Glue Protects From Insect Bites

a bright yellow and red toad with the shape of a crucifix on its body rests in red sands

Biological Strategy

Crucifix toad

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Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

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Capture, Absorb, or Filter Organisms

Many living systems must secure organisms for food. But just as one living system must capture its prey to survive, its prey must escape to survive. This results in capture and avoidance strategies that include trickery, speed, poisons, constructed traps, and more. For example, a carnivorous plant called the pitcher plant has leaves formed into a tube that collect water. Long, slippery hairs within the tube face downward. When insects enter the tube seeking nectar, they lose their footing and slide inside, unable to climb out and escape being eaten and digested by the plant.

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Attach Permanently

A living system can conserve energy by attaching permanently to a particular site because it can take advantage of resources that come its way, rather than expending energy to move to resources. A permanent attachment, intended to last the lifetime of the living system, creates special challenges. For example, physical mechanisms, such as the anchor that holds a marine algae to the ocean’s bottom, must be able to withstand forces that can pull it off its substrate. Chemical mechanisms, such as a barnacle’s glue, must avoid both physical and chemical breakdown, such as being dissolved by water.

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Amphibians

Class Amphibia (“to live a double life”): Frogs, salamanders, toads

Amphibians live a “double life” by being able to survive both on land and in water. Unlike reptiles with dry, scaly skin, amphibians have smooth skin that needs to be kept moist. Many live in the water through development and start life with gills, and then transition to breathing with lungs. They can get even more oxygen through cutaneous respiration, or breathing through their skin. This class represents the first animal group to be able to efficiently walk on land—aided by the well-defined limbs that gave them a “leg up” on their finned competitors.

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The skin of Australian frogs of the genus Notaden protects from insect bites via a secreted glue, which gums up insect mouthparts.

“Two species of Australian frogs secret a sticky substance over their skin to protect themselves from biting insects. The glue jams up the insects’ jaws, causes them to stick to the frogs’ skin, and the frogs can later eat the stuck insects. Mike Tyler, an environmental biologist from the University of Adelaide who discovered the skin glue, teamed up with orthopaedic surgeon George Murrell of the University of New South Wales to test the glue in sheep with torn knee cartilage. They found that the glue hardens within seconds and sticks well, even in moist environments. When set, it is flexible and has a porous structure that should make it permeable to gas and nutrients, which would encourage healing. When used on the sheep, it worked well at holding damaged knee cartilage together. Working with colleagues at the Commonwealth Scientific and Industrial Research Organisation in Melbourne, the scientists have characterized a key component of the glue and are now developing a genetically engineered version of this .” (Courtesy of the Guild)

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Australia

Last Updated October 27, 2016

References

“When provoked, Notaden bennetti frogs secrete an exudate which rapidly forms a tacky elastic solid (‘frog glue’). This protein-based material acts as a promiscuous pressure-sensitive adhesive that functions even in wet conditions. We conducted macroscopic tests in air to assess the tensile strength of moist glue (up to 78 + 8 kPa) and the shear strength of dry glue (1.7 + 0.3 MPa). We also performed nanomechanical measurements in water to determine the adhesion (1.9-7.2 nN or greater), resilience (43-56%), and elastic modulus (170-1035 kPa) of solid glue collected in different ways. Dry glue contains little carbohydrate and consists mainly of protein. The protein complement is rich in Gly (15.8 mol %), Pro (8.8 mol %), and Glu/Gln (14.1 mol %); it also contains some 4-hydroxyproline (4.6 mol %) but no 5-hydroxylysine or 3,4- dihydroxyphenylalanine (L-Dopa). Denaturing gel electrophoresis of the glue reveals a characteristic pattern of proteins spanning 13-400 kDa. The largest protein (Nb-1R, apparent molecular mass 350-500 kDa) is also the most abundant, and this protein appears to be the key structural component. The solid glue can be dissolved in dilute acids; raising the ionic strength causes the glue components to self-assemble spontaneously into a solid which resembles the starting material. We describe scattering studies on dissolved and solid glue and provide microscopy images of glue surfaces and sections, revealing a porous interior that is consistent with the high water content (85-90 wt %) of moist glue. In addition to compositional similarities with other biological adhesives and well-known elastomeric proteins, the circular dichroism spectrum of dissolved glue is almost identical to that for soluble elastin and electron and scanning probe microscopy images invite comparison with silk fibroins. Covalent cross-linking does not seem to be necessary for the glue to set.” (Graham et al. 2005:3300)

“Current surgical glues and sealants are either protein-based, in which case they exhibit low bond strength, or synthetic, in which case they form rigid and impervious barriers that hinder wound healing. Our results show that the exudate from N. bennetti frogs rapidly and spontaneously forms a proteinaceous pressure-sensitive adhesive that functions well in wet environments. The hydrated solid is highly elastic and consists of a porous mesh that in clinical contexts should allow the diffusion of gases and nutrients. Since some of the cavity radii…equate to pores with diameters in excess of 10 µm, it is likely that the hydrated material will also permit a degree of cellular infiltration. Initial experiments suggest that the glue is highly biocompatible, and it has been used successfully to bond severed cartilage tissue both ex vivo and in vivo.” (Graham et al. 2005:3311)

Journal article

Characterization of a protein-based adhesive elastomer secreted by the Australian frog Notaden bennetti

Graham LD; Glattauer V; Huson MG; Maxwell JM, Knott RB; White JW; Vaughan PR; Peng Y; Tyler MJ; Werkmeister JA; Ramshaw JA

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Reference

Journal article

Antipredator mechanisms of Australian frogs

Williams CR, Brodie ED, Tyler MJ, Walker SJ

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