Arthropods, including insects and spiders, make use of smooth adhesive pads as well as hairy pads for climbing and locomotion along non-horizontal surfaces.[1][2][3] Both types of pads in insects make use of liquid secretions and are considered 'wet'.[3] Dry adhesive mechanisms primarily rely on Van der Waals' forces and are also used by organisms other than insects.[4] The fluid provides capillary and viscous adhesion and appears to be present in all insect adhesive pads.[5] Little is known about the chemical properties of the adhesive fluids and the ultrastructure of the fluid-producing cells is currently not extensively studied.[4] Additionally, both hairy and smooth types of adhesion have evolved separately numerous times in insects.[3][6] Few comparative studies between the two types of adhesion mechanisms have been done, and there is a lack of information regarding the forces that can be supported by these systems in insects.[3] Additionally, tree frogs and some mammals such as the arboreal possum and bats also make use of smooth adhesive pads.[1][2] The use of adhesive pads for locomotion across non-horizontal surfaces is a trait that evolved separately in different species, making it an example of convergent evolution.[7] The power of adhesion allows these organisms to be able to climb on almost any substance.[2]
The exact mechanisms of arthropod adhesion are still unknown for some species, but this topic is of great importance to biologists, physicists, and engineers.[2][3][7] These highly specialized structures are not restricted to one particular area of the leg. They may be located on different parts, such as claws, derivatives of the pretarsus, tarsal apex, tarsomeres or tibia.[6] From the scaling analysis, it has been suggested that animal lineages relying on the dry adhesion, such as lizards and spiders, have a higher density of terminal contact elements compared to systems that use wet adhesive mechanisms, such as insects.[6] Since these effects are based on fundamental physical principles and highly related to the shape of the structure, they are also the same for artificial surfaces with similar geometry.[6] Adhesion and friction forces per-unit-pad area were very similar in smooth and hairy systems when tested.[3] Strong adhesion may be beneficial in many situations, but it also can create difficulties in locomotion.[3] Direction-dependence is an important and fundamental property of adhesive structures that are able to rapidly and controllably adhere during locomotion.[3] Researchers are unsure whether direction-dependence is achieved through changes in contact area or through a change in shear stress.[3] Friction and adhesion forces in most animal attachment organs are higher when they are pulled towards the body than when they push away from it.[3] This has been observed in geckos and spiders but also in the smooth adhesive pads of ants, bush-crickets and cockroaches.[3] Adhesive hairs of geckos are non-symmetrical and feature distally pointing setae and spatulae that are able to generate increased friction and adhesion when aligned with a proximal pull.[3] The adhesive hairs of some beetles behave similarly to those of geckos.[3] While directional-dependence is present in other animals, it has yet to be confirmed in insects with hairy adhesive pads.[3]
It has been observed that a surface micro-roughness asperity size of less than five micrometres can strongly reduce insect attachment and climbing ability, and this adhesion reducing effect has been put to use in a variety of plant species that create wax crystals.[5]
Adhesive chemical secretions are also used for predation defence, mating, holding substrates, anchor eggs, building retreats, prey capture, and self-grooming.[4]
Smooth adhesive pads for locomotion
Smooth adhesion has evolved in many families of organisms independently, which creates structures that appear unrelated to each other but generate the same function.[2]Phylogenetic analyses indicate that adhesive structures of arthropods evolved several times.[1] Organisms such as ants, bees, cockroaches and grasshoppers use smooth adhesive pads.[1] There are different types of smooth adhesive pads in these organisms such as the arolia, pulvilli, and euplantulae, all of which have a cuticle that is extremely soft and deformable.[1][2] The arolia of some ant species has been observed to be fluid-filled and is extended and contracted to provide adhesive force.[1] The euplantulae in crickets have a hexagonal microstructure which is similar to toe pads in tree frogs.[1] Generally, insects are able to adhere to surfaces through the contact between the insect adhesive organs and substrates that are mediated by nanometre-thin films of adhesive fluid.[2] Some functional principles of smooth pads (adaptability, viscoelasticity, pressure sensitivity) are similar to those known from industrial pressure-sensitive adhesion.[6] Smooth adhesive organs are ‘‘pillowlike,’’ which refers to the soft and fluid-filled, cuticular sac that moulds to the surface increasing the contact area on rough surfaces.[2] It appears that the fluid in smooth adhesive systems mainly serves to maximize contact on rough substrates.[3] The internal fibrous structure of smooth pads might be vital to their ability to deform, for shear-induced lateral increase in contact area, or for efficient transfer of tensile forces, yet at this point its specific function is unknown.[5]
