Friday, August 31, 2012

Drunella coloradensis: Making the Case

The nymph in the photo above -- spiny crawler, genus Drunella -- was featured in the entry I posted on August 18, "A Montana Sampler."  I found this nymph in Grant Creek, a small, freestone stream that flows through Missoula and is easily accessed from the motel where we were staying.  This is not the first time I have found this Drunella species -- the three nymphs in the following photos were found in the same stream, Grant Creek,  on August 20th last year (2011).

Obviously, this is a nymph that comes in various shades and colors.  In my entry of August 18, I noted that the species in question is either Drunella coloradensis or Drunella flavilinea: it's difficult to make a decision since they are described with the very same words in "Fly Fishing Entomology: Pacific Northwest Mayflies."  "Abdomen dark olive to brownish black; thorax a shade lighter; no head projections; pointed abdominal tubercles, tiny compared to grandis or spinifera [two other species]; femurs with bumps; gills all similar in size; dark tail band mid-length."

I'm not really sure which part of the abdomen we're supposed to compare with which part of the thorax, but that point aside, everything else fits.  There are no projections on the head -- as we would see, for example on Drunella cornutella.  These:

The abdominal tubercles are indeed pointed (more on this point in a moment); there are bumps/tubercles on the leading edge of the femora; the gills are the same size; and yes, the tail is banded.

Still, the issue remains which species did I find?  Are the nymphs in the photos above D. coloradensis or D. flavilinea?  This is something I've been wanting to know since the "flav" hatch (Western Green Drake, or Small Western Green Drake) is one of the major hatches out West: some fly fishermen regard it as the mayfly hatch of the year.  D. coloradensis also hatches as the Western/Small Western Green Drake, but it normally hatches later than the flavs, in some ways extending the Green Drake season.

So, what do we have?  Our nymphs are Drunella coloradensis, which we can verify in a number of ways.
One of the relevant factors is the timing of the hatch: the D. flavilinea hatch typically peaks in June and July; D. coloradensis adults emerge in August and September.  The nymphs that I've found are fairly mature; all have been found in August, by which time most of the "flavs" should be gone.

But let's go into greater detail.  And let me begin by noting what Knopp and Cormier (Mayflies: An Angler's Study of Trout Water Ephemeroptera, pp. 238-241) have to say about both of these species.

Drunella flavilinea:  "The small western green drake's emergence immediately follows the hatches of its sister species the western green drakes (Drunella doddsi and D. grandis) to extend the green drake season for several more weeks.  The timing of the duns' emergence is temperature-sensitive and is encouraged by steady water temperatures in the 55 to 57º F (13 to 14º C) range.  Depending upon local conditions, hatches may commence any time from mid-June to the middle of July." ... "The stocky D. flavilinea nymphs display a preference to inhabit sections of rivers with moderate to fast currents, in which the nymphs cling to the tops and undersides of riverbed rocks.  Mature nymphs measure from 7 to 11 mm and display body colors in shades of brown and brownish olive.  Distinctive black banding is easily visible on the three creamish yellow tails, and gills are present on abdominal segments three through seven.  Typical of the genus Drunella, the nymph's wide fore femora display serrations along the anterior margins.  Detailed inspection under magnification also reveals the presence of posterolateral spines and rounded tubercles on the posterior margin of each tergite. (underlined for emphasis)"  Rounded tubercles -- keep that in mind.

Drunella coloradensis: "This western species, also known as the small western green drake, illustrates the confusion that may occur in the naming of mayflies by descriptive rather than exact scientific names.  Drunella coloradensis is confined to high-altitude moderately fast flowing streams where the water temperature never exceeds 60º F (16º C).  The duns' emergence is a late-afternoon occurrence through August and September and may reinforce the flavs where the two mayflies coexist.  Drunella coloradensis nymphs are separated from those of the species Drunella flavilinea by the presence of sharp, rather than rounded, abdominal tubercles." (underlined for emphasis)  Sharp, rather than rounded...tubercles.

The emergence dates given for each of these species lends support for the D. coloradensis ID.  And Grant Creek is, in a way, a "high altitude" stream: it flows into Missoula (elevation: 3209 ft.) from Mt. Sentinel (elevation 5158 ft.)  But, the key question becomes -- are the abdominal tubercles "rounded" or "sharp."  Let's have a close look at the abdomen of the nymph in the photo at the top of the page.  (Click on the photo to enlarge it.)

  I'd say they're pointed and sharp: you can't say they're rounded and blunt.  (For an illustration of the "rounded" tubercles of D. flavilinea nymphs, see Knopp and Cormier, p. 239.)

Still, "sharp" and "rounded" are relative terms, and it would be best to have more data to go on.  And we do.  For this additional information, I am indebted to Roger Rohrbeck who this morning sent me the following key from the book Aquatic Insects of California (Robert L. Usinger, ed., 1973).

"Thumb" at distal end of fore tibia long and sharp; abdominal spines moderate; widespread, montane ... coloradensis
"Thumb" at distal end of fore tibia short and blunt; abdominal spines very short; widespread ... flavilinea.

The distal ends of the fore tibiae are unquestionably "long and sharp" on every one of the nymphs pictured above.

That solves it for me: the Drunella nymphs that I find each year in Grant Creek in Missoula are Drunella coloradensis.  This is a big nymph, by the way, those that I've collected are 12-14 mm long.

Below, a photo of Grant Creek behind Ruby's Inn and Convention Center in Missoula, MT -- 8/14/12.

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