Aquatic macroinvertebrates are not widely considered a “charismatic” group of animals. These are larva, nymphs, worms, and mollusks that happily blend in with the detritus of their watery habitats. The general morphology of most of them is “bug.” They’re large enough to see without a microscope, but they’re still pretty tiny—smaller than the scale of animals to which we’re used to giving much thought. As with most life on this planet, however, these bugs are fascinating if you look closely enough. They’re also handy indicators of water quality and are critical to their freshwater ecosystems and beyond. Ahead of our Marsh and Stream Explorers workshops later this month (more on those later, after I’ve sold you on these critters!), it’s my pleasure to introduce you to a few types of aquatic macroinvertebrates living in the streams, ponds, and rivers of Maine.
The builder
Aquatic macroinvertebrates lack a backbone, and some hatch without any built-in hard structure at all. Take for example, the Log Cabin Caddisfly. Who among us knows how to build our own shelter with found materials (and without YouTube tutorials)? Log Cabin Caddisflies do! Their larvae lack exoskeletons, so they fashion hard sleeping-bag-shaped cases out of sticks, sand, and other materials to protect and camouflage their soft bodies. The insects expend considerable energy producing silk to stick components together, and the results are pretty wicked-looking. Some artists have even supplied caddisfly larvae with jewels and precious metals to use in their cases. Somewhere in their lineage, this strategy became more advantageous than using energy to produce a chitinous exoskeleton. It certainly makes excellent camouflage. Caddisfly larvae spend one to two years underwater feeding on plant matter, breaking it down and recycling the nutrients. When they’re ready to pupate, their protective case doubles as a cocoon. They seal it up and emerge in the fall as winged adults, which float to the surface, molt, and then fly around looking for a mate. They might be alive as adults for only a couple of weeks.
The predator
Finding the larva of a darner dragonfly is sort of like seeing a celebrity’s high school yearbook photo. If they look familiar, it may be because dragonfly larvae were one of inspirations for the Xenomorph character from the Alien movie franchise (really!). From the spiky extendable jaws to the segmented abdomen tipped with pincer-like sensory organs, all the things that might seem offputting to us are adaptations that’ve been serving these nymphs quite well for about 300 million years. These critters look very different from us because they live very different lives than we do. For example, in the same way that reaching out and grabbing something is quicker and more accurate than throwing your whole body at a target, those extendable jaws allow dragonflies to effectively ambush prey. The quick strike is accomplished by taking in water through their rectum, then flexing muscles that create pressure in their bodies and push their jaws out. Using pressure to extend an appendage is a pretty simple concept, and it’s made dragonflies fierce predators both in the water and out.
A dragonfly nymph’s segmented exoskeleton might not conform to our human beauty standards, but it’s an incredibly popular adaptation in the natural world that gives hard, protective structures flexibility. Your own hair has an outer layer of overlapping hard cuticle cells that give it strength while allowing it to bend. The exoskeletons of dragonfly larvae have ten abdominal segments, which they’ll retain as adults, but in their classic svelte form. The thin abdomens of adult dragonflies are easier to maneuver when mating and allow them to fly fast. Large dragonflies like adult darners can reach top speeds above 30 mph and experience a gravitation force of 9Gs on a turn (that’s 9 times the force of gravity) and 4Gs in a straight line. For comparison, Artemis II astronauts experienced 3.9Gs on their way back into Earth’s atmosphere. There are approximately twenty species of darner dragonflies in Maine. Their larvae are more sensitive to pollution than some other types of macroinvertebrate, so their presence is an exciting sign of higher water quality.
The marsupial
Aquatic isopods, also called sow bugs, look like your average crawly bug, but they’re a unique member of the aquatic cleanup crew. Sow bugs are actually crustaceans. Their chitinous exoskeletons are strengthened by calcium carbonate just like the shells of their relatives, crabs and lobsters. They’re not particularly good swimmers, so you can find them crawling along a stream bed, munching away on fine sediment and dead animal and plant matter. Like caddisflies, isopods recycle nutrients back into living tissue, which moves along the food chain when they’re eaten—maybe by a dragonfly larva!
Sow bugs have a unique reproduction strategy. Males clasp themselves onto females and may remain there for months. Eventually eggs will be fertilized and then transferred to a brood pouch on the underside of the female’s abdomen. After hatching, they’ll spend about a month inside the brood pouch growing and molting. This strategy might remind you of mammalian marsupials, and in fact, the brood pouch is referred to as a marsupium on both types of animal.
There are 10,000 described species of isopods worldwide, including the roly polys, or “pill bugs,” you might find under your furniture. Five hundred species of isopod are found in freshwater, but they evolved in shallow seas 300 million years ago. This was the time of giant insects, thanks to oxygen levels 10% higher than they are today. There are still enormous species of aquatic isopods, including one, Bathynomus giganteus, that can grow up to fourteen inches long and lives on the ocean floor in the West Atlantic Ocean. Isopods have a pretty consistent “look,” so you’ll usually know when you see one. In freshwater habitats, they’re somewhat tolerant of pollution, making them a reliable indicator that a water body is recovering from the effects of pollution.
Bonus bug!
Water striders are those little insects that appear to walk on the surface of ponds, lakes, and streams. The mindblowing truth is that they are, in fact, walking on water. They accomplish this in two ways. First, the bottoms of their feet are covered in fine, hairlike projections called setae, made from the same material, chitin, as their exoskeletons. These setae are covered in wax and grooved, which causes a thin layer of air to be trapped between the foot and the surface of the water. They’re not actually getting wet, so they’re not stuck to the water. The second ingredient in this seemingly magical ability is surface tension. Water sticks to itself—it’s highly cohesive. This means that the water molecules on the surface are being pulled down by the water molecules under them and out by those next to them, but not up, because it’s only air above the surface. This stretches the surface taut like a skin of plastic wrap, and that tension holds objects on the surface. You can often spot motionless water striders by looking for the six dimples in the water created by each of their feet. Those feet are sensitive to vibration, which helps them find prey, mostly flying insects stuck to the surface of the water. Male water striders also send out vibrations through their forelegs to establish territory and attract mates. Turns out there’s a lot more going on with those little bugs zipping around on the water than it first appears!
Now you really want to go wade into a stream and meet some of these fine critters, right? You can—by becoming a Marsh and Stream Explorer! Since different species can tolerate various levels of pollution, aquatic macroinvertebrate surveys are an effective way to assess water quality and changes in aquatic habitats. Marsh and Stream Explorers started in 2020 as a partnership between Maine Audubon, Maine Department of Environmental Protection (DEP), Lakes Environmental Association, and the Portland Water District to recruit, train, and support volunteers interested in surveying streams in the Sebago Lake Watershed. Since then, we’ve expanded to have volunteers across the state who provide critical data in support of healthy watersheds in Maine. The data that volunteers collect is an invaluable screening tool to help Maine DEP prioritize where more in-depth water quality assessments should be conducted. Maine Audubon is hosting a Marsh and Stream Explorers online training on April 22 from 7-8 pm. Visit our Marsh and Stream Explorers page to learn more about how to get involved and about in-person trainings in your area.
