The Stream Doctor

September 15th, 2003

Email YOUR Questions directly to the Stream Doctor. This is your opportunity to get an experts professional opinion on anything stream related.

My recent column on whether or not mayflies emerge under water has generated some interesting responses:

Q. From Alan Shepherd: It would seem that a few rare species of mayfly actually do emerge as a dun underwater. To my way of thinking, if it actually happens, the angler will not know anyhow. The take home information is that duns can be fished underwater as well as on the surface, trout don't study entomology!

Mr. Shepherd submits the following extract from an article by John Claassen published on the New Mexico Trout website:

"An interesting aspect of the emergence is that the metamorphosis from nymph to subimago (dun) takes place entirely underwater rather than in the water-air interface as it does with so many other mayflies. This means that they transform directly from a nymph to a dun while at depth. The nymphal shuck is removed in the process. Either during the transformation or after the transformation, the flathead/clinger (his name for this group of mayflies) normally drifts for a long distance before reaching the surface. This gives the trout plenty of time to feed on them, perhaps without even breaking the surface. This implies that dun patterns can be fished successfully wet as well as dry during an emergence.

To read the entire article, go to:

From Matt Pierce: I thought I read about the bi-colors being the only mayfly to emerge under water. I believe it was in Selective Trout. I think I could have been wrong though. I know they can't inflate their wings but everything is supposed be ready by the time they get to the top. Please let me know what you find out.

From aacfish: (partial text) Thank you for your reply to my question on underwater mayfly emergence. So far, I have not been able to find anything that confirms total underwater wing extension. Check under Epeorus under entomology in

A. To Alan Shepherd: Thanks for your input. It brings up several interesting points. First of all, I do not know John Claassen or his credentials on the subject. Of interest, however, is the fact that there is a genus of stoneflies (Plecoptera) with the name of Claassenia. The name was coined by a man named Wu to honor P.W. Claassen, an early taxonomist. With such an unusual name, it would not be surprising if there was some connection. Anyway, back to the article.

Mr. Claassen's article is entitled "Mayfles in the Heptageniidae Family Live in the Fast Lane." and concerns only the single family. He mentions several genera in this family and then makes the quote shown above concerning metamorphosis from nymph to subimago; it gives the impression that the quote applies to all genera in this family.

To pursue this further, I consulted the book The Mayflies of North and Central America authored by George F. Edmunds, Jr., Steven L. Jensen, and Lewis Berner for information on the life histories of all genera in the family Heptageniidae; there are 13 different genera in the family (depending on which authority you trust). In the life history descriptions, I found the following: For the genus Epeorus - "The nymphs rise to the surface, and the subimagoes immediately escape from the turbulent water. The nymphs have been observed crawling to within an inch of the surface of the water, emerging there from the nymphal skin, and breaking through the surface film with wings fully expanded." For the genus Heptagenia - "When ready to emerge, the nymph rises to surface and the subimago bursts free. Some subimagoes emerge from nymphs attached to rocks beneath the water."

So, what can we deduce from this? Although Mr. Claassen seems to broadly infer that all Heptageniidae are subsurface emergers, it appears that only two genera, Epeorus and Heptagenia, sometimes emerge under water, and that this appears to be the exception rather than the rule. Remember, too, that there are 19 species of Epeorus and 13 species of Heptagenia. Edmunds et al. were presenting generic characteristics and we cannot be sure how many of these 32 species exhibit underwater emergence.

To Matt Pierce: I must admit that I'm not sure what you mean by "bi-colors," nor do I have a copy of Selective Trout. Thus, I hope this column satisfies your interest.

To aacfish: Thanks for the additional information. Whoever wrote the section on Epeorus implies that all species in this genus emerge underwater. Edmonds et al. seem to hedge the bet a bit and, in fact, imply that surface emergence is more likely.

Summary: How does this fit with my earlier column? The experts I talked to (4 universities, 1 state department of natural resources) and four of the books I consulted provided no support for subsurface emergence by mayflies. Only upon digging into the generic details in the Edmunds et al. book, did I find mention of two genera that occasionally emerge underwater. Additional information from the website lends credence, though painted with a broad brush.

I suspect an appropriate, generalized statement as to mayflies emerging underwater might read something like: Mayflies, in general, do not emerge underwater; however, there is some evidence of underwater emergence in two genera of the family Heptageniidae.

My apologies for misleading anyone in my earlier column. Seems like we never stop learning.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at

If you would like to comment on this or any other article please feel free to post your views on the FAOL Bulletin Board!

Previous Stream Doctor Columns

[ HOME ]

[ Search ] [ Contact FAOL ] [ Media Kit ] © Notice