Final instar nymph of an aeshnid dragonfly (College Station, TX).
Two years running our first planned field outing for the spring has coincided with one of Texas’s rare dips into below freezing weather. While fishing around in lakes is not the ideal pasttime for this kind of temperature, it is a decent way to get hold of some insects even in cold weather. One group of insects that’s a relatively common find are the aquatic immatures of dragonflies and damselflies. The dragonfly nymphs in particular are striking little creatures, with the earlier instars resembling squat little brown frogs. Like their adult incarnation, they’re also impressive predators, even tackling small fish.
Early and late instar dragonfly nymphs (Libellulidae, Aeshnidae).
Dragonflies lack a pupal stage, so the developing wings (wing pads or wing buds) can be seen in the late immature instars. I hadn’t caught a final instar dragonfly nymph before, so I was suitably excited to see one up close. The larger fellow above even obliged us by molting into a pretty blue darner during the middle of a lecture on dragonfly metamorphosis to a group of introductory entomology students.
*edit* Thanks to Jim Johnson for family corrections and additional information about nymphal characters.
6legs2many 
Lovely, I’ve never properly seen how dragonfly naiads change as they grow. I usually only think of them as big, brutal killers, because I think that’s cool.
Hi! Dragonfly nymphs are really cool, but I haven’t focused on them as much as I’d like (having spent nearly all of my time on the adults). That larger one is actually an aeshnid nymph. That long, torpedo-shaped body is characteristic of that family, and if you look at the labium under the head you’ll see that its shape is very different from the labium on the smaller libellulid (flat vs. spoon-shaped and covering the face).
You can also tell that the aeshnid is a late instar individual because of the length of the wing sheaths. The wing sheaths on the smaller libellulid are much smaller (relative to the head width), so that one has a few more molts to go.
Awesome, thanks!
I went back and checked the adult, and sure enough, aeshnid.
I didn’t know that about the nymphs–I’ve been having a lot of fun taking a closer look at them. Now that you point out the differences they are really distinct. I think I’ve primarily encountered the libellulid nymphs in the past, so I just assumed the aeshnids were later instars. I love those spoon-shaped labiums on the libellulids–they remind me of Darth Vader masks.
Again, thanks for the info! I think I may take some more pictures and make another post.
Have you ever heard of “super-instaration”? A process of re-introducing the hormones of the first instar just as it’s getting ready to molt, back into a second instar individual…which potentially causes the adult to be larger. As far as I know, no-one has reproduced the two foot wing-span of fossilized dragonflies…but it could be a goal. I mean, if you’re a mad-scientist type…or just really curious about internal organs and want them to be larger so you can see them better.
I know there are some interesting experiments (I believe with stored products pests?) where lack of food will cause them to drop back an instar. Theoretically they can go back and forth between instars for quite some time.
Some of the people in our department do some very cool selective breeding experiments for very large (and very small) fruitflies to look at outlier genotypes. I believe the limiting factor for the larger insects (like the two foot long prehistoric dragonflies) is the oxygen concentration in the air today. Insects use diffusion (rather than circulation) to move oxygen through their haemolymph. As the oxygen concentration has gone down over time, the maximum distance that oxygen will effectively diffuse has also decreased (and thus the maximum size of insects).
THat’s interesting, so, if a person puts such larvae in an oxygen rich(er) environment, do they get bigger? I don’t think it’s as simple as that, but if the experiment has been done, I’d like to hear about it
I think it’s more that they gradually evolved smaller maximum size as the oxygen levels decreased and constrained growth.
Higher oxygen levels can affect insect size, however:
http://www.jstor.org/pss/10.1086/322172
Interesting article. I hadn’t read it before. Wouldn’t it seem though, that the increased temperature I’ve always presumed was prevalent at the time the 2′ fossil dragonflies were laid down, would limit the size because of a reduced oxygen solubility… sort of balances out any gain in size or mass? Or maybe the body chemistry was slightly different and augmented the solubility of oxygen in the insect’s blood some way.
The question of how larger insects could come to be, has always fascinated me, even when I was a boy. I used to keep honey bees, until my dad became allergic, and one of my brothers kept praying mantis’…made some money selling them to pet stores, actually. The mantis’ seemed to be able to recognize people, some way and wouldn’t hurt us….though when they were adults were big enough to have drawn blood. And dragonflies; they’re just ‘cool’. I’ve spent some time drifting on a lake in a canoe, and I’ve had them land on me several times. They weren’t aggressive to me…but may not have recognized that I wasn’t just a handy solid object to rest on. I never pushed the issue; they’ve got some sizeable mandibles on them, look like they could take a chomp out of a finger. I’ve read stories of dragonflies on the Amazon river, taking small birds, and I beleive they could. So, whenever one blessed me by landing on an arm or such, I’d lift them up and have a look, but never try to touch them. It seemed quite often that they were looking back at me, by their head movements. Hard to be sure what they were looking at with the eyes they have.
Good memories.