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Trichoptera

(Caddisflies)

Class Insecta

Order Trichoptera

Number of families 45

Photo: A colorful caddisfly (Chimara albomaculata family Philoptomatidae) rests on a leaf in the rainforest of eastern Puerto Rico. (Photo by Rosser W. Garrison. Reproduced by permission.)

Evolution and systematics

The caddisflies that make up the order Trichoptera are small to medium-sized insects that resemble moths (order Lepidoptera) in appearance. Like the Lepidoptera, their sister group, members of the Trichoptera are holometabolous, meaning that they undergo a complete metamorphosis. Members of the two orders have similar structures, and the larvae of both orders spin silk from labial glands. However, there are characteristics that also separate these groups. For example, scales are present on the wings, body, and legs of adult Lepidopterans, but they are not present on adult Trichopterans.

The Trichoptera includes 7,000 species in two suborders: the Annulipalpia, or net spinners and retreat makers, which contains two superfamilies and nine families; and the Integripalpia, or tube case makers, which contains four superfamilies and 32 families. In addition, there are three additional superfamilies that have not been placed into suborders: the Rhyacophiloidea, or primitive caddisflies, containing two families; the Glossosomatoidea, or turtle case makers, containing one family; and the Hydroptiloidea, or purse case makers, also containing one family. This taxonomic division is justified by the deep differences in postembryonic development and larval morphology, among other aspects.

The oldest Trichoptera fossils are from Upper and Middle Triassic; they belong to the Necrotaulidae, a family that became extinct during Cretaceous. The current superfamilies appeared during the Jurassic.

Physical characteristics

Caddisfly larvae resemble caterpillars, with a heavily sclerotized (hardened) head and strong mandibles. The thorax has three pairs of legs; dorsal plates appear on the first or on all three thoracic segments. The abdomen is membranous,

sometimes with thread-like gills; the last segment has a pair of anal prolegs. Some larvae (e.g., Annulipalpia and Hydrobiosidae) are free-living and have a lengthened body and long anal prolegs. Other larvae (e.g., Integripalpia) are slow, with plump abdomens, and construct portable cases inside which they live and to which they take hold with their anal prolegs. The larvae of the family Glossosomatidae are of an intermediate type; they make portable cases, and the distal half of the anal prolegs is free from the abdomen. The prolegs are extended through the opening of the case to stabilize the larva on the substrate. The larvae of the family Hydroptilidae are miniscule and similar to free-living larvae in the first larval instar. In the last larval instar, they resemble the case makers as their size increases disproportionately, especially the abdomen, and they spin portable purse-shaped cases.

Caddisfly adults are not as diverse as larvae. They have a small, moth-like appearance, most with very long and slender antennae and reduced mouthparts. Setal warts are present on the head and thorax of adults. The wings are large, covered with hair, and rarely have scales. The wings are held tent-like at rest, and usually the antennae are placed together and directed forward; in most species, the antennae are longer than the body. Adult flight in most species is clumsy and slow. Adults range from 0.048 in (1.2 mm) to 1.76 in (40 mm) in length. Most have a drab coloration, although some brilliantly colored forms are known.

Distribution

Caddisflies are cosmopolitan, although some families are restricted to the Holarctic and Gondwanan regions. No individual species are cosmopolitan in distribution. Most species are restricted in distribution, with high numbers of endemics for several countries.

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Order: Trichoptera

Caddisfly larvae eating a fish. (Photo by A. Blank. Bruce Coleman, Inc. Reproduced by permission.)

Habitat

Most larvae and pupae live in freshwater, although there are some terrestrial (e.g., Enoicyla pusilla [Leptoceridae]) and marine (e.g., Philansius plebeius [Chathauiidae]) forms. Adults are generally nocturnal, hiding in cool, moist environments along river banks during the day and becoming active at night. Members of some species are active during the day.

Most freshwater individuals live in cold, unpolluted, welloxygenated rivers and streams, although some families are adapted to warm, slow-moving waters. The larvae are very sensitive to water temperature, dissolved oxygen, speed of current, illumination, size of silt, and chemical substances. Each species occupies a particular microhabitat in the water body, and several species may coexist in one place without competing. As a river’s qualities change from the headwaters to the mouth, there is a correlated replacement of species. Larvae use diverse elements of their natural habitat to build their cases. Those that build nets stick them to the rocks of the riverbed.

Behavior

For the most part, caddisflies are solitary creatures, although individuals in some species have been observed in aggregations. Some adult males defend a small territory such as a branch or stone. The net-making genus Smicridea and other members of the family Hydropsychidae defend the area where they deposit their nets; when other individuals approach, the net-makers emit alarm sounds.

All larvae spin silk when they build their nets, retreats, cases, or cocoons. At the end of their larval period, they build a cocoon in which they pupate. When they have completed

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their development, the adults cut the cocoon open and swim to the surface. The variety of building mechanisms of the larvae has allowed this order to invade different habitats with great success. The free-living larvae of Annulipalpia build retreats and nets to filter organisms or particles on which they feed; they pupate in a double case built with an external wall is made of loosely stacked stones and a closed silken inner case. The free-living larvae of Rhyacophiloidea build neither nets nor retreats; their pupal cases are similar to those of Annulipalpia. Larvae of Glossosomatidae build a turtle-shaped case, which they replace each time they molt. Before pupation, they remodel the last case, sticking it to the substrate by the border, and build a second closed silken case inside. Larvae of Hydroptilidae build a case in the last larval instar that is also used for pupation.

