Saturday, November 19, 2011

Another Perlodid Genus Shows Up -- and Other Good Things at the Rapidan River

Just when I said that, yes, I am finding some mayflies, but this is pretty much "stonefly season," I probably found more mayflies today -- four different families -- than anything else!  But let me begin with an insect that presents a bit of a challenge.  The Perlodid stonefly in the picture above -- what is the genus?

I found two of these nymphs.  Here is the other.

They were tiny -- as in VERY small -- and dichotomous keys point out right away that you need to use mature insects for proper identification.   So, I'm not 100% sure of my conclusions on this one, but I think the genus might be Yugus.  Let me remind you of what mature Yugus nymphs look like.  I found this nymph in the same spot in the Rapidan River where I was searching today back in March of this year.

So, am I nuts?!  Well, I have two pieces of evidence that I can use for support.   But I repeat, I'm not totally sure of the ID that I'm suggesting.

1) Yugus nymphs have a "labrum with [a] yellow mesal band" (pretty clear in the picture above), and 2) on a Yugus nymph, there is a "marginal lacinial setae row [that extends] from near [the] apical tooth to near [the] base (Peckarsky, Freshwater Macroinvertebrates, p. 71).    So, let's look at microscope shots of the heads, and before we do that, please note the yellow "M" on the head of the nymph in the picture above.

And here's a shot of the lacinia on the larger of the two nymphs, on which the setal row does indeed extend from the spine at the tip at least half way down to the base.

The evidence that I see -- keeping in mind how the colors and patterns change and develop on these nymphs as they grow -- argues in favor of Yugus.   But, for me, this is a tough one.

Let's move on to easier things.  I found another unusual insect today -- a type of crane fly larva that we see only rarely.  Have a look.

And let's have a close look at that rear end.

Now let me show you what happened to that rear end when this bug was preserved.

It turned into a bulb!  This is a crane fly larva (family: Tipulidae), genus Hexatoma.  The bulb gives it away, so too do the "ventral lobes," specifically the long hair on those lobes.  Very interesting find; it's so different than the Tipula that we normally see.

I guess for the rest of the insects I found, I should just stick to pictures.  I found small winter stoneflies -- a lot of them as I had expected -- and large winter stoneflies, some of the biggest ones that I've seen.  There were also a couple of Chloroperlids (green stoneflies), along with Giant stones and Common stones (genus Acroneuria).  But giving the small winter stonefly a run for its money in terms of dominating the sample today was the brushlegged mayfly!  They were all over the place, and they're starting to get big.  Let's begin there.

1. Brushlegged mayfly (Isonychia bicolor)

2. Pronggilled mayflies (genus Paraleptophlebia).  There were a lot of these in the leaf packs as well.

and another

3. One of many small winter stoneflies

4. A Chloroperlid -- and the Chloroperlid next to a Perlodid

5. A few beautiful large winter stoneflies -- and note how the wing pads are starting spread out and away from the body.

6. One of the Giants I found today (there are a lot of them in this stream), here giving the Chloroperlid a ride.

7.  And last but not least, a spiny crawler -- Ephemerella subvaria.  Remember that I found a tiny E. subvaria nymph in this stream on 9/13.

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