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Amazing Wildlife Sustainability and Conservation

Aquatic Insects: identification, examples, and use as bioindicators


Catching aquatic insects with a net on the shore of Devil’s Lake in Baraboo, WI.

If you flip over a few rocks in the shallow water at any healthy pond or stream, you’re sure to see a few aquatic insects! Many aquatic insects are the larvae or nymphs of well-known flying insects, like dragonflies and mayflies! They begin their lives in the water, then emerge from the surface of the pond or stream as flying adults.

These little creatures play an important role in pond and stream food webs. Aquatic insects help recycle nutrients from the pond’s bottom back into the food web! The juvenile life stages, called either larvae or nymphs, eat algae and waste that settles to the bottom of the water. Many other animals eat aquatic insects. Fish feed on aquatic insect larvae and nymphs living along the pond or stream bed. Some fish will snatch the emerging adult insects from the water’s surface or leap from the water to eat flying adult insects. Birds and amphibians may eat the aquatic insect adults as well.

Aquatic insects can teach us a lot about ecosystems – in other words, they are bioindicators. Some aquatic insects can be very sensitive to water pollution. An unhealthy pond or stream ecosystem cannot support a large and diverse population of aquatic insects. Healthy ecosystems should have lots of different aquatic insect species. Scientists use aquatic insects to check in on pond and stream ecosystem health. They will use nets to scoop up samples of aquatic insects from the water. Then back in a lab, they use microscopes to figure out the different species of aquatic insects that were living there. Usually more different species means a healthier ecosystem! Here are some examples of aquatic insects and a few tips on how to identify them!

Mayflies

Order: Ephemeroptera

Nymph Identification: Aquatic mayfly nymphs have bodies with a distinct head, thorax, and abdomen. They have three pairs of segmented legs, two antennae on their heads, and either two or three long strand-like tails on the ends of their abdomens. There will appear to be two miniature wings on the thorax – these are non-functional “wing pads”. Mayflies and stoneflies look quite similar. Mayflies, however, have claws on the end of each or their legs with a single hook, while stoneflies have claws with two hooks. Of course you might need a magnifying glass or even a microscope to count the number of hooks on these insects’ tiny claws!

About: Mayflies thrive in standing (lentic) and flowing (lotic) water habitats. As nymphs, they live underneath rocks along the bottoms of streams, ponds, and lakes. There are many mayfly species, which feed in different ways: gathering debris, scraping algae from rocks, and even hunting smaller aquatic creatures. After incomplete metamorphosis (no pupa stage) the winged adults emerge from the water’s surface. They molt their exoskeletons once during adulthood. Adulthood lasts, at most, a few days because adults don’t have mouths and cannot feed.

A mayfly nymph, Drunella grandis, or western green drake.

Stoneflies

Order: Plecoptera

Nymph Identification: Aquatic stonefly nymphs look very similar to mayflies (see above), with distinct heads, thoraxes, and abdomens. Like mayflies, they also have wing pads, three pairs of segmented legs and two antennae. However, stoneflies will always have just two long thin tails, and they always have two hooks on the end of each claw.

About: Stonefly nymphs live in clean flowing (lotic) water like streams and rivers. Nymphs live along the streambeds gathering debris and plant matter as food, or in some cases eating smaller aquatic insects. After their incomplete metamorphosis (no pupa stage), the winged adult stoneflies emerge from the water. Most species reach adulthood and emerge between late winter and early summer.

A stonefly nymph, Pteronarcys californica, or giant salmonfly

Caddisflies

Order: Trichoptera

Larvae Identification: Many aquatic caddisfly larvae are easily recognizable by their distinct cases. These larvae construct tiny cases out of little pieces of plant matter or rock which they live inside of, with only their heads, thoraxes, and legs exposed. Hidden within the case, the abdomen of the caddisfly is relatively soft, and ends in two anal claws instead of tails. There are some species of “free-living” caddisfly larvae that do not make cases. In all caddisfly larvae, the classic head, thorax, and abdomen body sections can seem less easily recognizable, compared to mayflies and stoneflies. Caddisflies may have a more maggot-like appearance, especially the free-living caseless ones.

