Previously, we’ve talked about the connections between snail anatomy and water quality [https://www.savethesound.org/2025/06/16/snail-anatomy-101-and-pollution-tracking/]. Next up on the topic of freshwater bodies, we’re sharing something stranger, tucked under rocks and crawling along streambeds. Enter—the EPT, Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies).
These freshwater macroinvertebrates— small, spineless aquatic animals visible to the naked eye—are the unsung heroes of streams and rivers. They not only keep ecosystems running smoothly but also tell us a lot about the health of our waterways. Their high sensitivity to pollutants makes them some of the best indicators of water quality.
Disclaimer: You may be tempted to shrink back from their spindly faces and unusual looks, reminiscent of both worms and roaches. But resist if you can because these insect larvae are simply baby bugs, thriving in their “ugly duckling” stage, while bringing huge, beautifying benefits to stream and river ecosystem health. Without further ado…
Meet the EPT: Indicators of River Health
Some of the most sensitive and informative residents of freshwater streams are the EPT taxa:
Ephemeroptera (mayflies)
Plecoptera (stoneflies)
Trichoptera (caddisflies)
These aquatic insects spend most of their lives as nymphs or larvae in streams and rivers before emerging as winged adults, during an event called the “hatch.” Because of their sensitivity to changes in dissolved oxygen, temperature, sedimentation, and pollutants, their presence (or absence) offers an insightful snapshot of water quality. Analyzing the macroinvertebrates in a body of water can help recognize signs of ecosystem health.
The EPT are prevalent in aquatic habitats and are frequently employed in biological monitoring, or “biomonitoring,” which studies organisms and their populations to assess the overall status of the health of an environment. The EPT are prime subjects in biomonitoring due to their limited mobility and sensitivities to chemical pollution and physical change. If something is changing the status quo of stream conditions, they let us know.
Reacting to minor changes may come off as dramatic, but we wouldn’t call them that; rather they are good communicators who express themselves clearly. A stream rich in EPT species with diverse populations typically signals clean, well-oxygenated water and a healthy, functioning ecosystem. Conversely, low EPT diversity often points to pollution, habitat degradation, or other environmental stressors.
Since stream bugs are affected by multiple natural factors such as temperature and streamflow, their absence or presence does not guarantee that the water quality is either poor or healthy. For a more comprehensive picture of the status, this data should be considered along with other indicators of water quality characteristics like pH, temperature, turbidity, dissolved oxygen, and nutrients.
Ecosystem Roles and Services of EPT Macroinvertebrates
Beyond their use as bioindicators, EPT macroinvertebrates are also important for habitat health. Though tiny, they play outsized roles in freshwater ecosystems:
- Nutrient Cycling: By feeding on leaf litter, algae, and detritus (organic debris), EPT species help break down organic material, accelerating nutrient cycling (releasing nutrients back into the ecosystem), and keeping the food web moving. Shredders, like stoneflies and some caddisflies, feed on larger pieces including leaves and woody debris. Grazers (or scrapers), like mayflies and some caddisflies, feed on detritus, algae, and aquatic plants.
- Energy Transfer: As prey for fish, birds, amphibians, and other wildlife, these insects are critical links in aquatic and terrestrial food webs. EPT species are a critical food source — transferring energy up the food chain from plants and detritus to larger predators.
- Water Quality Maintenance: Some species filter particles from the water, while others help decompose organic matter, improving water clarity and quality. Some caddisfly larvae are filter feeders, removing fine particles from the water column, improving water clarity.
How to Identify EPT Stream Bugs
You don’t need to be a professional entomologist to start recognizing these important insects. There’s a new online tool for beginners, experts, and everyone in between that makes it easy to identify stream insects in your local waterways including the EPT.
The site acts as a digital field guide for freshwater bugs living in the streams, rivers, and lakes of the eastern U.S., featuring detailed, zoomable images of 150 common species — from mayflies and dragonflies to beetles, snails, and mussels. Developed by Carnegie Mellon University along with partners like the Stroud Water Research Center, the guide is free and open to anyone to encourage more people to get involved in protecting freshwater ecosystems.
Protecting Our Streams by Listening to the Bugs
Monitoring macroinvertebrate populations — especially EPT taxa — gives us an early warning system for environmental change and a means to track the success of restoration efforts. Because EPT insects are sensitive to environmental stressors, monitoring their populations can provide early clues to changes in water quality before larger effects ripple through the ecosystem.
In the Long Island Sound watershed region, where rivers like the Connecticut, Housatonic, Quinnipiac, and Nissequogue, and Bronx flow from upland forests to estuarine waters, these insights are invaluable. Healthy rivers sustain not just aquatic life but human communities, recreation, clean drinking water, and resilience against climate change.
By paying attention to these tiny residents, we can protect freshwater ecosystems and the communities — human and wildlife — that depend on them.
Whether you’re a scientist, a student, or someone who loves wading in creeks, EPT insects are worth knowing. Although a little unsightly, these stream bugs make a beautiful impact on our habitats, and we offer them our admiration. After all, healthy streams start small — with clean water, intact habitats, and bugs that thrive.
Written by Betsy Painter, Science Writer and Communications Consultant for Save the Sound.