[2] George M. Thomson described a new species in the genus Pherusa in 1885, from specimens collected in a mountain stream in the Old Man Range of central Otago at an altitude of 3,000 ft (910 m).

Further populations were not found for several years, but were later discovered at a variety of locations, including "the bog-water on top of Swampy Hill, near Dunedin",[2] Otautau (together with P. subterraneus), Ruapuke Island, Drummond, and near Invercargill.

Unlike P. subterraneus, P. caeruleus is a deep blue colour, which it retains even after prolonged preservation.

[2] More recent work has unearthed a great deal of genetic variability within the specimens ascribed to the genus, which is likely to result in the description of a number of new species.

The centre of diversity is in the upper West Coast of the South Island, and the genus appears to be absent from the eastern portion of the North Island, (the Bay of Plenty, East Cape, and northern Hawke's Bay), presumably due to a recent history of volcanism.