Saturday, March 19, 2011
Whippoorwill Riches: Spring Insects Galore
I went to the Whippoorwill Branch of the Mechum this morning with a specific purpose in mind:
I wanted photos of common netspinners and fingernet caddis in their homes on the rocks. I was partly successful -- but let me turn to that later on.
The photo above is a nice shot of two flatheaded mayflies (family: Heptageniidae), but two different genera. The nymph on the left is the most common genus we see -- Maccaffertium (formerly Stenonema) -- while the one on the right is genus Stenacron, a genus that I've seen only rarely.
The Stenacron nymphs that I've seen all look like this -- they look slim and trim, especially when seen alongside their Stenonema (Maccaffertium) kin. These two genera are closely related in that they share a common gill structure: gills 1-6 are the same, while gill 7 is thin and fingerlike. But Maccaffertium gills are "truncated," i.e. squared off on the bottom, while the Stenacron gills come to a point. That shows up very clearly in this microscope shot of the Stenacron gills.
I also found a number of Ameletids, which I'd have to call fairly mature. One photo gives a good idea of just how big they've become.
But the "mayfly of the day" -- so to speak -- was the spiny crawler (family: Ephemerellidae). The stream was loaded with them. Most were small; but some were already mature. This is the main mayfly we find in the spring in our streams, so I've been waiting for them to arrive on the scene. Here's a shot of one of the larger ones that I found. This -- well, all of the spinys I found this morning -- is genus Ephemerella.
While this large (actual size, about 1/2") nymph was brown; the small nymphs that I found, by contrast, were more olive in color. I brought a couple home with me to confirm the genus, and I shot this picture of one:
I suspect that the two different colors -- the two photos above -- are actually two different species, but I have no way of knowing for sure.
Stoneflies? Quite a few actually. I found one common stonefly, genus Acroneuria, and 3-4 Perlodids (Diploperlas and small Isoperlas), and I again found two large winter stoneflies that I think are genus Taenionema.
Now -- to the main plan for the day. When you turn over rocks in a stream, it's fairly easy to pick out the homes of netspinners and fingernet caddis if you know what you're going to see. The "common netspinner" (family: Hydropsychidae) makes a "fixed retreat" out of small pieces of rock in which it stays hidden away for protection. Outside its retreat, it spins a net which it props up between some pebbles, in which it catches its food. Then, every so often, the larva emerges from its retreat to clean off the net. Here's a fixed retreat on a rock:
Just looks like a collection of pebbles and grains of sand. Note that in this case the larva is partly exposed (the green); it looks to me like part of his "tunnel" (and these retreats are often built in the shape of a tunnel) has collapsed to the left. When I pulled away even more of its cover -- don't worry, it will build a new home! -- the larva was more fully exposed and started crawling away.
And here's the larva as it looked in my tray, both dorsal and ventral views.
This is genus Cheumatopsyche -- the most common genus we see in our streams -- and it has a tolerance value of "6". But I should note that I also found 3-4 netspinners that were genus Diplectrona, with a tolerance value of "2", in this stream as well: sometimes their pebbly "fixed retreats" were on the same rocks!
Fingernet and Trumpetnet caddisflies do not make retreats out of stones. Rather, they spin nets out of silk in the shapes of fingers and trumpets respectively. These "homes" are not so easily recognized when you pick up a rock, because when you pick up the rock, their nets collapse into slime. The slime looks like its make out of fine grains of sand; it's not, it's made out of silk which the caddis produces.
Here's what you will see -- but this is a "trumpetnet" caddis in its collapsed home: I couldn't find any fingernets.
If you look closely, you can see the larva struggling inside of its goo; it was a small one, cream colored, almost transparent. This spot is, at the most, an inch square.
I tried to get a photo of this larva out at the stream -- but I couldn't get anything good. You'll have to settle for this microscope shot from my lab (which still isn't great).
That's what was crawling around inside of that mess. The defining traits of the trumpetnet caddis (family: Polycentropodidae), by the way, are the gray spots ("muscle scars") on the top of its head, and the sharply pointed fore trochantin.
One final picture to share. There were still a fair number of Uenoid case-makers stuck to the rocks in this stream, but I also saw "groups" lined up in pupation. Here's a good shot of the formation they take (the rock was carefully returned to the water after the photo was taken).