Monday, January 31, 2011
"Dragons and Damsels": Odonata, Pt. II.
The damsels. There are three damselfly families: "Broad-winged" (family: Calopterygidae), "Spread-winged" (Lestidae), and "Narrow-winged" (Coenagrionidae). I have never seen a "Spread-winged" damselfly(Lestidae), and I'm not convinced they exist in our streams, at least in those parts of the streams that we normally sample. Thus, our only concern in this entry will be the families Calopterygidae and Coenagrionidae.
Damselflies and dragonflies differ markedly in their appearance. However, they inhabit similar water, both preferring the warm, slow waters along the edges of rivers and streams, and streams with vegetation that grows on the bottom. They are also both predaceous in their eating behaviour, dropping and extending their lower lips to pick up their prey (remember the photo of the Cordulegastridae nymph from the previous entry). They are also cannibalistic: they eat their own! I've seen this with my own eyes both in samples brought into the lab, and while sampling out on the streams. But damselflies differ from dragonflies in having what appear to be three visible paddle-shaped tails: in reality, these are "caudal lamellae," and are actually gills used for breathing. (Close-up photo below.)
Calopterygidae (the "Broad-winged" damselfly"):
This is a beautiful nymph: long, slender, and delicate in appearance. I've found very small ones -- less than 1/2" in length -- but I also found one that was close to 2" long; that one is now in the StreamWatch reference collection. While their thin bodies really give them away, it is the antennae that are used for family identification. The first segment of each antenna is thicker and longer than all of the following segments. This can be seen in the photo above, but here is a close-up that focusses in on this feature.
The StreamWatch data available to me suggests that this type of damsel is only found rarely, and most often in the main stem sites of the Rivanna. Nonetheless, I think we're finding increasing numbers of them and not only in the Rivanna: on my own stream excursions I've seen them quite often in Buck Mt. Creek and in the North Fork of the Rivanna. When I find them, they're always in the tangled up vegetation that covers the rocks in the summer.
Coenagrionidae (the "Narrow-winged Damselfly"):
The body and head of the "Narrow-winged damselfly" are obviously wider/larger than those of the "Broad-winged damsel". Also, the segments of the antennae are pretty well equal in length. But the key feature to see for identification is the labium (lower lip). Like the labium of the Darner dragonfly, the labium of the "Narrow-winged damsel" is wide at the top but narrows down at the bottom.
So, these are easy to ID in the field if you flip the nymph onto its back and look at the lip with a loupe.
Narrow-winged damsels are found more frequently in StreamWatch samples than the Broad-winged damselflies. That's a little too bad since their TV is "9," while that of the Broad-winged is "5". That being said, they have been found in Ballinger Creek, Buck Mt. Creek, the Doyles River, Lickinghole Creek, the Lynch River, the Moormans River, Swift Run, the South Fork of the Rivanna, and at all of the sampling sites on the Rivanna itself.
As we all know, we see the adult dragons and damsels throughout the hot months of summer. That means we expect to see nymphs in late spring, all summer, and even into the fall (I found one in November at Buck Mt. Creek.)