Apterocyclus honoluluensis Waterhouse 1871

Apterocyclus honoluluensis

Apterocyclus honoluluensis, the Kauai flightless stag beetle, is a species of stag beetle in the family Lucanidae. It is found on the island of Kauai in the Hawaiian Islands.

Apterocyclus honoluluensis is historically and currently the most commonly found species of Apterocyclus, and is known from various locations on Kauai. It is the smallest member of the genus, at a size of 14-17 mm, rarely up to 21 mm.

The larvae were once found in large numbers in soil that contained the decomposing trunks of the tree Acacia koa. The larvae is sensitive to heat, and can be killed by high temperatures. Adults of Apterocyclus honoluluensis have been victims of heavy predation by the mice introduced to the Hawaiian Islands.

In 1994, Apterocyclus honoluluensis was a potential candidate for protection from the Environmental Species Act, but it was declined because of insufficient data on vulnerability, as published in Federal Register Volume 59, Number 219, Pages 58982 - 59028, November 15, 1994.

This smallest species (14–17 mm, rarely up to 21 mm) can be readily distinguished by the presence of a short but distinct ocular canthus (Fig. 1), which is not present in the remaining four species. Otherwise, as lamented by Van Dyke (1922), there is great morphological variability in the species. In most specimens the protibia lacks any external teeth proximal to the apex. A few specimens of each sex exhibit a single weak external tooth on the protibia, unlike the majority of specimens for which the entire external margin of the protibiae is otherwise a useful diagnostic character. Based on the specimens studied there is also pronounced variability in males in surface sculpture (shiny vs. alutaceous) and mandibular shape. Some male specimens have mandibles with traces of additional internal teeth. Females are more strongly shiny and can be distinguished by their mandibles completely lacking internal teeth and their more spherical abdomens. Without additional collecting to better define the distribution and pinpoint potentially isolated populations it is not possible to determine if the morphological variation present is actually taxonomically important.

This was historically, and has been recently, the most commonly collected species. It is known from various localities in Kōke’e State Park.