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Trichoptera, or caddisflies, comprise the most diverse insect order whose members are exclusively aquatic. Only aquatic Diptera outnumber them in species and ecological diversity. The larval stages are found in lakes, rivers, and streams around the world, and are important components of food webs in these freshwater ecosystems (Resh and Rosenberg 1984). A few species in the family Chathamiidae from New Zealand and Australia are unusual for the Insecta in having larvae that are truly marine, mostly restricted to tidal pools.
Adult Trichoptera, in contrast to the larvae, are terrestrial and look much like drab, fragile moths, often occurring in large numbers in lakeside or streamside habitats. The similarity is not incidental. Trichoptera are closely related to the order Lepidoptera and together the two orders comprise the superorder Amphiesmenoptera, or "dressed-up wings," the name referring to the dense clothing of scales or hairs on the wings. Monophyly of these two orders is strongly supported in both morphological and molecular analyses (Kristensen 1984, Wheeler, et al. 2001). Trichoptera possess the more primitive character state, having hairs rather than scales, and this character accounts for the name Trichoptera, meaning "hairy wings." Also, unlike moths and butterflies, which typically have a coiled, tubelike proboscis for feeding, adult caddisflies lack well-developed mouthparts, including the absence of mandibles, but have a well-developed haustellum (synapomorphic for the order) formed from a fusion of the hypopharynx and labium, and used in some species to imbibe liquids.
Unlike Lepidoptera larvae, which are predominantly terrestrial herbivores, Trichoptera larvae, with very few exceptions, are aquatic and primarily detritivorous. Like lepidopteran caterpillars, caddisfly larvae are capable of spinning silk from specially modified salivary glands. The diversity of microhabitats exploited by caddisfly larvae is a consequence of the many ways silk is used to construct retreats, nets, and cases and probably accounts for the success of the order as a whole (Mackay and Wiggins 1979, Wiggins 1996).
Almost 12,000 caddisfly species, placed in 45 families and about 600 genera, have been described from all faunal regions, but it has been estimated that the world fauna may contain as many as 50,000 species (Schmid 1984). The three currently recognized suborders are largely characterized by differences in the way silk is used (Ross 1944), whether to produce nets or tubes, or as glue to make various types of portable cases, often incorporating sand and small pebbles, or bits of leaves and twigs, each genus or even species building its own particular style of case. Some larvae are free-living and predaceous, but nevertheless lay down a strand of silk as they move, much like the larvae of Lepidoptera.
The larvae, and the fascinating nets and cases they produce, represents the life stage most familiar to the non-entomologist, and the case-making behavior of some species may account for the English common name, caddisfly. Although the origin of the word is obscure, it has been suggested to derive from cadaz or cadace (caddys), a word of variable spelling used in Shakespearean times to refer to a ribbon made from a certain kind of yarn sold by traveling vendors, who because of this were sometimes called "cadice men." Cadice men would pin samples of their wares to their clothing, a habit which may have suggested the name caddisfly or caddisworm for the aquatic larvae, who exhibit the analogous behavior of attaching bits of leaves and twigs to the outside of their cases (Hickin 1967).
Although caddisflies are not generally considered to be of great economic importance as pests, they are beneficially important in the trophic dynamics and energy flow in aquatic ecosystems. The larvae are also useful as biological indicator organisms for assessing water quality. Extensive use of them has been made for this purpose because larvae of different species vary in sensitivity to various types of pollution (Resh and Unzicker 1975, Resh 1993, Dohet 2002), and because the taxonomy of the group is relatively well known for temperate regions. Unfortunately, the larvae of many species, especially in the tropics, are unknown or have not been associated with their adult forms.