New weevil species discovered in pristine parts of the Ryuku Islands, study says

(CN) — On the Ryuku Islands, a chain of subtropical islands distributed between mainland Japan and Taiwan that have a relatively isolated evolutionary history, a team of scientists found a new, relatively isolated weevil species.

In 2015, researchers from the Okinawan Institute of Science and Technology (OIST) began placing net traps on Okinawa Island to monitor and collect a wide range of insects such as beetles, wasps and bees, according to their study published Friday in the Coleopterists Bulletin. In 2022, entomologist and OIST insect collection manager Jake H. Lewis was looking over OIST’s weevil collection when he noticed features so unique in one specimen that it potentially qualified as a new Acicnemis species.

Before declaring such, Lewis reached out to the Kyushu University Museum in Japan, the Natural History Museum in London, and several other institutions to compare his weevil to the 180 specimens in the Acicnemis genus, according to the study.

Lewis' weevil, Acicnemis ryukyuana, possesses yellow bands on its shoulders and a distinct pattern of grey, black and yellow scales on its tough forewings. Also, the study says that a microscopic view shows that A. ryukyuana has long scales (hairs) on its back and a unique shape of the last segment of the leg, among other features that Lewis explained via email distinguished it among the Acicnemis genus.

“Most species of Acicnemis have heart-shaped third tarsal segments (split in the middle). However, Acicnemis ryukyuana has truncated third tarsal segments. Furthermore, the elongated, erect scales on the head, pronotum ('neck') and back of Acicnemis ryukyuana are very long compared with the other Japanese species of Acicnemis. Just based on the length of those scales alone, you can identify Acicnemis ryukyuana easily,” wrote Lewis.

Per the study, Lewis chose to name the species A. ryukyuana, whose Japanese name translates into “Ryuku dead-tree weevil,” because all available evidence suggests that it is endemic to the Ryuku biodiversity.

“Based on specimens I’ve seen in museum collections, the taxonomic literature and the fact that Acicnemis ryukyuana has only been collected in the most pristine, natural forests in Okinawa, I think it is safe to say that this species is likely endemic to the Ryukyu Islands, and as such represents an important element of the native biodiversity. Although most Acicnemis species are widespread and relatively common throughout the Ryukyu Islands, this species is rare, range-restricted, and morphologically very unique, which makes it a pretty special species for me as an entomologist based in Japan,” wrote Lewis.

That aside, unknown factors remain concerning A. ryukyuana such as what it eats, as the study says that weevils generally feed on plants, and some weevil species have highly specialized diets. As of now, the study reports that the nature of A. ryukyuana's diet, specialized or otherwise, is unknown.

Furthermore, the study says that the researchers only captured A. ryukyuana in protected, undisturbed areas such as a pristine part of Yanbaru National Park and the well-preserved subtropical forest areas on Ishigaki Island as opposed to heavily populated and disturbed areas. Lewis further noted via email that the researchers spent the last decade running traps across 24 other sites without turning up A. ryukyuana specimens, which he wrote suggests a higher sensitivity to human disturbance among other factors.

“Why does Acicnemis ryukyuana only occur in these well-preserved forested areas? Nobody knows yet for sure, but it likely has something to do with the host plant(s) of Acicnemis ryukyuana, which may only occur in such forests. I hope to investigate this during future field work.”

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