Monday, April 23, 2012
Drunella tuberculata: Making the Case
On 4/17 I posted this photo -- and the one that follows -- of two genus Drunella spiny crawler nymphs that I found at Buck Mt. Creek.
I noted that although the two nymphs differed in color -- note the banding on the legs of the insect in the first photo -- anatomically, they were exactly the same: both have occipital tubercles, both have a single tubercle on the mesothorax, and both have prominent tubercles on tergites 5-7. On this side view of one of the nymphs, you can clearly see the occipital tubercles and the methathoracic tubercle: we'll see the abdominal tubercles in a later photo.
While I was undecided on 4/17 whether these nymphs were D. allegheniensis or D. tuberculata (citing Beaty's descriptions), I did additional work on this issue over the weekend, and I think I can now prove that both nymphs are Drunella tuberculata. Let me do this first, using the evidence Beaty provides ("The Ephemeroptera of North Carolina," p. 25), then add additional support from an article by Luke M. Jacobus and W. P. McCafferty ("Revisionary contributions to the genus Drunella," pp. 127-147 in the Journal of the New York Entomological Society, Vol. 112, No. 2/3, 2004.) This article is available on line at: http://findarticles.com/p/articles/mi_6920/is_2-3_112/ai_n28247003/?tag=content;col1.)
Using Beaty's descriptions, there are two things we have to decide: 1) is there a "dark transverse band" across the frons (face), and 2) do the occipital tubercles "diverge"? If the answer to both of these questions is "yes," then our nymphs are D. allegheniensis; if the answers are "no," then they're D. tuberculata. Let's look at the faces. First, the "brown" nymph, the one without leg banding.
Nothing I would call a dark transverse band. And here is the face of the nymph with the banding.
No banding. (Note: what appears to be a dark line between the eyes is actually a "seam" that appears darker in the photo than it actually is. In any event, it is posterior to what is normally considered the frons.)
Do the occipital tubercles "diverge" apically. To the contrary, I'd say that they "converge." Have a look.
So, using Beaty, I'd have to go with D. tuberculata. But the features on which Jacobus and McCafferty focus is even more convincing. For me what is critical is what they say about the denticles (teeth) on the tarsal claws and the setae (hair) on the terga.
D. tuberculata: "The claws usually have five or fewer denticles."
D. allegheniensis: "...usually has more claw denticles than sympatric congeners [like D. tuberculata], and in D. allegheniensis, these denticles are situated along most of the length of the claw, rather than basally only."
I can only see three denticles on the claws, and they are "basal only." Picture.
D. tuberculata: "Long, hairlike setae protrude dorsally on abdominal terga 8 and 9."
D. allegheniensis: "Drunella allegheniensis does not have long setae that protrude dorsally from the hind margins of abdominal terga 8 and 9." Have a look at the terga of one of our nymphs.
I think that's pretty convincing. Drunella tuberculata.
Let me note in conclusion that Donald Chandler has posted a photo on Discover Life (http://www.discoverlife.org/mp/20p?see=I_DSC427&res=640) of a Drunella tuberculata nymph, and it looks exactly like these.