Tuesday, February 1, 2011
How do we determine "genus" identification? A case study
Pictured above is a Perlodid stonefly, genus Clioperla. But we're going to pretend that we don't know that. We know for sure it's a stonefly: two tails, and no gills on top of, or sticking out from the sides of the abdominal segments. We've also eliminated a lot of the stoneflies, for making family identification, since it's just too big to be some of those critters, and it looks nothing at all like a Roach-like stonefly or a Giant stonefly, etc. So, we've narrowed it down to a "Common stonefly" (family: Perlidae) or a "Perlodid" stonefly (family: Perlodidae).
The question becomes -- does it, or does it not, have what Rose Brown so cleverly calls "hairy armpits"?
I.e. are there tufts of frilly gills behind each set of legs? If there are then it looks like this and it's a Perlid (Common stonefly).
Let's flip our critter over and look.
Nothing. Shaved clean (as it were!). Our nymph is a Perlodid stonefly.
But we want to know more. Can we determine the "genus" -- not just the "family" -- of this particular insect? Genus identification requires the use of a dichotomous key, and the standard text most of us use -- those of us working in this part of the country -- is Barbara L. Peckarsky, et.al., Freshwater Macroinvertebrates of Northeastern North America, Cornell University Press, 1990. Perlodidae genera are keyed out in this book on pp. 69-73.
Where does the key have us begin? It tells us to look at the "Y ridge of the mesosternum" (the "chest" between the first two pairs of legs). Let's take a look.
(Trust me -- that "Y" isn't always this clear!) The question is: do the end tips of the up-facing arms point up? Or do they re-curve and point down? Clearly, they're pointing up. That eliminates one of the genera.
The next thing we're told to do is to look at the "submental" gills. (Submental gills would stick out on either side of the neck.) Does it have them or not? And if it has them are they "at least twice as long as the greatest width"? Well, let's get a better idea of what these gills look like by looking at a different sample from my collection.
That little "finger" sticking out from the neck on the left -- from our point of view -- is a submental gill.
Now let's look at the neck of our nymph.
It looks like it's wearing a "choker" (!), but it does not have submental gills. That eliminates three more genera.
Now the focus becomes the nature of the lacinia (part of the maxilla -- supplementary jaws). In the picture above, the lacinia -- there's one on each side of the head and we're looking at the underside of the head -- is the dark orange point about 2/3's of the way up the head. Let's tear that away so we can see it much better.
We now have two questions that have to be answered: 1) are there any hairs along the top ridge that follow the two large spikes at the top, moving from right to left? (The answer is "Yes.") And 2) is the lacinia tapered and rounded from the second tooth/spike to the base? (Again the answer is "Yes.")
At this point, I will skip over some other details brought up by the key -- but we've already hit on everything that's important. Our answers have taken us to two possibilities: our nymph is either "Isoperla" or "Clioperla". The key sets out the following options:
1) Dorsal abdominal segments with alternating transverse or longitudinal light and dark stripes or bands..... Isoperla
2) Dorsal abdominal segments uniformly brownish except for a few small light spots (which may be in longitudinal rows)..... Clioperla
So, do we have this? ("alternating dark stripes or bands")
Or do we have this? ("uniformly brownish")
We have this. It's a Perlodid nymph, genus Clioperla. One other thing will confirm this. In their detailed study of stonefly genera -- Nymphs of North American Stonefly Genera (Plecoptera) -- Stewart and Stark say of the head of Clioperla "(it is)...mostly yellow with dark bands extending through or behind lateral ocelli to compound eyes and across anterior margin of frons [front of the head], enclosing a large yellow quadrangular area." (p. 381) Here is a close-up view of a Clioperla head.
Could anything be more fun than this? It's hard to imagine! But I should warn you that genus identification is rarely easy, especially when the subject is a Perlodid stonefly. I've worked with Rose Brown of StreamWatch on genus ID when, on occasion, we've read and re-read the options presented to us by the "keys," and still ended up despairing of reaching a final solution. Still, when you can determine genus identification for certain, it is very, very rewarding.