Benthic Invertebrates

The abundance of these aquatic larvae in our streams and rivers is a good thing.

July 3, 2019 | | Notes from the Wild

Turn a stone, almost any stone, in a Headwaters’ stream and you will find larval insects underneath. These include mayfly larvae, caddisfly larvae and the aptly named stonefly larvae. They are often compelling in form and colour. Stonefly larvae, for example, remind me of medieval samurai warriors.

Most of these insects cling fast to the undersides of the stones with hooked claws, at home in even the most turbulent waters. Their flattened physiques allow them to squeeze tightly between rock and stream bottom in an upside-down position. In this dark, oxygen-rich habitat, they feed variously on algae, organic detritus or other invertebrates smaller than themselves.

Two flatheaded mayflies from the Credit River in Alton. Photo by Don Scallen.

Two flatheaded mayflies from the Credit River in Alton. Photo by Don Scallen.

Freeliving caddisfly larva from the Credit River in Alton. Photo by Don Scallen.

Freeliving caddisfly larva from the Credit River in Alton. Photo by Don Scallen.

Common stonefly from the Credit River in Alton. Photo by Don Scallen.

Common stonefly from the Credit River in Alton. Photo by Don Scallen.

And they in turn feed larger creatures, from crayfish to minnows to the quintessential poster species of cool, clean, water – the iconic brook trout. Anglers have long known that these insect larvae are favourite trout prey. Fly fishing artisans model trout lures after them.

The abundance of these aquatic larvae in our streams and rivers is a good thing. It speaks to the health of those waterways. Stoneflies, mayflies and caddisflies are sensitive to pollution and low oxygen levels. Biologists monitor their numbers and their diversity to assess water quality.

The catch-all term for these aquatic larval insects is “benthic invertebrates”. Benthic means bottom dwelling; invertebrate of course, describes any animal without a backbone.

The stonefly, mayfly and caddisfly larvae that don’t become fish food eventually transform into flying adults. Some mayfly species do this all at once, rising in clouds from the water surface. At this stage in their life cycle they become food for birds like swallows, flycatchers and cedar waxwings.

These insects go largely unnoticed, especially in their larval stages. But their low profile belies their tremendous ecological significance.

Flatheaded mayfly from the Credit River in Alton. Photo by Don Scallen.

Flatheaded mayfly from the Credit River in Alton. Photo by Don Scallen.

Common stonefly from the Credit River in Alton. Photo by Don Scallen.

Common stonefly from the Credit River in Alton. Photo by Don Scallen.

Brushlegged mayfly from the Credit River in Alton. Photo by Don Scallen.

Brushlegged mayfly from the Credit River in Alton. Photo by Don Scallen.

About the Author More by Don Scallen

Don Scallen enjoys sharing his love of nature through his writing and presentations. Check out his blog "Notes from the Wild".

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