FNZ 6 - Hydraenidae (Insecta: Coleoptera) - Introduction
Ordish, RG 1984. Hydraenidae (Insecta: Coleoptera). Fauna of New Zealand 6, 64 pages.
( ISSN 0111-5383 (print), ; no. 06. ISBN 0-477-06747-6 (print), ). Published 12 Nov 1984
Small size, obscure habitats, and - in some countries - poor representation have impeded study of the hydraenids. Among early systematists to consider them were Kugelann, who in 1794 erected the type genus Hydraena, and Mulsant (1844), who proposed the name from which the family name derives. In more recent times our understanding of the family has been enhanced by the writings of A. d'Orchymont and E. Janssens in Brussels, C. Deane in Victoria, Australia, G. Enderlein in Berlin, R. Jeannel in Paris, P. Zwick in Schlitz, West Germany, J. Balfour-Browne in Surrey, H. B. Leech in California, and P. Perkins in Washington, D.C. Zwick (1977), working on Australian members of the genus Hydraena, found six species with valid names already described and added 23 new ones. In a personal communication dated 1978, discussing the South African fauna, J. Balfour-Browne wrote of 50 undescribed hydraenids; and in a very comprehensive review of the 'western hemisphere' hydraenids Perkins (1980) recorded 206 species, 142 of which were new. In New Zealand too the hydraenid fauna has proved to be much richer than was formerly supposed.
The first hydraenid to be described from New Zealand was Orchymontia spinipennis, which Captain T. Broun, a pioneer coleopterist, erected in 1919. One of the two specimens on which Broun's description was based is in the British Museum (Natural History), and bears the label "Moa Basin", a locality in mid Canterbury; the other is in Brussels. Broun had earlier chosen Hydraenodes spinipennis as a name for this species, but as d'Orchymont (1937) rightly points out, this name was not published until 1921, and so became a junior synonym. No further species of the aquatic mainland fauna were described until 1975, when P. Zwick described Orchymontia maclellani from a single specimen that he had obtained during a brief visit to Westport in 1973.
In the interim, however, some attention had been given to the subantarctic fauna. In a contribution to the Cape Expedition reports, A. E. Brookes (1951) described a terrestrial hydraenid from Campbell Island as Meropathus chuni campbellensis, which J. L. Gressitt and G. A. Samuelson (1964) elevated to species rank. In 1971 I described Meropathus aucklandicus from the Auckland Islands and M. johnsi from The Snares, and made passing reference to an undescribed species known from the Chatham Islands and Stewart Island.
From the literature, then, one might suppose that there are four species of Meropathus and two of Orchymontia in the New Zealand subregion. J. Balfour-Browne informs me, however, that as early as 1943 he was aware of an additional aquatic species. While examining a series of twelve specimens from Swanson, Auckland, collected in 1942, he had noted that one of them was different. He did not describe it because the specimen had been slidemounted before the difference was observed. I am indebted to Mr Balfour-Browne for drawing my attention to this, because in so doing he indirectly initiated an extensive collecting programme. This was undertaken mainly by Dr G. Kuschel and his colleagues in Entomology Division, DSIR, and myself. It has resulted in the acquisition of some 4860 specimens, which form the basis of this account of a fauna of 32 species.
Evidence from these collections indicates that 22 of the species occur in the South Island or the New Zealand subantarctic, 4 occur in the North Island only, and a further 6 occur in both the North and South islands. Many species appear to be quite localised in their distribution, though further collecting is needed before the ranges of some can be delimited with any confidence. All but four species of the New Zealand Hydraenidae are aquatic. The commoner species can be taken from major rivers, but smaller tributaries seem to offer preferred habitats. Even within a single stream, some parts yield more specimens and species than others. Many specimens have been taken from under stones in rapids, but collectors have also noted specimens associated with fallen leaves in the water.
Observation of living beetles shows that they normally surface only when disturbed, and that they have neutral buoyancy. Air is trapped on the pubescent ventral surface, and it seems that oxygen requirements are absorbed from this reserve. Probably, therefore, they require well aerated water.
From the beetles' distribution it seems that in the headwaters of the many streams that lace New Zealand's hilly terrain, considerable speciation of ancestral stock has occurred. Larvae have been collected from the same habitat as adults, so some at least are aquatic.
These species also presumably evolved under a canopy of native bush, and to a large extent they remain there, though some of the commoner species can occupy more exposed habitats. New Zealand's four terrestrial species are all in genus Meropathus, and are found on the sea shore.
Northern Hemisphere hydraenids, called 'cascade beetles' by some, are reported to inhabit matted vegetation along stream margins, in swampy regions, and in brackish pools. I have collected hydraenids under similar conditions in Australia.