Science Literature

It is well known that orb-weaving spiders put droplets of adhesive all over their webs to catch prey. Although there have been many attempts to study the nature of these adhesives, it is only recently that experiment designs have allowed the mechanism of adhesion to be analysed properly. Single adhesive droplets have been probed at varying extension rates.

"Here, by directly probing single adhesive droplets used by spiders, we demonstrate the importance of the mechanics of adhesive in dramatically enhancing adhesion. We show that glue drops function as a viscoelastic material instead of as a viscous material and that the elasticity of the principle adhesive in this system, the glycoproteins, increases adhesion by two orders of magnitude in comparison with capillary forces, thus putting to rest the old notion of the adhesive being viscous."

Lower Cretaceous spider's web. "The advanced structure of this fossilized web, along with the type of prey that the web caught, indicates that spiders have been fishing insects from the air for a very long time." (Credit: American Museum of Natural History, source here)

The glycoproteins are cross-linked (either physically or chemically) enabling forces to be transmitted efficiently. At slow speeds, the pull-off forces are low; but at high speeds, the forces rise dramatically. Thus, when an insect first impacts the web, it is moving fast - and the web adhesive produces its largest gripping force. This allows the spider time to subdue the insect - and by then the adhesive forces have reduced in magnitude so that the spider can remove the insect from the web with relative ease. As in all biomimetic studies, these are coveted properties that we wish to emulate:

"This finding should significantly benefit the development of synthetic adhesives for biomedical, orthopedics and wound-healing applications. The understanding of how spiders use this unique glue will allow scientists to develop reversible adhesives that work in the presence of water," says Dhinojwala.

Lee points out that spider adhesive is a smart material, and that we are only beginning to understand its secrets.

"Man-made glues are mono-functional - their material properties are designed to stick one thing to another, and that's it. But in Nature Communications, Sahni et al. report that the 'glue' droplets that coat spiders' webs are multi-functional. Depending on the rate at which they are extended, the droplets act either as a viscous adhesive or as a rubber-like elastic solid." [. . .]
"But in-depth knowledge of the molecular structure of the glue droplets on spiders' webs is lacking. [. . .] A series of studies investigating the molecular content and supramolecular assembly of glue droplets is therefore required."

Whilst this research is a splendid example of empirical science in action (understanding how the natural world operates), the issue of origins is raised in this paper. An evolutionary framework is adopted, not because the research is dependent on that framework, but because papers that refer to design (especially "well-designed" features) apparently need to affirm non-intelligent causation. So, the first paragraph of the paper begins with this:

"Nature has evolved a myriad of well-designed adhesives that assist in locomotion, self-defence and prey capture. Geckos use micron-sized hairs as dry adhesives for locomotion. Mussels secrete specialized proteins to stick under water. Modern orb-weaving spiders use micron-sized glue droplets laid on a pair of viscoelastic axial silk for catching prey."

The exquisite design apparent in spider silk does not just relate to its remarkable strength, but to its ability to manage water and also, as noted in this blog, to the properties of the applied adhesive drops. In all these cases, the designs are not easy to reproduce. Large amounts of research funds are devoted to mimicking the natural products. It is not a trivial exercise! Even with years of intelligent activity by teams of researchers, problems remain unsolved. No explanation of origins that attributes all this complexity to natural causation has succeeded in convincing anyone who has not already adopted the ideology of evolutionary theory. Add to this the evidence that the first spiders appeared abruptly in the fossil record: the Mesothelae appear in the Carboniferous essentially identical to living forms. Furthermore, although appearing in Mesozoic and Cenozoic deposits, other spider families are represented which are also comparable with modern specimens. The evolutionary ancestors of the whole group is unknown (see here). It is a shame that a paper that marks a significant step forward in understanding the complexity of spider web stickiness should be mixed up with assertions about origins that are so lacking in substance.

Viscoelastic solids explain spider web stickiness
Vasav Sahni, Todd A. Blackledge and Ali Dhinojwala
Nature Communications, 1, No.19, 1-4, (17 May 2010) | doi:10.1038/ncomms1019

Abstract: Modern orb-weaving spiders have evolved well-designed adhesives to capture preys. This adhesive is laid on a pair of axial silk fibres as micron-sized glue droplets that are composed of an aqueous coat of salts surrounding nodules made of glycoproteins. In this study, we measure the adhesive forces required to separate a small microscopic probe after bringing it in contact with a single glue droplet. These forces are highly rate-dependent and are two orders of magnitude higher than the capillary forces. The glycoproteins in the glue droplets behave as a viscoelastic solid and the elasticity is critical in enhancing adhesion caused by specific adhesive ligands. These results have important implications in mimicking bioadhesives.

See also:

Lee, H. Intelligent glue, Nature, 465, 298-299 (19 May 2010) doi:10.1038/465298a

Researchers discover spider webs' true 'sticking power', PhysOrg.com (May 17, 2010)

UA researchers discover true sticking power of spider webs (video)