South of our house, there is a little dead-end road called Springbrook Lane, and near the end of the road is a little pool, about ten feet across and maybe thirty feet long, and only a couple of feet deep. S_ thought it would be interesting to go down there with an insect net, and see what was there. She was right, it was interesting. A single scoop turned up a bunch of tadpoles and insect nymphs, including this one:
This is a dragonfly nymph. I’m not sure what species, but it was less than an inch long so it is probably one of the smaller ones. They all look a lot alike at this stage, and I guess the best way to find out the species is to let them grow up and see what they turn into.
Dragonfly nymphs are fully aquatic, and have gills to get their oxygen. One odd feature is that their gills are inside the abdomen, so they can pull water in and out of an opening in their rear. This helps them to move around, they can expell water fast enough for jet propulsion when they need to move quickly. This next picture is looking at the tip of the abdomen, and I think that the opening to the gill chamber is visible right there at the tip.
These pictures were all taken with the nymph in water, because when it came out of the water the legs were plastered to the side and it didn’t look like much. I wanted to get pictures of the underside, and was having trouble coming up with a good method to photograph it from underneath while it was in water. Then I remembered that, in the lab, we have these disposable cuvettes for the spectrophotometer. These are water-tight plastic chambers with two optically-flat sides, so if I put an aquatic insect into them, I can then photograph from the side (which I expect to be very helpful when I finally get around to scoring some mosquito larvae). In this case, the nymph was long enough that it had to orient up and down in the cuvette, so it was easy to get shots of the underside:
In that last picture you can see another interesting feature of dragonfly nymphs – the extendable lip. Adult dragonflies are voracious carnivores, and the nymphs are, too. But, it is hard work swimming through water to catch prey, so the nymphs have come up with an easier way. That structure under the head that looks like an extended chin is actually a greatly enlarged, articulated lip that the nymph can shoot out, as shown in this sketch (which comes from here):
And here is a closer view of the actual lip:
The lip has claspers on the end, that can grab onto the prey item and drag it back into the dragonfly nymph’s waiting maw. I think you can see the claspers on this picture:
The claspers fold out kind of like jacknife blades, and have a serrated edge for holding onto the prey item. As for what the dragonfly nymphs eat, that depends on how big they are. Newly-hached nymphs will go after things like mosquito larvae, and as they grow they work their way larger and larger. The bigger species of dragonflies will even tackle small fish.
When they mature, the nymphs will crawl out of the water (sometimes just up a reed stem, other times completely up on shore ambling across the beach) until they find a good spot. Then they crack open their backs, and the dragonfly emerges. This is one of the distinctions between nymphs and larvae: a nymph molts straight to the adult with no intermediate pupal stage, while a larva becomes a pupa first.
The smaller dragonfly nymphs are very effective at clearing out mosquito larvae, by the way: when S_ scooped with the net, one thing we did not find was mosquito wigglers. Dragonfly nymphs are also the subject of one of my favorite insects-as-food recipes. The recipe, in its entirety, is:
(a) Boil dragonfly nymphs.
(b) Eat them.
 This was actually a different nymph which was found, dead, in Coles Creek last year. It was badly decomposed and moldy, so most of it wasn’t worth looking at, but the lip structure was still in OK shape.
 From Unmentionable Cuisine, by Calvin Schwabe. Great book, great book.