Hairy adhesive pads for locomotion
Both hairy and smooth pads in arthropods act to maximize the amount of contact with a surface.[2] The foot pads of flies are densely covered with flexible hair-like structures called setae, and some lizards and spiders use similar hairy pads to create adhesive effects.[2] This indicates a favourable design for hairy pad adhesion.[2] Hairy pads can be classified as pulvillus, fossula spongiosa, and tenent hairs.[4] Hairy attachment pads employed few other features, such as flaw tolerance, lower sensitivity to contamination, and roughness.[6] Hairy attachment systems are typical for evolutionary younger and successful insect groups, such as Coleoptera and Diptera.[6] The density of hairs increases with increasing body weight.[6] An increase of the attachment strength in hairy systems is realized by increasing the number of single contact points.[6] Protuberances on the hairy pads of Coleoptera, Dermaptera, and Diptera belong to different types. Representatives of the first two lineages have socketed setae on their pads.[6] Setae can range in length from a few micrometers to several millimeters.[6]Dipteran outgrowths are acanthae, which are single sclerotized protuberances originating from a single cell.[6] The acanthae are hollow inside, and some have pores under the terminal plate, which presumably deliver an adhesive secretion directly to the contact area.[6] Hairy attachment pads of reduviid bugs,[8] flies [9] and beetles[10] secrete fluid into the contact area. The secretion contains non-volatile, lipid-like substances, but in some species it is two-phasic emulsion presumably containing water-soluble and lipid-soluble fractions.[6] Adhesion strongly decreases as the volume of the secretion decreases, which indicates that a layer of pad secretion that covers the terminal plates is crucial for generating a strong attractive force.[6] Data suggests that besides van der Waals and Coulomb forces flies rely on attractive capillary forces mediated by the pad secretion.[6] At low humidity, adhesion strongly depends on the amount of liquid deposited on the surface, and therefore contact duration.[11]
The hairy pad design has been argued to have a number of advantages over the smooth design, such as superior performance on rough substrates, effortless detachment, self-cleaning properties, and increased adhesion due to contact splitting.[3]
Unlike frog and lizard adhesive pads which are often dry, insects tend to have an associated fluid for adhesion. The fluid that is secreted has a special property of being composed of an immiscible mixture of hydrophilic and hydrophobic material.[12]
Adhesive foot pads only stick when pulled toward the body, but unstick when moved away from it, which allows for effortless and rapid detachment. Insects can do this actively through their claw flexor muscle, but in most cases, the foot is able to attach and detach passively, without the help of any nerves and muscles. (Bullock, Drechsler, & Federle, 2008)
Other uses for adhesives in insects
Adhesive chemical secretions are also used for predation defence, mating, holding substrates, anchoring eggs, building retreats, prey capture, and self grooming. Structures for use in repelling attackers or temporarily or permanently adhering to a substratum or a mating partner have been found in the developmental stages of the egg, larvae, pupae, and adult. Some species have developed adhesives for prey capture and some use adhesive glue for cocoon building. Adhesive glands of the head can involve mouthparts, antennae, the labial salivary glands, or species specific glands. A variety of glands, often located in the abdomen, can be used for defensive adhesion mechanisms.[4]
Insect adhesive glands
Epidermalglands and their secretions are highly diverse and vary in their function for: protection from adverse environmental conditions and microbialcontamination, regulation of water balance, communication with pheromones and alelochemicals, defense from predators and parasites, construction and making food accessible.[4]
Class 1 epidermal cells are the predominant glandular cell type for adhesive gland systems in insects with features that indicate either lipid or protein secretion. In class 1 cells for locomotion lipoidal secretion is most common, although the secretions are often mixtures of lipids with proteins and carbohydrates. Class 1 cells that are used for more permanent body or egg anchorage and for retreat building make use of protein-based secretions.[4]
Class 2 epidermal adhesive gland cells have only been found in the defence systems of Aphidoidea and Tingidae. Defensive adhesive secretions function mechanically and also develop a chemical irritant function caused by reactive substances of low molecular weight which combines within the sticky secretion to produce toxic glue.[4]
