FNZ 62 - Trechini (Insecta: Coleoptera: Carabidae: Trechinae) - Popular summary
Townsend, JI 2010. Trechini (Insecta: Coleoptera: Carabidae: Trechinae). Fauna of New Zealand 62, 101 pages.
( ISSN 0111-5383 (print), ISSN 1179-7193 (online) ; no. 62. ISBN 978-0-478-34717-9 (print) , ISBN 978-0-478-34716-6 (online) ). Published 16 Jun 2010
Trechini are small- to medium-sized ground beetles that live in habitats with high humidity. Many of the species that have become adapted for living in caves have reduced eyes, or have lost their eyes altogether. The true cave dwellers are pale brown in colour, having lost the characteristic dark pigmentation of most other ground beetles. Non-cave species live in damp habitats above ground such as beside forest streams, and others live buried in marine gravels below the high water mark. This latter group is best known from the Kaikoura Coast, although they have been collected more widely.
The most characteristic feature of trechines is that they have grooves on the head that curve around the eyes (or ocular area, if eyes are absent) and often meet grooves in the neck region, giving the head a broad inflated appearance. They also have a special arrangement of hairs around the edge of the elytra.
There are about 2500 species in the tribe Trechini worldwide (Ball & Bousquet 2001). Most trechines are found in the temperate regions of the world, or at higher altitudes in the tropics They are widespread in the northern hemisphere, especially in Europe, North America, China, and Japan. Most of the southern continents and New Zealand have diverse trechine faunas, but amongst our relatively few species are some lineages that are apparently not present in Australia, Tasmania, or South America, posing some interesting biogeographic questions.
Currently there are 33 species of trechines known from New Zealand, all of which are endemic except 1 (a widespread species occurring around islands in the Southern Ocean). The largest genus, Duvaliomimus, contains 15 taxa, of which 9 are in the North Island and 7 in the south, but none of these species occur in both islands.
Trechine ground beetles are widespread throughout New Zealand, but have not been found on Chatham or Three Kings Islands. A single coastal species occurs around most of the islands to the south of New Zealand. They are very rare in Northland as only a single specimen of a very tiny species has been found in a cave there. The South Island has just over twice as many species as the North Island, and of these over 50% are restricted to northwest Nelson. This may to a degree reflect collecting activity, especially in caves, but there is no denying that there is a natural concentration of species in this region. Marlborough and most of Canterbury appear to lack trechines, except for those living in sub-littoral gravel, probably because the relatively dry climate and low humidities means there is a lack of suitable habitats.
All trechines require high humidity so caves, which provide an environment close to 100% humidity, are ideal. Almost 1/2 the known New Zealand trechine species are found in caves and these are highly modified for an underground life. They are pale, often lack eyes and usually have lengthened antennae and legs, which are probably used for sensory purposes. Troglobite (obligate underground dwelling) trechines are assumed to be dependant on live prey for food so need to survive for prolonged periods when prey is scarce. Surface-dwelling species also require high humidity and are usually found along streamsides or in beach gravel below the high water mark. As many of these habitats are difficult to collect in, it is not surprising that specimens are uncommon in collections.
Almost 40% of the troglobitic species are known from only a single population, making them highly vulnerable to extinction from catastrophic events. A single polluted stream could conceivably cause the loss of nearly 3% of our known trechine fauna. Although the number of New Zealand’s trechine species is small on a world scale, they are a special part of our biota because they are one of our most diverse groups of cave insects, they occupy unusual habitats, and they show extreme adaptations to the hostile environments in which they live.