Monday, March 14, 2011
Megaloptera: Hellgrammites and Alderflies
When I started this blog, I decided I would not post an entry on Hellgrammites -- since everyone knows what they look like. But, with almost 6 inches of rain last week, our streams have been slow to recede, and here I sit with time on my hands. So.....
Hellgrammites (family: Corydalidae) and Alderflies (family: Sialidae) are the only two members of the order Megaloptera. I think there's a very good chance that if you tell someone you're lifting rocks in a stream looking for aquatic insects, they'll assume you're finding Hellgrammites -- or possibly Crayfish (which, of course, are not really insects). Hellgrammites are insects; they have three pairs of legs, and what we see in the stream are the larvae -- which hatch in the summer into quite interesting looking terrestrial adults (for some good photos of the adults, go to: http://bugguide.net/node/view/3609/bgimage (multiple pages of photos). After mating, the female afixes her eggs to the bottoms of bridges or trees overhanging a stream: when they hatch, the young larvae simply drop into the water.
If you've ever picked one up, you know they have nasty "pincers" (jaws), and they will happily bite into your finger! I'd urge caution in handling. And, they can grow to a pretty good size: 2" or more for a mature larva is not at all uncommon. There is probably no need for a detailed description since almost all of us recognize them when they are large. Two photos will help to focus on the key features.
As we can see from this photo, they have "paired lateral filaments" on abdominal segments 1-8 and 10, and
there is a pair of hooks on each of the prolegs at the end of the abdomen (this photo shows only one of the prolegs clearly). This detail is probably not necessary for identifying adults -- but for "baby" hellgys (a little "Bob" slang), the four hooks on the prolegs tell the tale (though we're normally looking at them through a microscope). We see "baby" hellgrammites in samples from July through September, and they can be confused -- when they're on your net -- with common netspinners. Here is a microscope photo of one:
But note the hooks:
Hellgrammites are common in our watershed: they are found in almost every stream sampled by StreamWatch. Normally, the numbers are small (i.e. 1-8 per sample), but large numbers have shown up in samples in Cary's Creek, Long Island Creek, Mechunk Creek (55, 58), and the Lynch River. StreamWatch (and VA DEQ) give Corydalidae (hellgrammites) a tolerance value of "5"; in some parts of the country they're given a value of "0" -- but a lot of this varies with genus.
The other member of the Megaloptera order -- Sialidae ("alderfly") -- has been found in our streams only rarely. In the StreamWatch data that I have reviewed, I see "1" listed in a sample for Cunningham Creek, "1" listed in a sample from Raccoon Creek, and we found 1 last year (year before last?) in a sample I helped with at the Mechunk. (Naturally, it took Rose to figure out what it was!) Sialidae has a tolerance value of "4". Picture:
I don't have a good picture. This is a microscope shot of the one we found at Mechunk Creek. Alderflies also have "pincers," and they can be mistaken for small hellgrammites -- but they don't look quite right. The big difference is that they don't have prolegs with hooks. Instead, abdominal segment ten "has a single long, tapering filament, fringed with fine hairs, that projects to the rear" (Voshell, Freshwater Invertebrates, p. 148).