caddisflies

by Jeff Shearer


An example of a caddisfly case constructed of plant material. The caddisfly larva in this picture are Agrypnia sp. (family Phryganeidae), often found in pond and wetland environments.

A net-spinning caddisfly, Hydropsyche sp., (family Hydropsychidae) common to many
streams and rivers in South Dakota.

My first experience with caddisflies came years ago as a kid at the edge of a pond. While peering into the water as any inquisitive boy would, I noticed tiny clusters of vegetation moving across the substrate. Further investigation revealed each of those vegetative clusters had an inhabitant - a caddisfly larva. At the time I learned an important lesson; several of those larvae on a hook were irresistible to bluegill. Years later I would learn I made my first observations that day into the world of aquatic insects.

Caddisflies (Order: Trichoptera) are among the most diverse groups of aquatic insects in North America. Over 1,350 species and 22 families are recognized. Larval caddisflies are similar in appearance to caterpillars and adult caddisflies resemble moths. In fact, from an evolutionary standpoint the orders Trichoptera and Lepidoptera (butterflies and moths) are closely related. The high diversity and success of caddisflies is attributed to their ability to produce silk, an evolutionary adaptation that aids many biological and ecological functions. Species diversity is highest in rivers and streams, but caddisflies are found in lakes, ponds, and wetlands as well. The Black Hills streams have the highest diversity of caddisflies within South Dakota.

Caddisflies have a holometabolous life history, meaning they go through complete metamorphosis. A holometabolous life history includes an egg, larval, pupal, and adult stage. Most caddisflies are univoltine, completing a life history cycle in one year. However, bivoltine and semivoltine life cycles do occur. The egg, larval, and pupal stages are completed underwater. Caddisfly larvae usually go through five instars, or development stages. The final instar constructs a pupal case of silk. In most species, the pupal stage lasts two to three weeks. Some caddisfly pupae go through a dormant stage (diapause) up to six months, an especially advantageous trait for those species inhabiting seasonal wetlands that become dry. When mature, the pupa breaks out of its case and emerges as an adult at the water's surface. Most adults live less than one month and are weak fliers, but strong runners. Females deposit their eggs in strands or masses over the water, though some species crawl beneath the water's surface to deposit eggs. Emergence and egg laying are especially vulnerable times for a caddisfly as many fish, especially trout, are quick to take notice of the readily available food source. Fly fishermen are also quick to take notice, matching their fly with the type of caddisfly hatch.

Perhaps the most intriguing feature of caddisflies is their case-building habit. All families, except Rhyacophilidae and Hydrobiosidae, construct cases with some sort of organic or inorganic material held together by silk. The shape and form of these cases vary widely among families but are mostly similar among genera. Most cases are portable (like those moving vegetative clusters I observed as a kid), but some are fixed to rocks or logs. Caddisflies of the family Helicopsychidae construct a spiral case very similar to a snail. Understandably so, early taxonomists misidentified Helicopsychid caddisflies as snails. Every suitable material found within an aquatic environment may be used. Examples include pebbles and sands of various sizes, sticks, grass, leaves, even tiny snail shells. The case-building instincts are so strong that caddisflies can be removed from their natural case, placed in an aquarium with artificial case material (e.g. beads, colored sand, etc), and observed building a case with the material provided. In some Black Hills streams, I have observed Brachycentrid caddisflies using flakes of mica, giving their case a bright sparkle under the water.

From an ecology perspective, caddisflies play a critical role in the trophic function of aquatic systems. Like many other aquatic insects, caddisflies are secondary producers. In this role, caddisflies form an important link between plant and organic material and higher level carnivores, such as fish. Aquatic insects are often categorized into functional feeding groups, depending on how they acquire their food. Generally speaking, a caddisfly larva's feeding group can be determined by its case. Species that use inorganic material for their case, such as sand or small pebbles, are often grazers and collectors, feeding on diatoms and other fine organic material in the water. Caddisflies that use plant material for their case are likely shredders, feeding on coarse organic material, such as leaves and twigs. Net-spinning caddisflies (Family Hydropsychidae) create silk nets to filter food from the stream. Free-living caddisflies, like Rhyacophilidae, are predaceous and feed upon smaller invertebrates. Given their high diversity, it is not surprising that caddisflies have representatives in most feeding groups.

An interesting ecological aspect about caddisflies is the level of resource partitioning displayed by these insects. Resource partitioning simply refers to the allocation of a resource (e.g. food) by closely related organisms to reduce competition. A prime example of resource partitioning is provided by caddisfly species in the family Hydropsychidae. As previously mentioned, Hydropsychid caddisflies construct silk nets (very similar to a spider's web) to filter food from the water. Mesh size of these silk nets varies specifically by species. A general tenant of stream ecology is that food particle size decreases as stream size increase. Where coarse particle material (e.g. leaves, sticks, grasses) are predominant in headwater streams, fine particle material (e.g. plankton, detritus) is much more abundant in larger rivers. In stream systems where Hydropsychid caddisflies are abundant, species can be found "partitioned" along a predictable gradient in accordance to available food particle size. That is, species that have a large mesh size are found near the headwaters where food particle size is the largest. As particle size decreases down the watershed, so too does the mesh size of Hydropsychid species.

Ecological observations aside, caddisflies do have practical value. Many stream and water quality specialists recognize caddisflies, along with mayflies and stoneflies, as valuable indicators of stream health. These three groups of insects are commonly referred to as EPT for Ephemeroptera, Plecoptera, Trichoptera, the orders of these three insect groups. While not as useful in lakes or ponds, the abundance and diversity of EPT provides a strong indication of stream (and watershed) health. Then again it does not take a stream ecologist to know the value of caddisflies. Ask any fly fisherman who has tied on an elk hair caddis and they will testify to the caddisfly's practical value. Whether indicators of stream health, models for ecological study, or fish bait for an inquisitive boy, caddisflies lead an inconspicuous, but important, life.

Jeff Shearer is an Aquatic Ecologist for South Dakota Game, Fish and Parks

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