Geodorcus helmsi

Geodorcus helmsi is a large flightless stag beetle that varies in colour from black to brownish-black. Its dorsal surface can vary from dull to glossy. 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.{{Cite journal |last=Grey |first=L. |last2=Trewick |first2=S. A. |last3=Johnson |first3=S. L. |date=2024 |title=Introduced mammalian predators influence demography and trait variation of a New Zealand stag beetle |url=https://link.springer.com/article/10.1007/s10841-024-00593-0 |journal=Journal of Insect Conservation |language=en |volume=28 |issue=4 |pages=725–736 |doi=10.1007/s10841-024-00593-0 |issn=1572-9753|doi-access=free }} In larger male specimens, the mandibles are long, slender and strongly arched with a conspicuous tooth near their base. G. helmsi is distinguished from other Geodorcus by its straight un-arched tibiae, and five setose ridges on the elytra.{{cite web|last1=Holloway|first1=B.A.|title=Lucanidae (Insecta: Coleoptera)|url=https://www.landcareresearch.co.nz/__data/assets/pdf_file/0019/26326/FNZ61Lucanidae.pdf|website=Landcare New Zealand|publisher=Landcare Research|access-date=3 November 2016|archive-date=17 May 2017|archive-url=https://web.archive.org/web/20170517231057/http://www.landcareresearch.co.nz/__data/assets/pdf_file/0019/26326/FNZ61Lucanidae.pdf|url-status=dead}}

File:Geodorcus helmsi (Sharp, 1881) male Syn. Lissotes helmis (Sharp, 1881) (3256464796).jpg|Geodorcus helmsi

File:Geodorcus helmsi m d.tif|Geodorcus helmsi

File:Geodorcus helmsi.jpg|A male (left) and female (right) Geodorcus helmsi demonstrating sexual dimorphism

File:NZAC Pare - Stag Beetle.jpg|Geodorcus helmsi on a pare

Geodorcus helmsi is the most widespread of the ten Geodorcus species, having 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. Adults have a wide altitudinal range, from sea level to 1400 m.{{Cite web |last=Concannon |first=Claire |date=2022-07-09 |title=The battling beetle |url=https://www.rnz.co.nz/national/programmes/ourchangingworld/audio/2018848839/the-battling-beetle |access-date=2022-07-13 |website=RNZ |language=en-nz}}

All Geodorcus species are mainly nocturnal and hide underneath fallen log stones or leaf litter on the forest floor.{{cite journal|last2=Green|first2=Chris|last3=Owen|first3=Keith|date=1994|title=Distribution, conservation status and some features of the natural history of Dorcus stag beetles (Coleoptera: Lucanidae)|url=http://www.doc.govt.nz/Documents/science-and-technical/sr75.pdf|journal=Science and Research Series|issue=75|last1=Sherley|first1=Greg|access-date=4 November 2016}} At night G. helmsi have been seen active on tree trunks, chewing at the bark to get access to the exudate. This species occupies a highly variable habitat, including forest and tussock-dominated high country.

The feeding ecology of adult G. helmsi may be highly variable: they occupy a wide range of habitats from forest to the tussock zone in the high country. Adults have been observed on tree trunks feeding on sappy exudate from wounds in the bark. Larvae of other lucanid beetles commonly eat the surface of rotting wood. Geodorcus larvae have been observed to have large quantities of humus inside their gut.

In New Zealand stag beetles, there is no published information about the duration or timing of oviposition, larval, pupal and adult stages of the life cycle. Copulation has been observed in October. Larvae of Geodorcus have been seen under decaying logs, occupying a gallery in the soil layer. They are C-shaped, slow moving, and avoid the light.

All Geodorcus species are protected under Schedule 7 of The 1953 Wildlife Act, making it illegal to hunt, kill, or possess a specimen.{{cite web|title=Wildlife Act 1953|url=http://www.legislation.govt.nz/act/public/1953/0031/latest/whole.html#DLM277090|website=New Zealand Legislation|publisher=Parliamentary Counsel Office|access-date=4 November 2016}} Predation by introduced rats has reduced the population density of G. helmsi; on islands where rats are present, only their remains can be found. On Rakiura (Stewart Island) rats eat more male Geodorcus helmsi than female specimens and the surviving individuals are significantly smaller than those killed by rats. This species has been found to make up to 27% of the dry weight of feral pig stomach contents.{{cite journal|last2=Easdale|first2=Tomas A.|last3=Williamson|first3=Wendy M.|last4=Forsyth|first4=David M.|date=2014|title=Causes and consequences of ground disturbance by feral pigs (Sus scrofa) in a lowland New Zealand conifer–angiosperm forest|url=http://newzealandecology.org/nzje/3212.pdf|journal=New Zealand Journal of Ecology|volume=39|issue=1|pages=34–42|last1=Parkes|first1=John P.|access-date=4 November 2016}}

File:Helms's Stag Beetle imported from iNaturalist photo 149337222 on 21 October 2024.jpg

File:Geodorcus helmsi 29828350.jpg

File:Helms's Stag Beetle imported from iNaturalist photo 116665520 on 21 October 2024.jpg

File:Geodorcus helmsi 1205877.jpg

File:Helms's Stag Beetle imported from iNaturalist photo 1781140 on 21 October 2024.jpg

File:Geodorcus helmsi 116577910.jpg

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• Geodorcus helmsi discussed on RNZ Critter of the Week, [https://www.rnz.co.nz/national/programmes/afternoons/audio/2018697670/critter-of-the-week 31 May 2019]

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Category:Lucaninae

Category:Endemic beetles of New Zealand

Category:Beetles described in 1881