Tuesday, April 10, 2012
"Plauditus dubius" Small Minnow Mayflies (male and female): Making the Case
The nymph in the photo above is a female Plauditus dubius small minnow mayfly (Doyles River, 4/9).
The nymph in the photo below -- clinging to the back of an E. vitreus flatheaded mayfly -- is a male Plauditus dubius small minnow mayfly (Buck Mt. Creek, 4/5).
Let me see if I can convince you.
While we have seen the small minnow mayfly genera Acentrella, Baetis, and Heterocloeon, this is the first time we've encountered a Plauditus nymph. For the genus ID, let's turn to Steven Beaty ("The Ephemeroptera of North Carolina," p. 11).
"Genus Diagnosis: Segment 3 of labial palpi subquadrate, with medial margin almost straight (not receding from base); tibia and tarsi without row of long hairs; no hind wing pads; two caudal filaments." He adds, "Found mostly in the Mountains with some Piedmont records."
That both of these nymphs have two caudal filaments (tails) is clear from a look at the photos. For the absence of long hair on the tibia and tarsi -- here is a close-up of one of the arms of the female.
No hair visible on the tibia or tarsus, only on the femora. (Remember that dense, long setae on the femorae, tibiae and tarsi is a key feature of Acentrella nymphs.)
But the critical feature that we have to see is the shape of the third segment of the labial palps. I hope you can see it in the following photo -- this is a very small insect with a very small head! Note that the inner edge (medial margin) of the labial palp is indeed virtually straight.
(I hope you'll trust me when I say that this nymph had "no hind wing pads" -- I forgot to take a photo.)
So, we are looking at Plauditus nymphs, but how do we know they are Plauditus dubius nymphs? Back to Beaty's descriptions (still on p. 11).
"P. dubius -- nymphs 3.0 - 3.5 mm; segment 9 with dark lateral dashes; caudal filaments with narrow dark median band; two color patterns: male - enlarged medial spot on tergite 2 with 5-7 often dark; paired submedian spots on 3, 4, 8, 9 (spots on 5 obscured); sternites with lateral tracheation and segments 3, 5-7 often tinged with reddish brown; female -- median spots on tergites 2 and 6, paired submedian spots on tergites 3-5, 7-9; tergites 5-7 may be slightly darker than other tergites which are generally pale overall."
We can clearly see the "narrow dark median band" on the caudal filaments on both of our nymphs. For the dark lateral dashes on segment 9 look at the photo below (this is of the female).
Sure enough. Now let's look at the color pattern on the abdominal tergites of the male.
If you enlarge the photo -- click on it -- I think you can see everything that Beaty describes. There is an enlarged medial spot on tergite 2; segments 5-7 are dark; there are paired submedian spots on segments 3, 4, 8, and 9, while the spots on 5 are obscured. I can even show you the lateral tracheation on the sternites -- though this is a view of the ventral side of the female.
What about the colors and patterns that we find on the female?
Again, please enlarge the photo for best results. There are "median spots" on tergites 2 and 6; there are "paired submedian spots" on tergites 3-5 and 7-9; and tergites 5-7 are slightly darker than the other tergites "which are generally pale overall."
Pretty cool. Another species of small minnow mayfly to add to our list and to continue to look for in our better streams. (The tolerance value of the Plauditus dubius group is 2.2.)
(Oh. Donald Chandler has also posted a photo of a female Plauditus dubius: it looks exactly like mine. Go to: http://www.discoverlife.org/mp/20p?see=I_DSC241&res=640.)