The World of Aquatic Insects

by Jeff Shearer

Royal Coachman, Pheasant tail, Hare’s Ear nymph, Light Cahill, Black gnat, Adams, Elk hair caddis… An avid fly fisherman would recognize these names as fly patterns that have fooled trout for years. Look in any fly shop and you’ll find a variety of hand-tied flies designed to catch trout, salmon, bass, panfish, and pike. But as diverse in name and shape as hand-tied flies may be, they only scratch the surface when compared to the diversity of real life aquatic insects they imitate.

Take your favorite trout stream in the Black Hills and what will you find… Brook trout, brown trout, maybe rainbow trout, longnose dace, white suckers, and possibly mountain suckers. Look under a few rocks, in a submerged leaf pack, and at the water’s surface in this same stream and you may find a hundred species of aquatic insects. While easily overlooked at first glance, the role aquatic insects play in the ecology of streams is immeasurable.

Just what types of organisms are classified under the group "aquatic insects"? Chances are you will recognize several common names, if not most. Ten orders of insects comprise this group. Almost all species in the orders Ephemeroptera (mayflies), Odonata (dragonflies / damselflies), Plecoptera (stoneflies), Trichoptera (caddisflies), and Megaloptera (alderflies / fishflies) have aquatic larvae. The remaining five orders, Heteroptera (true bugs), Coleoptera (beetles), Diptera (flies / midges), Lepidoptera (caterpillars), and Neuroptera (spongillaflies), are mostly terrestrial. However, these five orders have several species or entire families that have one or more life wwws adapted to aquatic environments. The term aquatic insects is often used synonymously with "aquatic macroinvertebrates," which also include crustaceans (e.g., copepods, crayfish, amphipods).

The life history of aquatic insects can be categorized into three forms of metamorphosis, which may take a few weeks to several years. Mayflies, dragonflies and damselflies, and stoneflies go through incomplete metamorphosis (hemimetabolis) with three wwws: egg, larva, and adult. Caddisflies, alderflies and fishflies, beetles, flies and midges, caterpillars, and spongillaflies go through complete metamorphosis (holometabolis) with an egg, larva, pupa, and adult www. True bugs (e.g., water boatmen, water striders, backswimmers) go through a gradual metamorphosis (paurometabolis) with an egg, nymph, and adult www.

Habitat preference can also differentiate aquatic insects. Select any habitat type (e.g., riffle, pool, run) in a stream and there will be a distinct group of insects adapted for that environment. Many species of stoneflies and caddisflies prefer clean, gravel substrates with swift current, and thus inhabit riffle areas. Certain species of mayflies and midges are well suited for burrowing in soft sediments of beaver dam pools. Water striders inhabit the water’s surface. Backswimmers and water boatmen can be found in the water column in slow moving pools or backwater areas. Even unique habitats do not go unnoticed in the eyes of an aquatic insect. For example, black fly larvae can be found on the vertical rock walls of waterfalls or the face of dam spillways.

But other than providing endless diversity to a stream’s biological community, what role do aquatic insects really play? Insects are a vital link between a stream’s energy sources and higher level consumers (i.e., fish). Energy inputs, whether from the riparian zone (e.g., leaves, sticks, pine needles) or within the stream (e.g., algae, aquatic plants, periphyton), drive stream systems. Aquatic insects aid in the transport of these energy sources up the food chain. Some insects are designed to shred leaves and twigs, others collect and filter fine organic matter, others graze on algae and periphyton, and still others are predators. This processing of energy is not only one of the primary ingredients to producing trophy trout, but also to ensuring a healthy stream system.

The health of a stream system is also directly tied to disturbance, both natural and manmade. The timing and magnitude of natural disturbance (e.g., floods, drought, photoperiod) is critical to maintaining the ecological functions of a stream system. For example, insect productivity may increase by several orders of magnitude following a flood pulse and the associated influx of energy inputs. Increased insect productivity, in turn, results in increased fish growth and abundance. Manmade disturbance, on the other hand, often has a detrimental impact to a stream’s ecological functions. Streams in South Dakota are exposed to a wide variety of potential impacts given the diverse land uses within our state. Sedimentation, water withdrawals, channelization, heavy metals from mine tailings, stormwater discharge, and riparian zone loss are just a sample of potential threats. Whether through direct changes to stream habitat or alterations to energy processing, human-induced impacts ultimately lead to changes in a stream’s aquatic insect community. Then again, this biological response to stream impacts may be the greatest contribution aquatic insects make from the standpoint of natural resource management.

Just like fish, aquatic insects are excellent indicators of stream health. Some species of mayflies are sensitive to heavy metal contamination, many stoneflies can not tolerate siltation or thermal pollution, whereas an overabundance of fly and midge larvae may indicate multiple stressors. Many tools, techniques, and models exist to evaluate changes in an aquatic insect community. Regardless of the methodology, the concept remains the same – aquatic insect communities in healthy streams should differ from aquatic insects communities in impacted streams. Measuring and evaluating these differences has been the focus of many water resource agencies. A recent report by the U.S. Environmental Protection Agency indicated that nearly every state water quality agency in the U.S. uses aquatic insects to some degree in evaluating surface water quality. Given the importance of clean water and healthy streams, aquatic insects certainly play a large role for such small creatures.

So the next time you tie a trout fly on the end of your line, see a dragonfly on a cattail, or a mayfly crawling across a rock, take a moment to reflect on the larger role aquatic insects play in a stream ecosystem. From energy processors to indicators of stream health, aquatic insects are truly diverse in every facet of structure and function. Whether your favorite stream is tucked away in a Black Hills canyon or trickles quietly through the open prairie, aquatic insects are playing an often overlooked, but integral role in that stream’s ecology.

Jeff Shearer is an aquatic ecologist for South Dakota Game, Fish & Parks.

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