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

Cryptorhynchinae Cryptorhynchinae is a large subfamily of weevils ( Curculionidae), with some 6000 species. They are found in most zoogeographic regions although they are most diverse in the Neotropics, Australia and Oceania. They are distinguished by having t ...

of Curculionidae. It is distributed in Australia,

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...

and

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

. 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''; id, Papua, or , historically ) is the world's second-largest island with an area of . Located in Oceania in the southwestern Pacific Ocean, the island is separated from Australia by the wide Torr ...

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 DNA barcoding is a method of species identification using a short section of DNA from a specific gene or genes. The premise of DNA barcoding is that by comparison with a reference library of such DNA sections (also called " sequences"), an indi ...

allow a safe identification. In January 2016, a paper was published that revised the Australian 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 New Britain ( tpi, Niu Briten) is the largest island in the Bismarck Archipelago, part of the Islands Region of Papua New Guinea. It is separated from New Guinea by a northwest corner of the Solomon Sea (or with an island hop of Umboi the D ...

. 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, 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 superfamily Curculionoidea, known for their elongated snouts. They are usually small, less than in length, and herbivorous. Approximately 97,000 species of weevils are known. They belong to several families, ...

by the absence of a metanepisternum and by a

synapomorphic In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...

structure of the tarsus with minute claws and a deeply incavated articulation of tarsomere 4. The metathoracic

spiracle Spiracle or spiraculum may refer to: * Spiracle (arthropods), opening in the exoskeletons of some arthropods * Spiracle (vertebrates), openings on the surface of some vertebrates * Spiraculum ''Spiraculum'' is a genus of operculate land snail ...

located externally at the side of the metaventrite is a unique feature and may ensure sufficient respiration during

thanatosis Apparent death, colloquially known as playing dead, feigning death, or playing possum, is a behavior in which animals take on the appearance of being dead. It is an immobile state most often triggered by a predatory attack and can be found in a ...

. ''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 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 found els ...

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

apparent death Apparent death, colloquially known as playing dead, feigning death, or playing possum, is a behavior in which animals take on the appearance of being dead. It is an immobile state most often triggered by a predatory attack and can be found in a ...

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 A trochanter is a Tubercle (human skeleton), tubercle of the femur near its joint with the hip bone. In humans and most mammals, the trochanters serve as important muscle attachment sites. Humans are known to have three trochanters, though the ...

(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, p ...

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...

). 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