So you look out at the pond. It's freezing outside, maybe there is even ice covering the pond and you wonder what could possibly be alive down there. Well, would you believe that there are still dragonflies, mayflies, beetles, flies and many other bugs, just like in the summer? Well, sort-of like in the summer: you see they are still there, but they are over-wintering in their larval forms, cleverly disguised as such things as nymphs, naiads, and larvae. There may even be frogs and salamanders, etc. In fact, I received e-mail recently from our editor, Pam Ingle, telling me about an experience she had with a frog under the ice. She wrote:
Did I tell you about seeing the bullfrog under the ice? We've had a thin sheet of ice over the pond for a couple of weeks, on and off. I was out removing leaves, pine needles and stuff, and must have startled him, but there he was, swimming to the deep end! We put in two bullfrog tads last spring, but hadn't seen em!
That must have been a quite a surprise! It's certainly a testament
to the hardiness of bullfrogs! And so you see, there IS life under the ice.
In this article, however, we will concentrate on the insect family.
All summer and fall those insects were busy laying their eggs (ovipositing) in your pond. Some of those eggs will wait till spring and warm temperatures to hatch, but most of them hatched during the summer and fall and they are now developing. All insects go through metamorphosis. Those in your pond started out as eggs and have hatched, but they now look nothing at all like their adult forms. Many will go through several different stages and appearances before they are ready to become adults. Dragonflies, for example, often go through 7- 10 instars or stages, and at each stage of development they grow a new exoskeleton. It's like they get to put on a different outfit every few weeks!
These are all Green Darner nymph. They don't look much like that huge green dragonfly you saw last summer. But believe me, by next summer they will or the summer after that! Some dragonfly larva spend more that a year in their aquatic habitat. In fact, in almost all cases, they actually live more of their lives underwater than above! These photos demonstrate the variations between instar stages within a species. At first the green darner nymph is small and brown with a black band, and then a little bigger and just brown, then towards the end its quite green. It is after its green stage that it becomes a dragonfly by crawling out of the water on a plant stem and doing its metamorphous into final adult form.
Perhaps you have found their excuviae (exoskeletons) about your pond. To the left is an example of a Blue-eyed Darner's excuviae on horsetail from our pond, and to the right is a Green Darner's excuviae from our pond which I scanned. The Green Darner excuviae is approximately to scale (2 in). Damselfly excuviae are smaller, daintier and thinner.
Other insects living under that ice in larval form are mayflies, stoneflies, caddis flies, and diving beetles. The National Audubon Society Field Guide to North American Insects & Spiders has great pictures of all of these in a section in front called "Water Nymphs and Larvae". Here are a few sketches I found on the web:
Mayfly larvae (see two below)
Stonefly larvae (next two)
So how do they stay alive out there under the ice? Well, yesterday I called a local entomologist (one who studies insects) who read to me from a book about how the nymphs burrow in the mud and go into a kind of hibernation. They do this by completely changing their metabolism. This occurs thru a change in their hormones and is triggered by the photo period, in other words they are light sensitive. The changes in the length of daylight triggers the hormonal change. This all occurs in the brain, that's where the 'switch' is, and its all passed on from the female to her offspring, so its a maternal trait. Apparently this maternal form of hereditary is why the insect order in general is so successful. It's the females who disperse to lay their eggs, and if she finds habitat that turns out to be suitable for her nymph, then all her female offspring are already programmed to be able to survive there and for their offspring to also! The entomologist also mentioned that some dragonfly instars (stages in the development of the nymphs) are more capable of surviving under freezing conditions than others, so the timing of egg laying and the weather all interact in this struggle for survival.
Other organisms, such as frogs and salamanders do a similar semi-hibernation in the mud when the pond freezes over. Nature in her wisdom has programmed them all to be able to survive even freezing ponds, and as we saw above, she even has even given them the capability to pass on this ability to their offspring. To the casual observer, frogs, salamanders and other larger critters may appear to be the only creatures emerging when your pond begins to thaw. But, in actuality, a myriad of organisms have been under that ice for the long cold winter months. Yes, Virginia, there IS life under that ice!