(The flatheaded mayfly in the photo above was found this morning in the Rapidan River. The genus is Maccaffertium; I don't know the species ID.)
The most common mayfly we find in our streams -- virtually all of our streams -- is the flatheaded mayfly (family: Heptageniidae), and it can be found in our streams almost any time of the year -- though the species will vary, of course. And, the most common genus of flatheaded mayfly we find in our streams is Maccaffertium (formerly Stenonema). (In 2004, all Stenonema species, save one, became Maccaffertium species, the exception is Stenonema fuscum.)
There are numerous Mac species that inhabit our streams. The North Carolina Division of Water Quality assigns tolerance values to no less than 13. Six of these species are important to fly fishermen:
M. vicarium (March Brown); M. pulchellum (Light Cahill); M. ithaca (Light Cahill and Gray Fox); M. modestum (Cream Cahill); M. mexicanum (Cream Cahill); and M. mediopunctatum (Cream Cahill).
Establishing genus ID for Maccaffertium nymphs isn't a problem -- in fact, if you know what you're looking for, you don't even need magnification. The key? Look at the gills. Those on abdominal segments 1-6 are "truncated," i.e. they're short and flat, not rounded or pointed. And, the gill on segment 7 is a thin filament that angles out from the body.
Since flatheaded mayflies are fairly large when mature (up to 20mm), and since the dorsal patterns seem to differ in significant ways, I thought species ID would also be easy -- it isn't! Before I explain the Mac "problem," let me first post just a few of the Mac photos that I've taken over the spring and summer.
The flatheaded mayfly in this last photo is a Maccaffertium vicarium nymph (March Brown), and the photo was taken on March 28th at Powells Creek. M. vicarium is one of the few Maccaffertium species that can be identified using visible patterns and features. There are dark bands on the posterior margins of the abdominal tergum (ventral side of the nymph), and there are "posterolateral spines" present on segments 3-9 (most Mac species only have spines on segments 7-9). (See, for example, Knopp and Cormier, Mayflies, p. 177.) Here's a ventral view of our M. vicarium nymph.
But for most other Mac species, dorsal and ventral "patterns" are too variable to be helpful. And to explain what has to be done to safely ID the other M. species, I have to take apart the head of a Maccaffertium nymph.
When the nymph is flipped over and we look at the head, what we see is called the "labium," the lower lip. This whole "lip" can be "picked" off of the head and examined.
The labium parts, from outside in, are the labial palps, the paraglossae, and the glossae. What part do they play in mastication? I'm not really sure. The key players in "eating" are beneath the labium, and having removed it, here's what we see.
Underneath the labium are the maxillary palps and the maxillae. A "maxilla" is a supplementary jaw. Note that each maxilla looks like a meat cleaver, though the leading edge looks a lot like a comb. Behind the maxillae and the palps are the mandibles -- the jaws -- which play the key role in "eating". A mandible looks like this.
The mandibles are not our concern -- it's the maxillae on which we need to focus attention. Specifically, we have to look at the top of each maxilla, the "maxillary crown".
I wish my photo were better -- but we're looking at something really small. Here's the thing. There are
"hairs" (setae) and "spines" on the maxillary crown, and the key to species ID is to know how many of each there are on that crown!
I don't have the equipment I need to make the determination that's needed. The maxilla needs to be "slide-mounted," with high magnification, so those hairs and spines can be counted.
So, at the moment I'm at an impasse, and I have a "big Mac" problem. Do I -- as an amateur who has nothing to gain (but knowledge, hmm, is anything worth more?!) -- buy another microscope so I can determine Maccaffertium species ID? Or, do I accept my limitations and move on? Have to think about that one.
I turn to two things when I'm working on insect anatomy.
1) R.E. Snodgrass, Principles of Insect Morphology. This was first published in 1935 by McGraw-Hill. The edition I'm using was published in 1993 by Cornell University Press. My understanding is that this is the "classic" work on the subject.
2) Also very helpful to me when working on mayflies, is an article that's available on the internet --
Herman T. Spieth, "The Phylogeny of Some Mayfly Genera," Journal of the New York Entomological Society, Vol. XLI, September, 1933, pp. 327-391. (Go to: http://www.famu.org/mayfly/pubs/pub_s/pubspiethh1933p327.pdf). See especially the plates on pp. 379-381.