''Trigonopterus'' is a genus of flightless weevils placed in the Cryptorhynchinae of

Curculionidae The Curculionidae are a family of weevils, commonly called snout beetles or true weevils. They are one of the largest animal families with 6,800 genera and 83,000 species described worldwide. They are the sister group to the family Brentidae. Th ...

. It is distributed in

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...

and

Melanesia Melanesia (, ) is a subregion of Oceania in the southwestern Pacific Ocean. It extends from New Guinea in the west to the Fiji Islands in the east, and includes the Arafura Sea. The region includes the four independent countries of Fiji, Vanu ...

. About 90 species had been formally described until March 2013, when a single paper more than doubled this number, agreeing with previous studies and a systematic barcoding study that many more species have yet to be described. As of October 2021, there were 480 described species. The center of its diversity appears to be

New Guinea New Guinea (; Hiri Motu: ''Niu Gini''; , fossilized , also known as Papua or historically ) is the List of islands by area, world's second-largest island, with an area of . Located in Melanesia in the southwestern Pacific Ocean, the island is ...

where 51 or more species can be found in a single locality. Many of them are very similar to each other, but male genital characters and DNA barcoding allow a safe identification. In January 2016, a paper was published that revised the

n species within this genus. That paper described 24 new species and indicated the potential for more research into undescribed Australian species within this genus. In April 2016, a paper was published that described four new species in this genus from the island of New Britain. The paper postulated that more species of this genus presently unknown to science may exist on the island. It emphasised the importance of prioritising further research due to the ongoing destruction of the habitat of these species. Further papers described 133 new species from

Sulawesi Sulawesi ( ), also known as Celebes ( ), is an island in Indonesia. One of the four Greater Sunda Islands, and the List of islands by area, world's 11th-largest island, it is situated east of Borneo, west of the Maluku Islands, and south of Min ...

, where only one species (''T. fulvicornis'') had previously been recorded. The genus can be diagnosed among wingless cryptorhynchine

weevils Weevils are beetles belonging to the Taxonomic rank, superfamily Curculionoidea, known for their elongated snouts. They are usually small – less than in length – and Herbivore, herbivorous. Approximately 97,000 species of weevils are known. ...

by the absence of a metanepisternum and by a synapomorphic structure of the tarsus with minute claws and a deeply incavated articulation of tarsomere 4. The metathoracic spiracle located externally at the side of the metaventrite is a unique feature and may ensure sufficient respiration during thanatosis. ''Trigonopterus'' species inhabit primary tropical forests, both on foliage and edaphic in the litter layer. They have a marked tendency to

endemism Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...

with many species only known from a single locality. Their primary defence against predators is

apparent death Apparent death is a behavior in which animals take on the appearance of being death, dead. It is an immobile state most often triggered by a predatory attack and can be found in a wide range of animals from insects and crustaceans to mammals, b ...

or thanatosis. An animated 3D model of a ''Trigonopterus'' weevil reveals a number of mechanisms to maintain a stable defensive position.

Biological screw joint

Compilation of 100 Trigonopterus species - 1742-9994-10-15-3

The arthropod hip-leg joint consists of two parts - the coxa (or the hip) and the trochanter (or the head of the

arthropod leg The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip (a ...

femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

). The coxa, in the case of ''Trigonopterus oblongus'', resembles a nut, and it has a thread running along its inner surface with an angular span of 345°. The trochanter resembles the screw. It is rod-shaped with a large external spiral flange, having an angular span of 410°, in excess of a full circle, which functions as a thread. When the leg muscles of a beetle are stretched, the screw turns. Though the screw-thread provide for very large angular rotation, the front legs are capable of rotating by 90°, while their hind legs can rotate by 130°.

Evolution

The screw-and-nut system has now been found to be present in all 15 weevil species examined by the scientists and appears to be a hitherto unknown anatomical feature of weevils. It has been estimated that weevils evolved this system about 100 million years ago. It is surmised that the development of this feature provided additional flexibility which permitted weevils to improve their climbing abilities, helped them keep steady when at rest, and to give a stronger leverage for piercing by the snout.

Species

Gallery

References

{{Use dmy dates, date=April 2020 Beetles of Asia Taxa named by Charles Adolphe Albert Fauvel Curculionidae genera