The "D" in PMD

I was explaining fly fishing terms to my sister and she asked an interesting question - does the "D" in PMD stand for the adult mayfly or the color? I immediately said adult mayfly, but she said if that is the case then the terms "PMD nymph" or "PMD emerger" didn't make sense. Then I started thinking that I have seen the term "PMD adult", which would be redundant.

Just another thing to confuse me. Does anybody know the true origin of the term "PMD"?

Joe

Joe,
I have always taken the "D" to refer to the stage of insect, but do recognize the redundancy and, actually, contradiction; i.e., PMD nymph as you mention.

This is something explained on a great site: Troutnut.com:

Duns

: Mayflies have two adult stages. They first emerge from the water as duns (scientifically known as the subimago stage). They then molt into the spinner (imago) stage, in which they mate and die. Sometimes the word "dun" is confusingly used to refer to a brownish gray color in fly tying materials.

Here is my take. The term “dun” is used to describe the subimago stage of Mayflies because of their drab color. It has been used in the common names of many Mayfly species, regardless of whether they are dun in color. I think that’s probably because the subimago/dun is the most familiar stage to most people. When used in a common name it usually gets attached to all stages of the species, hence PMD nymph, PMD spinner etc.

"Dun" is this case part of the common name of the insect, as it is in some other (evening dun, blue dun , etc). The word is clearly overloaded in flyfishing; one of the British publication pointed out the phrase "dun dun dun" actually make sense as the sub-imago of an insect called the dun (color) dun (part of the name).

She then said the "B" in BWO must refer to their mood and not the color, either. This is what happens when you have a sister who is a lawyer.

Joe

Here it means "Pale Morning Dun". Though I don't know if it is the same insect (or insects) your side of the pond.
Dun doesn't refer to the colour in this usage. In the UK if you say "Mayfly" you are talking about one of the Ephemerella family. Not any up winged fly, as is the case in the US. Other up-wing flies are refereed to generically by anglers as "Duns". Hence the name PMD.
Cheers,
A.