The Trichoptera, or caddis-flies, comprise one of the largest orders of aquatic insects, with a world fauna of more than 7000 species known to date. About 470 species representing 24 families are known from Australia.
Caddis-flies are holometabolous, and most closely related to the Lepidoptera. The adults are small to moderate sized, winged insects with moth-like appearance and range from 2 to 40 mm in length. They have slender, filiform antennae and large compound eyes. Body, legs and wings are covered with fine hair. However, the structure of their mouth-parts and details of wing venation distinguish them from all but a few (e.g. Agathiphagidae) Lepidoptera. Most adults are drab and crepuscular, although a few are quite distinctly coloured and some are diurnal. Caddis-flies mate in flight, on the ground or on riparian vegetation. They are generally oviparous, and eggs are deposited in or near water shortly after mating. The larvae are aquatic, and morphologically resemble lepidopteran caterpillars, but lack prolegs on abdominal segments 1-8. They live not only in all types of freshwater habitat, but also in saline coastal lakes and streams (
) and intertidal rockpools (
Endopterygote Neoptera with reduced mouth-parts, two pairs of subequal functional membranous wings; fore wing with anal veins looped; body and wings more or less densely covered with hairs and occasionally with groups of scales. Larvae aquatic; mouth-parts with well developed mandibles, strongly developed functional legs and pair of abdominal prolegs; free living, constructing fixed shelters or portable cases. Pupae exarate with strong mandibles.
Larval Trichoptera are usually easily distinguished by their characteristic morphology, their freshwater habit, and their concealment within cases. Some glossatan Lepidoptera also occur underwater (e.g. certain Pyralidae), however abdominal prolegs with crotchets, and short thoracic legs should serve to distinguish them from caddis-fly larvae.
Wiggins (1984a) surmises the diversification at family level in Rhyacophiloidea and Hydropsychoidea preceded separation of Laurasia and Gondwanaland, as almost all families in these two superfamilies are represented on modern fragments of both supercontinents. In contrast, divergence within the case-building Limnephiloidea can be attributed to breakup of Pangaea as, with few exceptions, the families are endemic to land masses of either Laurasian or Gondwanan origin (Wiggins 1984a).
The world fauna is divided into 43 families, of which 24 families, representing all three superfamilies, are recognised in the Australian fauna (Neboiss 1986). The number of species recognised in Australia is rapidly increasing and at the beginning of 1987 stood at 476. This number includes some undescribed species. Preliminary work on several genera indicates considerable additional new species, but it is too early to present an estimate. Unavoidably this work will change the proportional family representation, but at present the highest number of species (101) is known from Hydroptilidae, constituting about 21% of the Australian Trichoptera fauna; Leptoceridae-83 species (17%), Ecnomidae-57 species (12%) and Hydrobiosidae-57 species (12%). Of the remaining 20 families, 14 represent between 1 and 5% each, and 6 families are below 1% each.
The Australian caddis-fly fauna has close affinities with the families and genera in New Zealand and South America. However, most species are endemic to Australia. A high proportion of endemism is also recorded from small, geographically isolated areas like Tasmania with 74% endemic species (Neboiss 1977) and south-western Australia 79% (Neboiss 1982). The northern Australian fauna shows strong Oriental and Papuan influences. It is dominated by Leptoceridae, particularly the genera
(Neboiss 1981). Of the other families, Hydroptilidae, with more than 100 species, is the largest in Australia and besides the endemic component has affinities with New Zealand and Papuan regions.
Two endemic families occur in Australia-Plectrotarsidae and Antipodoeciidae although the latter may be only a branch of Conoesucidae. Four families-Conoesucidae, Calocidae, Oeconesidae and Chathamiidae-are shared with New Zealand, but the Australian-South American connections over Antarctica are demonstrated in the distribution of Helicophidae, Kokiriidae, Tasimiidae and Philorheithridae. The single Australian limnephilid genus,
, belongs to the primitive subfamily Dicosmoecinae, which also includes certain South American genera (Wiggins 1984a). An unusual distribution is recorded for the family Atriplectididae, which occurs in southern Australia and the Seychelles islands. The Chathamiidae is the only family with marine larvae and is found in water of normal oceanic salinity, in the intertidal zone of rocky sections of the New South Wales. It also occurs in New Zealand and on the Chatham and Kermadec Island.