form acts. In other words, try your darndest to find differences. New molecular tools can help sort things out (or murky the water), but often the old, name-bearing, specimens cannot be defaced or destroyed to take tissue necessary for molecular analysis. So you're left with the expert's *opinion*, based on what he or she has seen.

Gotta run. Happy Friday, everyone!
WiflyfisherAugust 4th, 2007, 5:13 am

Posts: 604
Konchu, thank you for elaborating on how you go about trying to determine the various types of aquatic insects. Now for my next question. :-)

However, bicolor has a lot of variation in its appearance

When does variation in appearance dictate that the insect is another specie in the same family/genus rather than calling it the same specie as others?
John S.
KonchuAugust 4th, 2007, 5:53 am
Site Editor

Posts: 496
As with the organisms themselves, this answer can vary.

Long story short, though: if there is not a smooth range of variation and some forms are consistently distinctive in at least one life stage, then perhaps these represent a new species. Sometimes different species are not that different in form, but have big differences in behavior (e.g., some crickets' songs, fireflies' flash pattern, perhaps some aquatic insects' courtship behavior). DNA differences can make strong suggestions about this, too, and the DNA is really useful because it doesn't change between life stages (egg, larva... adult). But we still need to develop our DNA technology further.
MillcreekJuly 25th, 2014, 11:22 am
Healdsburg, CA

Posts: 332
John -

Kondratieff BC; Voshell JR Jr. 1984. The North and Central American species of Isonychia (Ephemeroptera: Oligoneuriidae). Transactions of the American Entomological Society 110:129-244.

You can find this paper, if you haven't already, at this web address:
"If we knew what it was we were doing, it would not be called research, would it?"
-Albert Einstein

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