Class 3 epidermal adhesive glands are usually bicellular and consist of a terminal secretarily active cell and an adjacent canal cell that surrounds the cuticular conducting duct.[4]
Hundreds of gland cells and glandular units are contained in class 1 or 3 and might aggregate to form whole gland organs so as to discharge large amounts of a secretion. Adhesive cells used for locomotion are all class 1 epidermal adhesive cells. Class 3 epidermal adhesive cells may play a role in some hairy adhesive pads, but this has not yet been confirmed. Some adhesive glands that are used for locomotion are also used for capturing or holding on to prey (Fac, 2010). The secretion of some class 1 cells and class 3 cells are mixed in the subcuticular or intracuticular spaces. They may also be mixed in the larger glandular reservoirs before being discharged, which allows the formation of complex structural mixtures as well as chemical reactions between the components of the mixture. Gland cells used by female insects for gluing eggs to a substratum during oviposition have not been well studied. Glands used for sticking eggs to surfaces have been observed to be of the class 1 type. Adhesive glands are involved in the production of silks, which are produced by a variety of dermal glands for building shelters, cocoons, and supporting sperm.[4] Class 1 cells are often applied for this purpose.
Adhesive secretions
Most bioadhesives use polymers (carbohydrates and proteins) to create the adhesive and cohesive strength.[4] Natural adhesives used by both plants and animals are composed of only a few basic components, such as proteins, polysaccharides, polyphenols, and lipids that are mixed in various combinations.[4] Natural adhesive chemical and micromechanical functions are often not well understood.[4] Adhesives that are for mechanical work are often composed of high-molecular compounds containing proteins, resins, mixtures of long-chain hydrocarbons and mucopolysaccharides, or waxes.[4] Defensive adhesive secretions often combine their mechanical effect with a low molecular weight chemical irritant to deter predators.[4]
There is a great diversity of aliphatic compounds in insect adhesive secretions. Aliphatic compounds are a major constituent of secretions for some locomotion organs in insects, and they are also involved in the formation of defence secretions. Adhesives of this type contain only limited amounts or no polar components such as fatty acids, esters, and alcohols. Often these compounds are temperature sensitive.[4] Very little research has been done on classifying and identifying carbohydrates within insect adhesive secretions. So far, glucose, trehalose and mucopolysaccharides that contain glucose, galactose, mannose, beta-glucopyranose, and/or (N-acetyl-beta-) glucosamine have been identified as components of insect adhesives. Carbohydrates have been found in defense secretions as well as for sticking eggs together.[4]Aromatic compounds have been identified in the adhesive defence secretion of termites and ants. It is also thought to be used by butterflies to secure eggs.[4] Insect adhesives contain a broad spectrum of isoprenoids. These compounds have been found in defense mechanisms in some species such as termites.[4]Amino acids, peptides, and proteins are nearly always found in insects' adhesive secretions. They are employed for adhesion across many functions such as defense, locomotion and cocoon building.[4]
Arachnid adhesion
Spiders have independently evolved hairy adhesive pads. Their pads do not use an associated fluid and are much similar to many lizards, not like the hairy pads that are used by insects.[3]
Convergence with tetrapods
Smooth adhesive pads are an example of convergent evolution between amphibians (geckos and frogs), arthropods, and mammals (possum).[7] The mechanisms involved even appear to be similar.[1] This could indicate that this method of locomotion has found its optimal form in many species of animals.[7] Hairy attachment systems of the gekkonid lizards and spiders do not produce fluids; these organisms rely on van der Waals interactions for the generation of strong attractive forces.[3][6]Tree frog toe-pads are made of columnar epithelial cells that are separated from each other at the apices.[1] Pores for mucous glands open into the channels that are between the cells which create a toe pad epithelium that has an array of flat topped cells with mucus-filled grooves between them.[1] The purpose of having cells separated at the tip is to allow the toe to conform to the structure it will adhere to.[1] The hexagonal design around the outside of the cells (similar to the crickets) is likely to allow for the mucus to spread evenly over the cell.[1] Smooth adhesive pads are found in arboreal possums, which are marsupials that glide between trees.[1] The possum is also capable of using smooth adhesive pads to climb vertically, making use of large toe pads.[1] The pads consist of an epidermal layer of stratified squamous epithelium, with the outer most layer's cells being flattened.[1] The pad has alternating ridges and grooves with sweat glands emptying into the grooves providing fluid for wet adhesion.[1]Bats have also evolved adhesive pads separately. Some bats make use of an adhesive appendage, while others have suctioning adhesive organs.[12]
