Caddisfliesby Karl Blankenship


Caddisflies are a tolerant lot. As a rule of thumb, they tend to put up with more pollution than the stoneflies and mayflies that anglers often associate with high-quality streams. It is a tolerance bred from diversity; and, of the three major groups of aquatic insects, none is as numerous as the caddisfly.


Medium dark-olive sedge (Macronema zebratum) photo-David H. Funk

The numbers say it all. Pennsylvania has 134 species of stoneflies and 230 species of mayflies. But it has at least 320 species of caddisflies, according to surveys compiled by Edwin Masteller, a professor emeritus of biology at Penn State, Erie. (Only Alabama has more caddisflies, with 342.) Even that figure probably leaves some species uncounted. "I am sure there are more," Masteller said. "I cannot cover all of the diverse habitat in the state, and if others look, we will find more."

Indeed, between 1991, when he completed the first statewide survey, and 1998, when he updated it, the list grew by nine species. Masteller is encouraging others to join the search: He has created a World Wide Web page where people can review results and report findings. One visitor to the site informed Masteller of a watershed where he collected 63 species of caddisflies over the course of a year, some of which are rare in Pennsylvania.

The survey is not just an academic exercise. "This information could be used in evaluating high-quality watersheds and the need to protect or manage these areas," Masteller said. "It may be useful in determining the streams for fish stocking. Trout fishermen may be interested in the flight periods of certain species to coordinate their fly tying and subsequent fly fishing trips."

Giant red sedge

Giant red sedge (Pycnopsyche scabripennis) photo-David H. Funk

Masteller's work was backed by the Fish and Boat Commission and the Wild Resource Conservation Fund, which finances scientific and educational projects related to Pennsylvania's nongame wildlife. The Commission intends to incorporate information about caddisflies and other aquatic insects into a computerized mapping system that will help identify, and protect, particularly valuable stream systems. With statewide inventories, people in the future will be able to track how aquatic insect populations­which are critical to the aquatic food chain­are faring. Declines of certain species, or a change in the aquatic insect diversity found in a particular stream, could be a signal that something is wrong in the environment.

Though typically considered moderately tolerant of pollution, some caddisfly species are more sensitive than others, making them potential sentinels of water quality, said Andrew Shiels, Fish and Boat Commission Nongame and Endangered Species Unit Leader. "Some caddisfly species, I think, could be used as very good indicators," he said. "Some individual species are in more pristine waters and headwater areas, while others are in areas that receive some silt."

Their ability to survive in many areas reflects their strength in numbers. What's true about caddisfly diversity in Pennsylvania is true throughout the continent. In North America, there are more species of caddisflies­1,369­than of mayflies and stoneflies combined, making them an important insect species in almost every freshwater ecosystem. Worldwide, there are an estimated 10,000 species of caddisflies.

Their great diversity stems from the fact that caddisflies have adapted to many habitat "niches" in waterways. Put another way, some caddisflies tolerate poorer water quality better than many mayfly and stonefly species. As one moves from pristine conditions, as a rule of thumb, the more important caddisflies become.

Because mayflies, stoneflies, and caddisflies often serve as the primary food for many forage and game fish species, the tolerance of caddisflies­also known as sedges, shadflies, or sandflies­can be particularly important for fishermen in many streams. "From that standpoint, in any marginal trout water, you would be more likely to need imitations of caddisflies than mayflies," said Greg Hoover, an entomologist at Penn State. "If we were to rank those orders, stoneflies as an insect order are more sensitive than mayflies are, and third would be caddisflies."

This tolerance to less-than-pristine conditions appears to be something caddisflies gained over time. The oldest, most primitive species of caddisflies are those found in cool, flowing streams. Today, many caddisfly species have adapted to other conditions; some are even found in ponds. One family of caddisfly, the Hydropsychidae, have so many tolerant that some aquatic biologists consider them to be the "white suckers" of the insect world. Still, that doesn't mean all species are tolerant; some are quite picky. In his surveys, Masteller found 51 caddisfly species that could be found in no more than one county.

Because of their diversity, caddisflies also exhibit the greatest range of behavior among the three major aquatic groups. Caddisfly larvae are the engineers of the aquatic world. Larvae of many species build shelters, or cases, out of twigs, stones, bits of leaf, bark, or other materials, binding the parts together with a glue-like substance. They often add to the cases as they grow, and eventually the cases become the shelters in which the larvae transform into pupae. The design of each case is characteristic of a particular genus. Some cases are even portable. Some species weave webs, attaching them to rocks in fast-flowing streams, to catch food as it flows by. And then, some species­typically the most primitive­do neither.

After spending six to 10 months in the larval stage, caddisflies form cocoons. Once transformed into a pupa, the insect cuts its way out of the case­if it built one­and makes its way to the surface of the water, where it breaks out of its pupal exoskeleton and emerges as a winged adult.


The larvae of many species build shelters, or cases, out of twigs, stones,
bits of leaf, bark and other materials.
They bind the parts together with a glue-like substance.
photo-David H. Funk

Adults have four wings of nearly equal length and are covered with fine hairs; in fact, the Greek name for the caddisfly insect order, Trichoptera, means "hairy wings." When at rest, the wings are in an inverted "V" over the back, similar to a pup tent. The adults look like drab moths, and tend to be poor fliers that are attracted to light. Collectors often lure them into traps using black lights.

Like most aquatic insects, the caddisfly adult stage is short, averaging only one to four weeks. Adults mate on streamside foliage, after which the females return to the water to lay their eggs. Some deposit eggs as they skim across the surface. Others dive to the bottom where they attach their eggs to rocks or other submerged objects. Eggs usually hatch within two to four weeks, and the cycle begins again.

Left, Freeliving caddis, Ryacophila sp., at right, Pycnopsyche sp. photos-David H. Funk
 Caddis  Pycnopsyche sp.

Although this cycle has been going on for millions of years, the significance of caddisflies to healthy stream systems has only lately been appreciated, which is why­nearly three centuries after Pennsylvania was settled­aquatic insects are only now being comprehensively surveyed. When comprehensive aquatic insect surveys began a decade ago, caddisflies were chosen first, because of their diversity. "The caddisfly study was the building block to work with invertebrates to determine water quality in the Commonwealth," said Frank Felbaum, Executive Director of the Wild Resource Conservation Fund. "Let's face it, for years, we have not conducted any kind of biological inventory in the state's rivers on species other than fishes."

Biologists have begun to consider the presence and diversity of many different types of organisms to further our knowledge of aquatic systems. Knowledge of the aquatic insects, molluscs, crustaceans, and fishes in a waterway helps us understand habitat quality and its relationship to species diversity.

Dr. Edwin Masteller's web site is at

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