Arachnid locomotion is the various means by which

arachnids Arachnida () is a Class (biology), class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, opiliones, harvestmen, Solifugae, came ...

walk, run, or jump; they make use of more than

muscle contraction Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...

, employing additional methods like

hydraulic Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counter ...

compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression *Gas compression *Compression ratio, of a c ...

. Another adaptation seen especially in larger arachnid variants is inclusion of elastic

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...

Hydraulics

In most arachnids, hydraulic compression acts as the primary means of extension in several of their

hinged A hinge is a mechanical bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation: all other ...

leg joints, namely the femur–patella joint and tibia–metatarsus joints or second and third leg joints respectively. Instead of blood,

hemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...

is used to move nutrients around inside of the arachnid, and has the secondary function of acting as a hydraulic fluid. When compressed by the body of the arachnid, the hemolymph applies compressive force through channels in the limbs that cause them to extend. This motion is then balanced by flexor muscle to retract the leg joints as needed. Due to hydraulics being used for extension, the flexor muscle is able to be significantly larger than would otherwise be possible without impacting size or weight. Measurable core body volume change can occur during periods of higher compression to the legs, as the sinuses of the body contract to achieve pressurization in specific legs. Aside from the normal gait of the arachnid, in some variants, extremely high pressures are used as a means of jumping, propelling rear legs and allowing for much greater and more sudden motion.

Elastics

In larger variants of arachnids, such as the tarantulas and hairy desert spiders, another mechanism used for locomotion is an elastic

sclerite A sclerite (Greek , ', meaning "hard") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instead it refers most commonly ...

. These sclerites are semi-rigid connectors between leg segments that allow storage and expending of potential energy. This is used as a supplement or in conjunction with the hydraulics normally employed in those joints, allowing for greater weights to be carried, more rapid and sudden movement when combined with the already pronounced flexor muscle acting in those joints, as well as fine motor control with reduced sudden disruption of hemolymph flow. At higher compression of the joint the stiffness of the sclerite has been found to increase significantly, denoting support even outside of normal tension.

Influence on biomimetic design

Hydraulic locomotion in arachnids has acted as an inspiration for many modern

biomimetic Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from grc, βίος (''bios''), life, and μίμησ ...

concepts in robotics intended for use by or with people, especially in the field of

soft robotics Soft robotics is a subfield of robotics that concerns the design, control, and fabrication of robots composed of compliant materials, instead of rigid links. In contrast to rigid-bodied robots built from metals, ceramics and hard plastics, the ...

. The use of hydraulics in robotic joints is aimed at replacing the more control heavy nature of modern robotics with a more passive system developed in soft actuation. Various forms of actuation and force transmission can be achieved through these inspired designs, including rotation, lifting, and even

damping Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples incl ...

effects. The passive nature of the hydraulic and elastic extensor mechanisms employed have found use in

orthotics Orthotics ( el, Ορθός, translit=ortho, lit=to straighten, to align) is a medical specialty that focuses on the design and application of orthoses, or braces. An is "an externally applied device used to influence the structural and functio ...

projects aimed at assisting joints weakened by age or disease.

Fluid secretion

An additional method used by some arachnids to improve locomotion is to secrete fluids, characterized by a

hydrophobic effect The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in an aqueous solution and exclude water molecules. The word hydrophobic literally means "water-fearing", and it describes the segregation of water and nonpolar ...

, through the pads on the ends of their legs that are in contact with the walking surface. It has been shown that the arachnid is capable of using the adhesive fluid selectively, meaning it can choose to not secrete the fluid in certain circumstances where it would be unwarranted such as in moist conditions. The use of fluids allow the arachnid better traction through improved shear force for both standard locomotion and also sudden movements such as in jumping and leaping.

Challenges in modelling

Modelling the hydraulic system used by arachnids has been a challenge in the past due to scale and complexity. Simplified models focusing on individual joints and flow channels using modern imaging such as

Micro-CT X-ray microtomography, like tomography and X-ray computed tomography, uses X-rays to create cross-sections of a physical object that can be used to recreate a virtual model (3D model) without destroying the original object. The prefix ''micro-'' ...

has allowed for mathematical expressions of pressure and flow acting on the joints. Visualizing the flow of hemolymph in small bodies directly has been difficult due to resolution constraints and lack of contrast causing fluid and soft tissue being indistinguishable, but techniques have been employed using a combination of injected microbubbles as tracers in the hemolymph and synchrotron x-ray contrast imaging to track them.

References

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Hydraulics

Elastics

Influence on biomimetic design

Fluid secretion

Challenges in modelling

See also

References