The Stream Doctor

September 1st, 2003

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

Q. From aacfish: In your last column on FAOL you mentioned that mayflies couldn't inflate their wings in the water column. Why not? I was under the impression that some Heptagenia species actually emerged under water. Is that an erroneous impression? I'm sure I've seen this in fly fishing books somewhere.

A. You asked two questions, but I'm going to answer them in reverse order. First the question as to whether mayflies can inflate their wings in the water column; I'm afraid you've found some misleading information. I spent the morning consulting several books in my library about aquatic insects (one just on mayflies) and found no evidence in the life history descriptions of mayflies that they inflated their wings prior to reaching the water surface. To confirm this, I phoned several aquatic entomologist colleagues to see if any of them knew of any mayflies that inflated their wings underwater. Nobody I talked to knew of any evidence that mayflies inflated their wings underwater; this is done at the water/air interface or on land after they've crawled out of the water.

Some adult female mayflies, especially in the genus Baetis, reenter the water after mating by crawling along rocks or twigs to deposit their eggs directly onto the substrate. If one didn't know that the mayflies had emerged prior to their underwater journey, they might mistakenly infer that they had inflated their wings and were just emerging. Once they deposit their eggs, they do not reemerge into the air; they die and become part of the drift.

If you can find the reference to this happening in a fly fishing book, let me know the book's title; I'd like to see it.

The "Why is that?" question is harder, if not impossible, to answer. It may be related to physiological problems in that they can't circulate blood (haemolymph) into the new wing veins. Further, it would seem that inflating the wings prior to reaching the surface would be disadvantageous because (1) it would present a larger morsel to predators, and (2) it would decrease their ability to resist the current, thus probably extending their time in the drift and susceptibility to drift feeders. This is speculation; they don't inflate their wings under water, but I'm not sure there is an answer to "why not."
~ 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

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