Screw-and-nut Leg Joint Assists Climbing

Biological Strategy

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Move in/on Solids

To obtain needed resources or escape predators, some living systems must move on solid substances, some must move within them, and others must do both. Solids vary in their form; they can be soft or porous like leaves, sand, skin, and snow, or hard like rock, ice, or tree bark. Movement can involve a whole living system, such as an ostrich running across the ground or an earthworm burrowing through the soil. It can also involve just part of a living system, such as a mosquito poking its mouthparts into skin. Solids vary in smoothness, stickiness, moisture content, density, etc, each of which presents different challenges. As a result, living systems have adaptations to meet one, and sometimes multiple, challenges. For example, some insects must be able to hold onto both rough and slippery leaf surfaces due to the diversity in their environment.

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Manage Mechanical Wear

A living system is subject to mechanical wear when two parts rub against each other or when the living system comes in contact with abrasive components in its environment, such as sand or coral. Some abrasive components are a constant force, such as finger joints moving, while others occur infrequently, such as a sand storm moving across a desert. Living systems protect from mechanical wear using strategies appropriate to the level and frequency of the source, such as having abrasion-resistant surfaces, replaceable parts, or lubricants. For example, human joints like shoulders and knees move against each other all day, every day. To protect from mechanical wear, a lubricant reduces friction between the cartilage and the joint.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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Screw-and-nut leg joint of the Papuan weevil allows rotational movement combined with single axis translation.

Image: Alex2Guess /

Image: Alexander Riedel /

Coxa-Trochanter joint of weevil Trigonopterus oblongus. Trochanter functions as biological screw, coxa as corresponding nut.

“It is remarkable that in the case of the weevil leg a rotary movement is accomplished by a screw-and-nut system. In engineering, such systems are mainly used for fixing connections, whereas an axle would be used for a simple rotation. The possible advantages and disadvantages of a screw-and-nut system over an axle in a leg joint may be most apparent at its end points. In ventral position, when fully tightened, the joint comes to a dead stop. One possible disadvantage, the danger of overwinding the thread, then is negligible, because the depressor muscles are already fully contracted at that point. We suggest that an advantage of this construction is that the leg comes to a stable resting position, preventing passive straining of leg muscles, which would not be accomplished by an axle construction. If the weevil’s leg is pulled in the opposite direction over its back, the muscles are torn off, and the trochanter can be neatly dislodged from the coxal nut. Such dislocation can be produced with the help of delicate forceps. Under natural circumstances, this is a highly unlikely event, because the beetle’s natural defensive reflex is to bring the legs into a ventral position if attacked by a predator.” (van de Kamp et al 2011:52)

“As a rule, weevils are clumsier than many other beetles, e.g.
carabidae. Transformation of a hinge joint into a screw joint, however,
allowed them to move their legs further down, which made them better
climbers. The Trigonopterus oblongus weevil analyzed here lives on twigs
and foliage in the jungle of Papua New Guinea. For feeding, the thorn
is pushed into the plant tissue, while the hind legs provide strong
foothold. Presumably, the screw joint also is advantageous in this
respect.

“‘Meanwhile, we have also studied other weevil species and always found
screw joints,’ explain Riedel and van de Kamp. ‘Obviously, this joint
exists in all weevils, of which more than 50,000 species exist
worldwide.’ In this case, the researchers would have identified the
screw joint to be a so far unknown basic feature of the weevil family.
The best known species in Germany are Curculio nucum and curculio
glandium as well as the corn weevil, a grain pest.” (Karlsruhe Institute of Technology press release)

Last Updated August 18, 2016

References

Journal article

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