Torrent midges, as their name suggests, make their homes in the fastest-flowing parts of rivers and streams. Their larvae have evolved remarkable and unique adaptations, including suckers on their underside. Their pupae attach to rocks until the adult flies emerge. The adults are slender with long legs, superficially resembling crane flies.
Larvae feed exclusively on thin films of algae and detritus, especially diatoms, known as periphyton. The larvae change into pupae on rocks in riffle or just above the waterline, still in reach of waves. Adult torrent midges show a diversity of habits. Females of some species are insect predators, sucking their haemolymph. The food of males and other females is unknown, but nectarivory is likely. Adults of most species are short-lived and rarely venture beyond the river valley. Mating occurs shortly after emergence, and copulating individuals are often seen on vegetation or rocks. Mated females place small clusters of eggs on wet or submerged rocks.
Although often considered rare, torrent midges can be locally abundant and of considerable trophic value, including an important food for trout. Because torrent midges generally require clean, cool, well-oxygenated streams, they are potentially useful bioindicators of water quality.
The Tasmanian Torrent Midge (Edwardsina tasmaniensis), known only from Cataract Gorge of the South Esk River, Launceston, is a highly endemic species that is most likely extinct. All records of this fly are from 1923 to 1933, prior to the 1955 construction of Trevallyn Dam.
In 2010 the South Esk River in Cataract Gorge had a high turbidity, poor flow, and rocks covered with algae. All of these characteristics make the river poor habitat for torrent midges.
These observations confirmed a 1972 survey of the river, which found “only moderate flow was escaping from a pondage above the gorge, and there were definitely no [torrent midges]”. Because of the hydroelectric power station, most of the water from South Esk River is diverted into tunnels above the gorge and returns to the river in its lower tidal reach, reducing the flow of the river.
It seems unlikely that more Ed. tasmaniensis will be discovered. Habitats comparable to those historically in Cataract Gorge (a large, rocky, pristine, “torrential” river near sea level) probably do not exist elsewhere in northern Tasmania.
The main reason for the endangered status of the Tasmanian Torrent Midge was (is) its restricted geographic range. The likely extinction of Ed. tasmaniensis is probably related to the construction of Trevallyn Dam and resulting changes to water flow, temperature, and turbidity in Cataract Gorge.
Given the sensitivity of torrent midges to stream conditions, they may be quite vulnerable to climate change. Increasing regional temperatures and decreasing rainfall will likely lead to increasing water temperatures and decreasing flow. Both of these could have negative impacts on torrent midge populations, such as more algal blooms.
Deforestation poses another threat to torrent midges because loss of riverside vegetation may accelerate rising temperatures and sedimentation in the river. Failure to protect riparian corridors and significant portions of watersheds can ultimately result in other impacts, such as catastrophic debris torrents. All could have devastating consequences to species with highly restricted distributions.
There currently is no plan for the management of Tasmanian torrent midges. For most species, details on their distribution and ecology are limited to two surveys during the past half century. Although both suggest the demise of Ed. tasmaniensis, the remaining six Tasmanian species include four that are widespread and even locally abundant. The other two species are of greater concern because of restricted distributions: Ed. plicata is known only from three locations in western Tasmania, while Ed. reticulata is known only from two rivers in southeastern Tasmania. One of the latter streams was seemingly degraded and lacked torrent midges in 2010.
Farther afield, several species from mainland Australia are of concern because of restricted distributions. Ed. confusa is endemic to Barrington Tops National Park in New South Wales, and both Ed. bison and Ed. bubalus are endemic to alpine Mt Buffalo National Park in Victoria. Perhaps the most endemic species belongs to another genus, Theischingeria rieki, known from only two streams on Mt Bartle Frere in Queensland.
The continued survival of these and other torrent midges will depend on protection of mountain stream ecosystems, including river corridors. Streams lower down, which historically have been most accessible and heavily impacted by human activity, should be a high priority.
Lack of knowledge about the distribution and ecology of torrent midges may slow efforts to provide a specific management plan. However, these flies are among many species restricted to cool, clean streams. They are sensitive to rising temperatures, diminishing flow, increasing sedimentation, and other impacts. It follows that conservation efforts should strive to protect river corridors and watersheds, which will then protect torrent midges and other members of this unique community.
The Conversation is running a series on Australian endangered species. See it here