Sunday, July 30, 2017

But was it really Drunella tuberculata?

I've been working my way through this new book on species ID of mayflies, caddisflies, and stoneflies -- Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species (Clemson University Press) -- excited to see that we might be able to ID a number of things to the level of species, things that we've been stuck at genus ID up until now.  But when I came to the section on Drunella spiny crawlers (pp. 86-87) I was taken back by the illustrations I saw.  Specifically, the illustrations of the heads of D. tuberculata and D. allegheniensis.  Here they are, tuberculata on the left, allegheniensis on the right.

Problem.  The heads of the nymphs that I've been calling D. tuberculata -- including the nymph at the top of the page --look nothing like that illustration.  Rather, they seem to match D. allegheniensis. The heads on my nymphs have long tubercles like those on allegheniensis (not the tiny bumps of tuberculata), a dark, longitudinal band behind the lateral ocelli, and what appear to be two rows of setae at the front of the head, not simply one.

Better post a new entry, I thought, showing the error I made and correctly keying this out to D. allegheniensis.  And, I decided I'd use both the key in the new book and Steve Beaty's description in "The Ephemeroptera of North Carolina," p. 47-48.   To my surprise I found out that I had been right all along!

Let's start with our new key (Larvae of the Southeastern USA, p. 86).

139 Pair of submedian spines present on abdominal terga and well developed on at least terga 5-7; head roughened with pair of spines or tubercles between eyes above ocelli.....140
139' Pair of submedian spines absent on abdominal terga; head relatively smooth between eyes and above ocelli, not roughened as above...142

We've already seen that our nymph has tubercles on the head, it also has submedian spines on terga 5-7.

So on to 140.

140  Moderately long, somewhat coarse setae densely rowed at distal margin of clypeus....141
140' Clypeus not as above.... Drunella walkeri

Well, our nymph appears to have two rows of setae a the clypeal margin, but it clearly isn't D. walkeri.  Walkeri looks like this.

On to 141.

141  Posterior margins of abdominal terga 8 and 9 with long hairlike setae protruding dorsally...claws with few denticles situated basally....Drunella tuberculata
141' Posterior margins of abdominal terga 8 and 9 without long hairlike setae protruding dorsally...claws with denticles along most of length....Drunella allegheniensis

The moment of truth.  As it turns out there are long hairlike setae on the posterior margins of abdominal terga 8 and 9 -- very long on 8.

And the denticles on the tarsal claws are just at the base, not along the full length of the claw.

I'll be damned.  If we ignore that illustration and rely on the key, we have to conclude that our nymph is D. tuberculata.  But does our ID hold up when we move on to Beaty's description?  Let's take a look.

tuberculata -- nymphs 7-9 mm; head with long occipital tubercles not divergent apically; frons with dark transverse band interrupted medially, if present; small, blunt posteromedial tubercle on mesothorax; abdomen with long paired dorsal tubercles on segments 5-7; ... segments 8-9 with long hair absent medially; dorsal surface brown to dark brown, segments 8 or 8-9 with extensive pale areas; ventral surface pale. ... Uncommon.

1. Our nymph was 8 mm long.

2. We've already seen that the tubercles on the head are very long, and they seem to converge apically, not diverge.  (Note: on allegheniensis they do diverge, as in the illustration at the top of the page.)

3.  There is a dark tranverse band on the frons, but it seems to be interrupted.

4. The mesothoracic tubercle is both small and blunt.

5. We've already seen the long tubercles on terga 5-7, and the long hair on 8 and 9 is indeed absent medially.

6. Dorsal surface "brown to dark brown" -- not so much on this nymph.  But certainly true of previous tuberculatas I've found, especially those that were fully mature (which is what counts in making ID's).

7. And the ventral surface is pale.  (The dark dots are unmentioned by Beaty, but they also show up in Don Chandler's photo [].)

Drunella tuberculata for sure.

All in all a very instructive exercise.  Lesson learned?  As always, rely on the keys and descriptions: illustrations can be helpful, they can also mislead.

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