Stream Insects Go Through Life on the Edge
Durango Herald, May 9, 2004
Chester Anderson

Stoneflies are abundant along regional rivers. Adults live near the water's edge, while nymphs are aquatic. Source: Forest Secrets (J. Gooderham, E. Tsyrlin)

Unless you are an avid fly fisher or are involved in stream protection or monitoring, your only encounter with stream insects may be driving or rafting through a thick swarm of adults. Mayflies, caddisflies, stoneflies, and other stream insects are important to fly fishers because ninety percent of a trout’s diet consists of aquatic insects. These insects are important to all stream users because their presense (or absence) indicates stream health.

Like all insects, aquatic insects go through a lifecycle that progresses from egg, to larva, to winged adult. The larval or “nymph” stage of an aquatic insect occurs underwater, and depending upon the type of insect, spans a few weeks to several years. Nymphs live mostly on or under rocks in riffles and rapids, spending their time feeding and avoiding the open current, where they can become trout food. Some nymphs feed on algae, others shred and eat leaves, and still others prey on smaller insect nymphs.

Getting to the winged adult stage is a perilous journey for aquatic nymphs. They must reach the surface, break through the water’s surface tension, shed their skin (exoskeleton), unfold their wings, warm up flight muscles, and fly away, all before being eaten by a trout. The various aquatic insects incorporate impressive adaptations to accomplish this “hatch”. Mayflies have a sub-adult “dun” or subimago” stage, with hydrophobic hairs that break the surface tension. Caddisflies capture bubbles to propel them to the surface and use the aerated white water to break the surface film.

As adults, aquatic insects fly or scramble for the trees and bushes, their mission to find a mate. A common approach involves males swarming around the landmarks to which the females fly. Females are then literally ‘picked up’ by the males. Stoneflies have a more romantic courtship. Males drum on hollow sticks or vibrate willow branches. Female stoneflies assess the males’ drumming vitality before revealing their presence with an in-kind response, resulting in a drum session in which they seek each other out.

Because aquatic insects live year round in streams they are excellent indicators of the overall health of a stream. Mayflies stoneflies, and caddisflies show little tolerance for stream disturbance, and laws exist to monitor and protect these and other pollution intolerant aquatic insects. The variety and number of each type of aquatic insect found in a stream reveals the type and degree of pollution in the stream, and this knowledge is a first step towards eliminating sources of pollution. The Animas River is the site of an ongoing intensive aquatic insects study being conducted to assess the effectiveness of the clean up of mine sites in the Silverton area.

Chester Anderson is a Durango Nature Studies volunteer naturalist who owns and operates B.U.G.S. Consulting. To learn more about stream insects contact him at bugsconsult@earthlink.net.