Geodorcus ithaginis

Including their large mandibles (used for fighting over mates), male specimens range in length from 25.5 to 32.8 mm and are 10.4–12.4 mm wide. Females range in length from 20.0–22.5 mm. Their exoskeleton is black and ranges from dull to moderately glossy. The thorax is back, glossy, and wider than it is long. The elytra are rounded at the posterior, almost equally long and wide, and covered in short branching hairs. Male beetles have three teeth at the apex of the mandibles; females have two. What distinguishes this species from all other New Zealand stag beetles is a long conical vertical tooth, on the top of the mandible, in both sexes.

This species is only known from the Mokohinau Islands in New Zealand. It was one of 130 beetle species collected on the Mokohinaus by lighthouse keeper Andreas Stewart Sandager (stationed on Burgess Island 1883–1889); a third of these species turned out to be new to science, and were described by Thomas Broun. The stag beetle's type locality was given by Broun as "Halodroma Islet", but there is no island by that name. "Halodroma" may be another name for Lizard Island, a small flat island southeast of Burgess Island; Halodroma urinatrix is the former name of Pelacanoides urinatrix, the diving petrel, which used to nest in burrows on Lizard and two other low Mokohinau islands. A few more specimens were collected in 1902, and then none until the species was rediscovered in 1984 on "Stack H", a 1.2 ha island southwest of Burgess Island. All recent surveys have only found this species on Stack H, in a "living-room sized" patch of New Zealand ice plant or horokaka (Disphyma australe); it has not been seen on Lizard Island since its description in 1893.

These beetles are nocturnal, and their feeding behaviour is unknown. Adult stag beetles worldwide have mouthparts suited to feeding on liquids. Geodorcus helmsi has been seen feeding on sappy exudations on tree trunks. Rotting wood, tree roots or humus are the most common food sources for stag beetle larvae, and G. ithaginis lives in an environment of enriched humus.