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A Closer Look at Chironomids

By Emma Brentjens, NCRN I&M Science Communications Intern

Multiple worm-like chironomidae of different sizes and colors.

Chironomidae (midges). Stroud Water Research Center

Key Takeaways

Chironomids make up more than half of the macroinvertebrates identified in NCR streams
They’re a diverse insect family that thrive in a wide range of stream conditions
Chironomids are a staple of fish diets, especially in their pupal phase
In their adult flying insect phase they don’t bite or sting, even though they look similar to mosquitoes

Good Things in Small Packages

A diverse wildlife community lives in streams of the National Capital Region (NCR). While fish, amphibians, and other aquatic animals are easiest to see, none would survive without macroinvertebrates—organisms that do not have a backbone and are large enough to see with the naked eye. Macroinvertebrates, including chironomids, are a critical food source of aquatic ecosystems and are key to understanding stream health.

Two people sit on rocks in a stream and monitor macroinvertebrates. One person holds a bucket while the other writes on a clipboard.

Macroinvertebrate monitoring in Catoctin Mountain Park. NPS

The National Capital Region Inventory & Monitoring Network (NCRN I&M) monitors macroinvertebrate communities in 37 streams across ten parks. Nearly all the organisms in these communities are insects, and chironomids are the most commonly identified group. Also known as non-biting midges, the various life stages of chironomids have at least a dozen different common names including “bloodworms,” “midges,” “lake flies,” and “chizzywinks.”

The Biology of Chironomids

Say: “Kai-rah-no-mids”

A circular diagram shows the life cycle of chironomids. They begin at the water surface as egg masses. They then enter their instar larval stage at the bottom of the streambed. Next they move through the water as pupae. Finally, they become flying adults.

Chironomid life cycle. Porinchu, D.F. and MacDonald, G.M., 2003. The use and application of freshwater midges (Chironomidae: Insecta: Diptera) in geographical research. Progress in Physical Geography, 27(3), p. 381.

Larva: After hatching from eggs deposited in water, chironomids spend their early larval life underwater. Many types of chironomids can produce their own hemoglobin which allows them to survive in low oxygen (hypoxic) conditions. This gives them a pink or red color and explains the common name “bloodworms.”

On the left, a red, worm-like chironomid larva. On the right, several small tubes protruding from the stream floor.

Chironomid larva and chironomid larval sediment tubes. Left: B. Schoenmakers. Right: J.K. Lindsey

These larvae live at the bottom of streams and encase themselves in sediment tubes which provide camouflage, protect against pollution, and allow them to filter feed. They release nutrients like phosphorus by cycling water through their sediment tubes and keep their habitat clean by eating algae and detritus.

Flyfishing fly with a puffy white and gray head and a red and black striped body.

Fishing fly resembling a chironomid pupa. Justin Hanson

Pupa: After a few weeks, the larvae grow into pupae. At this stage, adult features begin to emerge, like wings and eyes.

Pupae then swim up to the surface where the chironomids emerge from their exoskeleton as adult flies. Many fishing flies resemble chironomid pupae as this is the stage when they are most likely to be eaten by game fish, like trout.

Adult: As adults, chironomids are tiny insects less than three millimeters long that fly in large, dense swarms. Although they resemble mosquitos, they do not have a mouth and cannot eat, bite, or transmit diseases. The adults only stay alive long enough to mate (usually three to five days). Males form swarms to be spotted more easily by females. Females lay their eggs in the water, and the whole cycle starts again.

Chironomid Biodiversity

Chironomids are common, making up more than half of the macroinvertebrates observed in NCR streams since 2008.

They are part of a very biodiverse family of freshwater organisms, with an estimated 2,000 species in North America and 10,000 worldwide. In the NCR, scientists mostly identify macroinvertebrates to the genus level. (A genus is a classification that contains multiple species.) In the NCR, 60 chironomid genera (plural of genus) have been identified in stream biota monitoring since 2008. The chironomid genera found in the NCR range from highly tolerant (e.g., Cricotopus, Orthocladius) to sensitive (e.g., Parakiefferiella).

Fun Fact!

Parakiefferiella, the most common sensitive chironomid in the NCR, was found almost exclusively in Flowing Springs Run at Harpers Ferry National Historical Park between 2019 and 2022

Research shows that chironomids tend to prefer cleaner rural streams over those surrounded by more densely developed urban land. Additionally, some species tolerate certain pollutants more than others.

Dr. Susan Gresens, an aquatic ecologist and taxonomist at Towson University, observed that some species of Cricotopus can inhabit streams with high heavy metal concentrations, but are not especially tolerant of high salinity streams. Other chironomids, including a species of Eukiefferiella (another common genus in the NCR), were found in waters with high conductivity, but not high heavy metals.

Gresens studied different tolerance levels between chironomid species by collecting pupal skins, which have more identifying characteristics than larvae. This practice of using chironomids as water quality indicators is widely used in Europe. In the U.S., non-chironomids like mayflies (Ephemeroptera), stoneflies (Plecoptera), and caddisflies (Trichoptera) are typically looked to as indicators of good water quality due to their sensitivity to pollution and low oxygen levels.

NPS scientists use a benthic macroinvertebrate index score created by the Maryland Biological Stream Survey to assess the health of NCR streams based on macroinvertebrate populations. The BIBI, or Benthic Index of Biotic Integrity, uses primarily aquatic insect biodiversity data—including chironomid data—in several different ways. For streams in the eastern piedmont region, the higher the percent of chironomids found, the poorer the score for the stream. This reflects the wide tolerance range of chironomids that leans toward more tolerant organisms. However, scoring for coastal plain region streams, looks at the sensitive side of chironomids. There, the higher the number of macroinvertebrates in the order Diptera (true flies)—a group that includes chironomids—the better a stream scores. Also, the higher the percent of sensitive chironomid taxa Tanytarsini, the better a stream scores.

So as it turns out, chironomids in all their shapes and forms, are key to a more comprehensive understanding of stream health in our parks. Learn more about macroinvertebrate monitoring in NCR by visiting the NCRN stream biota monitoring webpage.

A person sits in a stream and rinses a sample over a bucket.

Macroinvertebrate monitoring in Turkey Run at George Washington Memorial Parkway. NPS

Learn More about the National Park Service's Inventory & Monitoring Efforts

To help protect natural resources ranging from bird populations to forest health to water quality, National Park Service scientists perform ecological Inventory & Monitoring (I&M) work in parks across the country. The National Capital Region Network, Inventory & Monitoring program (NCRN I&M) serves national parks in the greater Washington, DC area. Visit the NCRN stream biota monitoring webpage to learn more.

Where this belongs

Antietam National Battlefield
Catoctin Mountain Park
Chesapeake & Ohio Canal National Historical Park
George Washington Memorial Parkway
Harpers Ferry National Historical Park
Manassas National Battlefield Park

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