Spider bares its fangs
(appeared in June 2014)

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The spider packs a lean machine in its stinging hook, says S.Ananthanarayanan.

The spider feeds almost entirely on insects and other spiders, usually trapped in the spidersí web and incapacitated by a jab of the spiderís fang, which injects the victim with venom. Apart from this organ, the spider does not have a real jaw or teeth and it partly digests its food in situ, and takes only liquids into its proper digestive system.

The spidersí prey, in turn, use ways to escape from the web, which the spider impedes by throwing out further strands while the victim struggles, and the prey also have hard body covering, to save themselves from the deadly jab of the spidersí fang. The fang, then, needs to be hard and strong, and also not too rigid, so that it can take the compression, bending and twisting in the course of an attack. In a report in the journal, Nature Communications, Benny Bar-On, Friedrich G. Barth, Peter Fratzl and Yael Politi at the Max Planck Institute of Colloids and Interfaces, at Potsdam, Germany and the Faculty of Life Sciences at Vienna, examine the design and architecture of the spider fang and suggest that the understanding may ďlead to the development of novel bio-inspired engineering materials with superior characteristics.Ē

The researchers note that natural structures are adapted to the difficult and conflicting demands placed on them, primarily by managing how the material of their components is distributed. This could be both by having the most suitable shape and also by using different grades of material in the construction of portions of limbs, as well as differences in the microstructure. The threads by which some species of clam, for instance, attach themselves to rocks, are made up of two kinds of structural protein, and the ratio of the mix changes from the nearer end to the farther end, making the farther end stiffer, while the nearer end is flexible, so that the thread can stand heavy impact loads.

The composition of the turtle shell and the beaks of the squid similarly vary the materials and the way they are laid out, to optimize functions of hardness and lightweight. The Potsdam and Vienna group modeled the same features of the fang of a large, wandering species of spider and tried out different combinations of structure and distribution of materials, to understand how the actual form in the natural spider fang made it more functional.

The spiderís fang, which is made of the material as horns and nails, is a part of the spiderís mouth parts and is connected to the body by a hinge joint and a set of muscles. While the fang can be used like a limb, for cleaning or widening the egg sac to release baby spiders, its main function is to pierce the hard armour of insects and inject them with venom, as also to hold down the prey, like a claw. For this exacting purpose, the tip of the fang needs to be hard, and the whole fang needs to resist damage that could arise over repeated encounters with insect prey.

The fang itself is a hollow, curved cone, curved to be able to strike with the greatest impact, hollow to contain the venom duct and conical for mechanical strength. The material of construction is chitin, a fibrous polymer, embedded in a protein rich framework. The researchers used models of the fang, based on data already available, and analysed the mechanical properties, of stiffness and hardiness, which the model assumed with different forms of internal architecture.

The first observation was that the shape of the centre line of the natural fang is that of a circle, with the venom duct lying along the curve. The width of the fang reduces uniformly, over the length of the quarter circle, to end in a sharp point at the tip. The effect of the curved shape was to lend maximum impact, like the striking of a hammer, for its particular function, and unlike the mosquito stylet or the bee stinger.

And then there was the tapering, conical profile.

Trying out the model the spider fang revealed that the hollow, conical shape of the natural fang, where the width of the fang increased uniformly as one moved from the tip to the base, as the picture shows, is one which provides the best possible stiffness, while using the least material. The combination of the curve and the tapering, hollow cone gives the fang the highest efficiency in being able to deliver a powerful blow, with a weapon of the greatest strength, and yet at the least cost

The widening of the shape as one moves away from the tip also increases the capacity of the fang to bear the load of the strike. While the high pressure of impact is borne by the tip, which has a harder material composition, the load is distributed over a larger surface towards the base of the fang, permitting softer and less rigid structures. This design permits ten times the load that a needle-like structure can bear.

In addition to high impact, the fang is also subject to shear and twisting forces. This challenge is met by a graded pattern in the way the material of the fang is arrayed. The tip of the fang, whose composition is rich in atoms of metals, to give it greater stiffness, has fibres predominantly oriented in parallel, as opposed to a plywood type orientation. This provides for greater ability to penetrate and moderate resistance to shear forces, as arise in twisting. The fibre orientation gradually changes to more of the plywood type as one moves towards the base, which makes for damage resilience.

The study, carried out with models, where different factors, like the thickness of the wall, the chitin-protein mix, or the parallel-plywood structure could be varied, has revealed that at the relevant scale, the natural spider fang architecture is the best adaptation for its bio-mechanical function. Understanding the mechanics of the spider fang would help better appreciate what pressures drove the evolution of other sharp-edge structures, like the scorpion stinger, or the elephant tusk. While this study was at the larger scale distribution of material and structure, the authors of the paper say that further studies at the molecular level may lead to greater insight.

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