About: Caddisflies live in flowing (lotic) water and sometimes in standing (lentic)water. Caddisfly larvae produce silk. The larvae will use the silk to fasten bits of plant matter or sand together into a case (looks like a shell). Some larvae don’t make shells and instead use silk to tether themselves to streambeds or build tiny nets to catch prey (like a spider’s web). Before adulthood, caddisflies transition through a pupa stage, during a process called complete metamorphosis. Many caddisfly pupas seal themselves in their cases for safety during this stage. The winged adults emerge from the water.

A caddisfly larva in the family brachycendridae

Dragonflies and Damselflies

Order: Odonata

Nymph Identification: One of the best characteristics for identifying dragonfly and damselfly nymphs is their specialized labium, or lower jaw. The labium is designed to extend in front of the body and retract as it grasps prey. Like all insects, members of the family Odonata have a head, thorax, abdomen, and three pairs of segmented legs. You may also notice their wing pads. You can tell damselflies and dragonflies apart by the endings of their abdomens. Damselflies have three long but flat tails on the ends of their abdomens, while dragonflies have no tails, but instead short triangular features, creating a pyramid-like shape on the rear tip of their abdomen.

About: The order Odonata includes dragonflies and damselflies. Nymphs are often found in ponds. Dragonflies are predators of other insects during both nymph and adult life stages. Dragonflies have a modified labium (lower jaw) that rapidly extends and pulls in prey. After incomplete metamorphosis (no pupa phase), the winged dragonfly adults emerge from the water. Female dragonflies have specialized ovipositors on the tips of their tails, an organ that allows them to dip their tails below the surface of the water to safely lay their eggs in aquatic plants.

A dragonfly nymph, Ophiogomphus severus, or pale snaketail

Other Aquatic Insects

Fly larvae – (Order Diptera) There are numerous species of aquatic fly larvae. Horse flies, mosquitoes, black flies, and midges are just a few examples. As larvae, flies have an elongated body which looks almost like a segmented worm. They have three pairs of legs which may be small and hard to see. Aquatic flies undergo a complete metamorphosis (i.e., with pupa stage) before emerging from the water as flying adults.

Water beetles – (Order Coleoptera) Their are several families of beetles that live in water during their larval, pupal, and adult stages. Beetle larvae may look somewhat similar to fly larvae, with long and segmented bodies. Adult water beetles will have a rounded, hardened forewing which covers most of their bodies.

Water striders and water scorpions – (Order Hemiptera) Water striders and water scorpions are two common aquatic insects in the order Hemiptera (“true bugs”). Water striders have elongate bodies and thin, widely spread out legs, which allow them to stay on the surface of the water. Water scorpions have elongate and sometimes flat bodies. Water scorpions use their front pair of legs to capture prey; the front pair of legs are noticeably bulkier than the other legs.

Scientist sits on the shore of a lake while sorting through a pan containing a lake benthic sample, looking for aquatic insects.
Looking through a lake sample for aquatic insects.

Are you interested in aquatic insects or want to teach children about aquatic insects? Check out the printable educational resources featuring my artwork, available in my Etsy shop and on my Teachers Pay Teachers page!


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References and Further Reading

  1. Merritt, R. W., Cummins, K. W., & Berg, M. B. (2008). Aquatic insects of North America (4th edition). Kendall Hunt, Dubuque.
  2. Neuswanger, J. (n.d.). Mayfly species Drunella grandis (Western Green Drake). Troutnut.com. Retrieved from: http://www.troutnut.com/hatch/446/Mayfly-Drunella-grandis-Western-Green-Drake
  3. Ward, J. V., Kondratieff, B. C., & Zuellig, R. E. (2002). Mountain stream insects of Colorado (2nd edition). University Press of Colorado.

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