The case-making larvae of Integripalpia build tubular cases of silk to which they stick parts of plants or minerals arranged in different ways; they remain inside these cases, with only the anterior part of the body protruding; they drag the case when they crawl and use the case to pupate. The adults are shortlived; their primary activity consists of looking for a mate and, in the case of females, laying eggs. To compensate for larval drift that occurs downstream, many species fly upstream for oviposition. Adults exhibit only short dispersal flights.

Communication between the sexes involves chemicals of sexual attraction (pheromones) and visual and auditory signals. Some of these signals have different meanings in different species; for instance, in some species drumming sounds stimulate mating, but in others it precedes an attack. Likewise, some species flap or spread their wings before mating, but for others such a display is an attack sign toward other males. Some pheromones produce swarming in males, which then exhibit dancing behavior in order to attract females.

Feeding ecology and diet

Adults consume only liquids, such as nectar or sap. There is a strong omnivorous tendency among larvae, which feed on detritus, suspended particles, algae, vascular plants, and whole animals or parts of animals.

The free-living larvae are filter-feeders (feeding on particles) or predators, while the case-maker larvae are shredders, grazing on plants or scraping algae from surfaces. Some Oxyethira (Hydroptilidae) quickly pierce and suck fluids from filamentous algae, cell by cell. Several types may occur in the same family, each one represented by different genera. Both larvae and adults are an important food source for fishes and other aquatic animals.

Reproductive biology

Mating takes place almost immediately after individuals emerge from their cocoons; they mate in flight, on riparian vegetation, or on the ground. Males are attracted from long distances by female pheromones. When they are close to each other, one or both of them move their wings, producing vibrations or scraping the substrate; others produce shakes in the body or stimulate mating by touching each other with

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their antennae. In some species the female raises her wings when she is ready to copulate. Mating may last a few minutes to several hours. Both males and females may copulate several times during their short life. Insemination is internal by means of the male’s genital organ transferring sperm in free form or via a spermatophore.

Eggs are cemented to stones or plants in the water. Sometimes the female dives for nine to 30 minutes to lay them. In other species, the female carries the eggs at the tip of her abdomen, and she spreads the eggs by dipping her abdomen in the water as she flies upstream. Limnephilidae deposit their eggs on vegetation that hangs over the water. There is no nest-building or parental care.

Adults are active mainly in spring and summer, but some species (such as Verger bruchinus from Argentina) are active only during winter.

Conservation status

The 2002 IUCN Red List includes four caddisfly species, all of which are categorized as Extinct: Hydropsyche tobiasi, Rhyacophila amabilis, Triaenodes phalacris, and T. tridonata. Caddisflies do not tolerate strong variations in their aquatic habitats; organic or chemical wastes can lead to a decrease in their density or their extinction. At the same time, most species have a restricted distribution, so drastic alterations in a particular region may extinguish the species that inhabit that area. The sensitivity of caddisflies to polluting substances can serve as an indicator of water quality in a given habitat. No direct efforts at conservation are being made, but caddisflies may benefit indirectly when aquatic ecosystems are protected.

Significance to humans

Some legends in central Japan were based on the doll-like cases of the genus Goera; these legends revolved around a young girl offered as a sacrifice. Beginning in the 1980s, the visual artist Hubert Duprat utilized caddisflies to create unique sculptural forms. He first removed larvae from their natural habitat, and then he provided the larvae with precious materials, prompting the caddisflies to construct “jeweled” cocoons. Some South American natives use larval cases as earrings and as beads for necklaces.

Order: Trichoptera

Caddisfly larvae live in a tubelike case of leaves, twigs, and sand. Most of the life cycle is spent in the tube. (Photo by ©Patrick Grace/ Photo Researchers, Inc. Reproduced by permission.)

Salmon and other fishes are attracted by caddisfly larvae, pupae, and adults, leading to the creation of a series of sophisticated patterns for fly-fishing that imitate the different stages of species from different regions.

Some species gnaw on wood structures in the water, while other species cause damage to rice fields and to aquatic ornamental and commercial plants. Adults are often attracted to lights, where thousands of them may appear simultaneously and cause damage to air conditioners and other devices. Caddiflies also may reduce visibility when they lay eggs on roads; apparently they confuse the shine of roads at night with water. The cementing substance of the eggs and eggs broken by tires can be transformed into a gelatin that is hazardous for drivers.

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1. Triaenodes bicolor adult and larva in larval case; 2. Abtrichia antennata adult and dorsal views of larva; 3. October caddisfly (Dicosmoecus gilvipes) adult and larva; 4. Glossosoma nigrior larva; 5. Stenopsyche siamensis larva. (Illustration by Wendy Baker)

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