The closer to the glacier, the less species are found in a glacial stream. This is due to the rough conditions: The temperature of the water, for example, at the direct source of the glacial stream, the glacier mouth, is only 0 to 1 °C. Only one species can tolerate this: the glacial stream non-biting midge Diamesa steinboecki. It’s only further down the stream, when more and more groundwater and tributary streams feed the glacial stream and the water temperature rises, that other species appear.
In order to survive in such an extreme location, the larvae of the glacial stream non-biting midge exhibit special adaptations. The larvae have not formed any body appendices and have large claws on their strong, stubby legs in order to get a good grip on rocks even in strong currents. The small body size and the slim, vermicular body structure are further adaptions. They can make good use even of low concentrations of nutrients in the water. Their life cycle is also optimally adapted to the conditions in the highlands: They populate glacial streams especially in winter, when the water level is at its lowest. In spring the adult flying insects hatch and leave the stream, before it carries too much meltwater.
Non-biting midges owe their German name to the fact that their forward-facing legs repeatedly jerk, even at rest. The reason for these movements is unknown.
Up to several thousand larvae can be found in one cubic metre of water in a glacial stream under good conditions. But if the water temperature rises by just a couple of tenths of a degree, the glacial stream non-biting midge immediately becomes much rarer. Climate warming thus presents a threat to this species.