Importance to humans
Some researchers propose using the advanced locomotive mechanisms seen in arthropods for modelling robotic movement to create maximally efficient movement.[3][6][7] Currently insect adhesive pads still outperform most artificial adhesives with respect to rapid controllability.[2] Some researchers also suggest using arthropod-based adhesive mechanisms for more effective tape and binding tools.[4][6] Additionally, some research indicates that the wrinkling effect that occurs in human fingers when submerged in water acts to increase grip on wet objects.[13] The mechanism is unknown but it may be due to changes in adhesion properties of the finger pads. By examining the properties of bioadhesion, finger pad adhesion can be better understood. However, this study on increased finger pad dexterity from wrinkling has been heavily disputed.[14] Despite this, it can be argued that a better understanding of insect adhesion mechanisms can help guide the development of better adhesives for human mobility and technology, as well as inform a better understanding of human finger function.
Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remnants of the old, empty exoskeleton are called exuviae.
Geckos are small, mostly carnivorous lizards that have a wide distribution, found on every continent except Antarctica. Belonging to the infraorder Gekkota, geckos are found in warm climates throughout the world. They range from 1.6 to 60 centimetres.
The gastrotrichs, commonly referred to as hairybellies or hairybacks, are a group of microscopic (0.06–3.0 mm), cylindrical, acoelomate animals, and are widely distributed and abundant in freshwater and marine environments. They are mostly benthic and live within the periphyton, the layer of tiny organisms and detritus that is found on the seabed and the beds of other water bodies. The majority live on and between particles of sediment or on other submerged surfaces, but a few species are terrestrial and live on land in the film of water surrounding grains of soil. Gastrotrichs are divided into two orders, the Macrodasyida which are marine, and the Chaetonotida, some of which are marine and some freshwater. Nearly 800 species of gastrotrich have been described.
In molecular physics and chemistry, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and therefore more susceptible to disturbance. The van der Waals force quickly vanishes at longer distances between interacting molecules.
In biology, setae are any of a number of different bristle- or hair-like structures on living organisms.
Adhesion is the tendency of dissimilar particles or surfaces to cling to one another.
In surface anatomy, a lamella is a thin plate-like structure, often one amongst many lamellae very close to one another, with open space between. Aside from respiratory organs, they appear in other biological roles including filter feeding and the traction surfaces of geckos.
Synthetic setae emulate the setae found on the toes of a gecko and scientific research in this area is driven towards the development of dry adhesives. Geckos have no difficulty mastering vertical walls and are apparently capable of adhering themselves to just about any surface. The five-toed feet of a gecko are covered with elastic hairs called setae and the ends of these hairs are split into nanoscale structures called spatulae. The sheer abundance and proximity to the surface of these spatulae make it sufficient for van der Waals forces alone to provide the required adhesive strength. Following the discovery of the gecko's adhesion mechanism in 2002, which is based on van der Waals forces, biomimetic adhesives have become the topic of a major research effort. These developments are poised to yield families of novel adhesive materials with superior properties which are likely to find uses in industries ranging from defense and nanotechnology to healthcare and sport.
Scopulae, or scopula pads, are dense tufts of hair at the end of a spiders's legs. They are found mostly on hunting spiders, for example Salticidae and Sparassidae. Scopulae consist of microscopic hairs, known as setae, which are each covered in even smaller hairs called setules or "end feet", resulting in a large contact area.
Calliphora vomitoria, known as the blue bottle fly, orange-bearded blue bottle, or bottlebee, is a species of blow fly, a species in the family Calliphoridae. Calliphora vomitoria is the type species of the genus Calliphora. It is common throughout many continents including Europe, Americas, and Africa. They are fairly large flies, nearly twice the size of the housefly, with a metallic blue abdomen and long orange setae on the gena.
