During my internship at the Natural History Museum in Los Angeles last summer, I often found myself spending my lunch hour sitting by the small pond in the Nature Garden.
Upon visiting the Malacology department, Jann, a researcher, revealed to me her collection of exoskeletons shed by a dragonfly nymphs of the pond. I took it upon myself to collect some of my own. They’re actually quite hard to find, but Jann was kind enough to give me one as a gift. I now keep it on my desk in my room, constantly marveling at the perfect retained shape of the nymph the exoskeleton once protected.
Apparently, the exoskeleton of insects does not change in size and shape, thus it does not grow along with the insect. At some point, the exoskeleton which encases the entire insect becomes too much of a tight fit and is molted in order to make way for a larger one. The molting process is catalyzed by the release of hormones; the inner layer of the skeleton is broken down and the nutrients are reabsorbed, while the outer layer is left behind. An additional substance is also produced that allows nymphs to slip out of their exoskeleton with more ease. Yet, it is still astounding how dragonfly nymphs undergo complete metamorphosis and emerge from these small exoskeletons as large, winged adults; surely, the emergence process is not an easy one, especially if the only flaw of the exoskeleton left behind is a relatively small whole from which the nymph emerges.
I thought it would be interesting to take a closer look at the very narrow, tiny spaces in which the smallest limbs of nymph once were.
This is a closer look at the part of the exoskeleton that used to encase the lower part of one of the nymph’s 6-jointed legs and corresponding “toes”, which is relatively puny even under the microscope. I was curious to see how well the shape of the tiniest aspect of nymph anatomy was retained in the exoskeleton structure post-emergence. You can even make out the shape or outline of the area (what seems to be a joint) where the “toes” join with the leg. The darker brown is where the front and back of the exoskeleton join. The detail is really wonderful.
I imagine it takes quite awhile for molting nymphs to successfully emerge; I wonder why insects have evolved to emerge rather than, say, peel off their old exoskeleton. While the exoskeleton surely has its protective advantage, the process seems rather costly in terms of time and vulnerability. It may be possible that evolutionary constraints, such as the limited ability of their leg appendages to be used as a peeling mechanism, limit insects to the process of molting.
I conducted this project as part of Professor Pringle’s EEB321 class at Princeton University.