By Watershed Conservation Research and Data Specialist Anna Willig
Aquatic macroinvertebrates — animals without a backbone that are visible to the naked eye — are some of the most fascinating creatures found in streams. We tend to lump all macroinvertebrates into one category, but there is tremendous diversity found underwater.
Some macroinvertebrates live for a few weeks, others for a few years. Some stay burrowed in the streambed their entire lives, others emerge as flying adults and migrate across the country. Some scrape algae off of rocks, others are voracious hunters. To illustrate this diversity, we are highlighting three sample orders from the class Insecta (the insects) that have vastly different life histories. While there are generalities that are shared between all species of each order, this is akin to generalizing the life history of all passerine birds (order Passeriformes, which includes most songbirds).
Odonata Spotlight | The Dragonflies
First up is the order Odonata, which includes dragonflies and damselflies. Dragonflies and damselflies are similar, but we focus on dragonflies, which are unique among aquatic macroinvertebrates for their large size, long lifespan, and hunting prowess. Dragonflies start their lives as eggs laid near the surface of the water. Eggs hatch within a few weeks, but some dragonflies can delay hatching for a year if they sense stream conditions are suboptimal. Once they hatch, they typically spend less than a year underwater in the larval (juvenile) stage, though some species spend up to six years developing in streams. Larval dragonflies are fearsome hunters that eat anything they can catch, including other insects, small fish, and tadpoles.
In their final days as larvae, dragonflies stop eating! During their metamorphosis from larvae to adult, their mouthparts change and, for a few days, are not functional. They crawl onto land at the end of their metamorphosis and climb out of their larval skin, emerging as adults. Once their wings dry and harden, adults take to the air, where they are as ferocious as they are underwater. They have remarkable vision and can predict where their prey will go, a feat not seen in other insects. Males establish territories over water, keeping other males out and luring females in.
Adult dragonflies live for several months to a year, and a few species, including the Common Green Darner (Anax junius), migrate across the country. Much remains unknown about the migration of Common Green Darners, but, like Monarch Butterflies, a migratory cycle likely lasts a few generations. The first generation emerges in the late winter in southern states and flies north to mate and lay eggs. The second generation hatches in late spring and migrates in late summer, making their way south to a place they have never been. They mate and lay eggs in the south, and the final generation emerges in the fall. This generation overwinters in the south, mating and laying eggs for the next migratory generation.
Ephemeroptera Spotlight | The Mayflies
Our next order is Ephemeroptera, the mayflies. Like dragonflies, mayflies have an aquatic larval stage and a terrestrial adult stage, but they share few other similarities. Mayflies are the only insect with an extra phase of metamorphosis called the subimago life stage. They are short-lived as adults and, in ideal conditions, can form massive swarms. Mayfly eggs often hatch within a few weeks, though some species hatch immediately and others delay hatching for a year in suboptimal stream conditions. Mayflies spend up to two years as aquatic larvae, shedding their skin dozens of times before adulthood. While most larval mayflies eat algae, leaves, and other plant matter, a few are predators (though not nearly as efficient as dragonflies).
At the end of their larval stage, mayflies crawl out the stream and shed their larval skin. However, they are not yet adults. This post-emergent, pre-adult form is a life stage unique to mayflies called a subimago. Subimagos have a hairy covering over their wings and must molt one more time to remove this covering, after which point they are adults. Adults only live for a few hours to days. During this ephemeral time, they have one task: reproduce. They lose the ability to eat during their metamorphosis into adults, forcing them to focus all their energy on finding a mate instead of finding food. Males form massive swarms, sometimes large enough to be detectable by weather radar, twirling and dancing to attract females. Females fly into the swarm, find a mate, and speed off to the nearest body of water, where their final act is laying eggs.
Coleoptera Spotlight | The Water Beetles
Another unique order is Coleoptera, the beetles, which is the largest order in the animal kingdom, containing an estimated 25% of all animal species. Unlike Odonata and Ephemeroptera, which include exclusively insects with an aquatic life stage, only a small fraction of Coleopterans have an aquatic life stage. Common water beetles include water pennies, diving beetles, and riffle beetles. Water beetles are aquatic for their larval and adult stages, only emerging for a few weeks to pupate. They hatch from eggs laid in the water, typically within two weeks, as most cannot delay hatching. Larval water beetles have diverse diets; some scrape algae off of rocks, some shred leaves, and some are predatory. They spend anywhere from a few months to a year in their larval stage and crawl onto the banks to metamorphose.
On the banks, larvae burrow into soft sand and mud to metamorphose, emerging as adults several weeks later. They fly briefly until they find a suitable place to enter the water. Once they re-enter the water, many water beetles never return to the surface. Some even lose their wings. Adults do not have gills and carry an air bubble to breathe, maintaining an equilibrium with the oxygen in the water that constantly replenishes their air. Some adults are short-lived and may not have mouthparts, while others can live for up to three years and have a variety of feeding strategies. Adults mate and lay their eggs underwater, setting the stage for the next generation.
Each of these orders contains multiple families, each family contains multiple genera, and each genus contains multiple species, each with a unique life history. Each stream contains its own community of macroinvertebrates, an unknown diversity hiding just beneath the waters. And these insects are not exclusive to local streams — there are thousands of species of dragonflies, mayflies, and water beetles found on all continents except Antarctica. Since 2018, the Watershed Team has collected annual macroinvertebrate samples from the headwaters of Ridley, Crum, and Darby Creeks. We are busy sorting and identifying our samples and are looking forward to sharing what these unique creatures can tell us about local stream health.
References and Resources
Interested in learning more about macroinvertebrates? Check out macroinvertebrates.org for information about and pictures of many of our local macroinvertebrates.