We've already talked about midges and black flies, the two members of the order Diptera with which we're all quite familiar. Of the other families in this order -- I have not done a lot with them: my main interest in stream work from the very beginning has been the study mayflies, stoneflies, and caddisflies. Still, I do have close-up photos of most of these insects and can indicate the main features we use for family identification. With that in mind...
1. The insect pictured above is a "mosquito larva" (family: Culicidae). This is the only one that I've ever seen. I found this last summer in Ballinger Creek when the water was low, when the riffles had essentially turned into pools. Key features for identification are 1) the brushy hairs in front of the head (the "moustache"?) on either side of the mouth, and 2) the fact that the three thoracic segments are fused to form a large square that is wider than the abdominal segments.
2. This is a "dance fly" (family: Empididae). These larvae are very, very small and hard to ID in the field. They have a tolerance value of "6". The key for identification? There are prolegs present on abdominal segments 1-8 (sometimes just 2-8). Those on segment 8 are longer than the rest.
3. This is an "aquatic snipe fly" (family: Athericidae). It's the only one I've ever seen, and it is not from our watershed. I found this last summer in a little stream named Back Creek near Lyndhurst, VA. Using Voshell (A Guide to Common Freshwater Invertebrates of North America, pp. 184-185), you might think that the dance fly and the aquatic snipe fly could be confused. But note that the tail ends are quite different, and when I picked this one up, I thought I had mistakenly picked up a small crane fly. The tolerance value is "2".
4. This is a "horse fly larva" (family: Tabanidae): tolerance value of "6". If you look closely, you'll see that the first seven abdominal segments are encircled by pairs of welts. There are no prolegs at the end of the abdomen.
5. This is an "aquatic moth" (family: Pyralidae). It belongs in the order Lepidoptera; it is not a Diptera. I hope the reader won't mind if I fit it in here. Note that, as with most other insects, there are pairs of legs on each of the three thoracic segments (head is pointing to the right). There are also pairs of prolegs on abdominal segments 3-6; they look like crochet hooks with magnification.
6. Finally, one that everyone knows -- the "crane fly larva" (family: Tipulidae). This has a tolerance value of "3". As ugly as they are as larvae, they turn into quite lovely terrestrial insects: sort of like Buddha-nature: a lotus rising out of the mud! The larva pictured above is genus Tipula: most of the time, this is the genus we see. But we do, on occasion, see two other genera -- Antocha and Hexatoma.
This is the genus Antocha, and note the "creeping welts" on abdominal segments 2-7. I regret to say that I have no photo to show you of genus Hexatoma. It's a cool looking larva. It has a "bulb" on the end that looks like an onion or red beet. Neither Antocha nor Hexatoma is commonly found in our streams.