A water strider has pierced this damselfly behind the head in order to suck out the body fluids of the prey.

A water strider has pierced this damselfly behind the head in order to suck out the body fluids of the prey.

Off the Trails: Muskeg ponds

Many of our muskegs are dotted with small ponds that are wonderful microcosms; I like to prowl around them, just to see what is going on there, at least among some of the larger forms of life. The organisms I happen to mention in this essay are not necessarily found only in muskeg ponds, but these ponds are a great place to observe them.

The most obvious signs of life are certain plants: the yellow-flowered pond lily with broad, floating leaves and the white-flowered, three-leaved buckbean. Much less obvious is an insectivorous floating plant called bladderwort. Its frail stems are hard to see, and I don’t find it very often. Its tiny bladders are traps for very small invertebrates; when the hairs at the opening of the trap are disturbed, the trap opens suddenly and sucks in the hapless prey. All of these plants produce flowers that are held above the surface of the water, so that pollination can be effected by insects.

All around the muskeg are the little round-leaf sundews. Some muskeg ponds, usually those with relatively barren shores, also have the long-leaf sundew. Both species trap insects on leaves with sticky hairs and digest them on the spot. And both make small, white flowers that are pollinated by insects (one can hope that these beneficial insects don’t get caught on the leaves, later). In general, insectivorous plants are found in nutrient-poor habitats, where there is a real advantage in supplementing the income with captured prey.

I enjoy watching the water striders skate about on the surface. Adult striders have long, thin bodies and wings, so they can move from pond to pond. Although they have six legs, only the long middle and hind pairs are used in locomotion on the ponds. Most of the propulsion comes from the middle legs, and most of the steering is done by the hind legs. Each foot is covered by small hairs that trap air bubbles, keeping the strider buoyant. The feet make a dimple in the water surface, and on a sunny day you can see the rounded shadows of the foot-dimples on the bottom of the pond, as if the strider were walking on snowshoes.

The striders’ small front legs are used only for grasping prey. These predators feed mainly on insects that fall into or land on the pond, preferring live over dead prey. Prey is detected by surface waves made by struggling insects. Striders pierce the captured insect and suck out the juices, leaving just a husk. Sometimes they are cannibalistic … and eat each other.

I recently went out to look at a few ponds, but by the time I got to the muskeg, a hard rain was pounding down, and no striders were visible. Hmm, are they hiding, out of the deluge? Yes! I stepped too close to a raised pond edge, and out from under that edge scooted a strider, looking very confused and agitated. It darted from one slim sedge stem to another, clinging closely to each one, and finally came to roost between two sedges, where it was almost invisible. I left it in peace, so it could go back into hiding.

With a little patience and luck, you might see some other interesting critters in the water. There are water boatmen, which use the hind legs as oars to propel them through the pond; they can carry an air bubble under the wings and around the abdomen as a private oxygen supply. They feed by piercing small animals or plants and sucking up the fluids.

Recently reported from many places in the Juneau area, and first reported in Alaska by the Bioblitz in Sitka in 2012, are the back-swimmers. It is interesting that experienced and knowledgeable observers did not see them at all until a few years ago, and now they seem to be quite widespread! These insects swim on their backs, as the name indicates; their hind legs are oars, and they can control their buoyancy by carrying and regulating the size of an air bubble under the abdomen. These are predators that grab and hold their prey with the front and middle pairs of legs, then stab it and slurp up the juices. They are said to fly well, moving from pond to pond, but they have to tip themselves right-side-up in order to extract themselves from the water. I am still waiting to see one!

Predaceous diving beetles are carnivores too; adults tear their prey to bits; the larvae grab prey with their large mandibles, inject a digestive fluid that liquefies the contents of the prey’s body, and slurp up the resulting soup. Both adults and larvae breathe air by poking their rear ends up above the water surface and store it in special internal chambers. These carnivores swim with their hind legs (what is it about hind legs?? Are the forward pairs just busy with other things?). Adults can fly well.

Among my favorites are the larvae of dragonflies and damselflies. They have a very cool extendable toothy jaw (it folds up under the body when not in use) that lets them lurk quietly and suddenly reach out to snag a passing prey. Large dragons can even take prey such as little fish and tadpoles. The aquatic larvae use all six legs for crawling around in a pond—with that fantastic jaw, who needs legs for feeding? They get their oxygen from the water and do not breathe air or carry air bubbles. When ready to transform into the airborne flying adult, the mature larva commonly crawls up on a plant stem or the shore, breaks open the back of its exoskeleton, and the soft-bodied adult squeezes out. A very recently emerged dragon or damsel takes a while to harden its new exoskeleton and expand and harden its new wings, and during this time it is a favorite prey of birds. Adult dragons and damsels feed on aerial insect prey.

There are other enchanting insects to be found, such as whirligig beetles that frenetically mill about on the surface in gangs, but I have not seen them recently. They swim with the middle and hind legs, using the front legs to grab prey, which are detected by the surface vibrations they make. At the water surface, the body is half submerged; one set of eyes functions above the water and another set works under water. When these beetles dive, they carry a bubble of air. If conditions in the pond deteriorate, the adults take wing for a better site. They are said to be distasteful to would-be predators.

That’s just a sample of some of the more visible active players in the pond ecosystem. You may notice that carnivory appears to be a standard dietary style.

For more information, check out the book on Aquatic Insects in Alaska by Hudson, Hocker, and Armstrong.

• Mary F. Willson is a retired professor of ecology.

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