Insect physiology includes the physiology and biochemistry of insect organ systems.
Bioadhesives are natural polymeric materials that act as adhesives. The term is sometimes used more loosely to describe a glue formed synthetically from biological monomers such as sugars, or to mean a synthetic material designed to adhere to biological tissue.
Entobdella soleae is a monogenean (Platyhelminth) skin parasite of the common sole, Solea solea, an important food fish. They are approximately 2 to 6 mm in length. It is flat, translucent, and has a large, disc-shaped haptor, a posterior organ used for semi-permanent attachment to the host. Typically, 2-6 parasites are found on wild sole, but in intensive fish farms this can rise to 200-300 parasites per fish, causing skin inflammation and sometimes death of the sole. E. soleae can live up to 120 days in seawater.
The feet of geckos have a number of specializations. Their surfaces can adhere to any type of material with the exception of Teflon (PTFE). This phenomenon can be explained with three elements:
Snail slime is a kind of mucus produced by snails, which are gastropod mollusks. Land snails and slugs both produce mucus, as does every other kind of gastropod, from marine, freshwater, and terrestrial habitats. The reproductive system of gastropods also produces mucus internally from special glands.
Bio-inspired robotic locomotion is a fairly new subcategory of bio-inspired design. It is about learning concepts from nature and applying them to the design of real-world engineered systems. More specifically, this field is about making robots that are inspired by biological systems, including Biomimicry. Biomimicry is copying from nature while bio-inspired design is learning from nature and making a mechanism that is simpler and more effective than the system observed in nature. Biomimicry has led to the development of a different branch of robotics called soft robotics. The biological systems have been optimized for specific tasks according to their habitat. However, they are multifunctional and are not designed for only one specific functionality. Bio-inspired robotics is about studying biological systems, and looking for the mechanisms that may solve a problem in the engineering field. The designer should then try to simplify and enhance that mechanism for the specific task of interest. Bio-inspired roboticists are usually interested in biosensors, bioactuators, or biomaterials. Most of the robots have some type of locomotion system. Thus, in this article different modes of animal locomotion and few examples of the corresponding bio-inspired robots are introduced.
Collocyte is a term variously applied in botany and zoology to cells that produce gluey substances, or that bind or capture prey or assorted objects by securing them with gluey materials and structures, or that simply look smooth and gelatinous. Literally the word means "glue cell", and it has a number of poorly distinguished synonyms, such as colloblast.
Sticky pads are friction devices used to prevent objects from sliding on a surface, by effectively increasing the friction between the object and the surface.
Microsuction tape is a material for sticking objects to surfaces such as furniture, dashboards, walls, etc. One side is usually attached to the base surface by a classical adhesive. Objects are attached to the other side by pressing them against the tape. They stick to the tape due to small bubbles (cavities) on the surface of the tape. These contain air, which is squeezed out when the surface of an object is pressed against the surface of the tape. Due to sealing properties of the material, when the object is pulled off the surface, a vacuum is created in the cavities. Due to external air pressure, this creates a force that prevents the object from being removed from the surface, a mechanism similar to that of a suction cup.
Nano tape, also called gecko tape; marketed under the name Insanity Tape, is a synthetic adhesive tape consisting of arrays of carbon nanotubes transferred onto a backing material of flexible polymer tape. These arrays are called synthetic setae and mimic the nanostructures found on the toes of a gecko; this is an example of biomimicry. The adhesion is achieved not with chemical adhesives, but via van der Waals forces, which are weak electric forces generated between two atoms or molecules that are very close to each other. So far there is little evidence to support nano tape being recyclable in the same way as plastic bottles, but it is reusable. More data is needed to know how environmentally safe nano tape is.
↑ Edwards, John S.; Tarkanian, M. (1970). "The adhesive pads of Heteroptera: a re-examination". Proceedings of the Royal Entomological Society of London A. 45 (1–3): 1–5. doi:10.1111/j.1365-3032.1970.tb00691.x.
↑ Walker, G.; Yulf, A. B.; Ratcliffe, J. (1985). "The adhesive organ of the blowfly, Calliphora vomitoria: a functional approach (Diptera: Calliphoridae)". Journal of Zoology. 205 (2): 297–307. doi:10.1111/j.1469-7998.1985.tb03536.x.
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