Kingdom: Animalia
Clade: Euarthropoda
Class: Insecta
Order: Coleoptera
Family: Lucanidae
Genus: Geodorcus
Species: G. helmsi
Binomial name: Geodorcus helmsi
Synonyms: Lissotes aemulus, Lissotes acmenus
Common names: NZ giant stag beetle, Helms's stag beetle, mohau
Geodorcus helmsi is a large, slow-moving, flightless stag beetle in the family Lucanidae. It is endemic to New Zealand. They have been collected from Karamea on the northern West Coast of New Zealand to the south of the South Island, as far as Tapanui in West Otago. It has also been collected from islands in Fiordland and from Stewart Island and Codfish Island. They have a wide altitudinal range, from sea level to 1400 m. This species occupies a highly variable habitat, including forest and tussock-dominated high country. It is mainly nocturnal hiding during the day under fallen logs, stones and leaf litter. The feeding ecology of adult G. helmsi may be highly variable. At night adults have been seen on tree trunks feeding on sappy exudate from wounds that they have chewed in the bark. Their larvae have been observed to have large quantities of hummus inside their gut.
Geodorcus helmsi varies in colour from a dull to glossy black/brownish-black. Like other stag beetles, they show sexual dimorphism: males range in size from 17.5–44.0 mm, including their large mandibles, while females are smaller (16.5–27.5 mm) with less conspicuous mandibles. In larger male specimens, the mandibles are long, slender and strongly arched with a conspicuous tooth near their base.
Since they walk very slowly and cannot fly, they are particularly vulnerable to predation by introduced pig, rats and mice. This species has been found to make up to 27% of the dry weight of feral pig stomach contents.
All Geodorcus species are protected under Schedule 7 of the 1953 Wildlife Act, making it an offence to hunt, kill or possess a specimen.
A male (left) and female (right) Geodorcus helmsi demonstrating sexual dimorphism.
A male
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