Spiders: Mystery of the biting claws revealed

Optimal shape and layered construction makes Bei klauen extremely resistant and hard

Hunting thanks to material projection: Cupiennius salei, a tropical hunting spider, pierces the shell of prey insects with a poison claw. © R. Barth
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Strong bite: With their sharp, curved mouth claws, spiders can pierce even hard insect tanks. What makes the claws so stable, researchers have now revealed: A clever combination of optimal form and complex microstructure makes the curved biter stiff and resistant at the same time. This could also give valuable ideas for artificial materials, according to the researchers in the journal "Nature Communications".

From a close-up, the spider's mouthparts are pretty awesome: large, curved claws end in a sharp point that effortlessly pierces even the hard armor of an insect. Once the armor has been breached, the spider injects its crippling and decomposing poison into its prey, the fate of which is sealed. The exciting thing about it: The curved claws of the spiders are basically made of the same material as the tanks of their prey: made of chitin, embedded in a protein matrix. Nevertheless, they are harder and more resistant.

Enormous powers

"During an attack, the tips of these claws experience mechanical forces that are strong enough to compress, bend and twist them, " explain Benjamin Bar-On of the Max Planck Institute for Colloids Research in Potsdam and his colleagues. The bite strength of a spider can reach up to a Newton, as experiments show. Nevertheless, the claw does not break off and proves to be stable even after repeated use. After all, the claw also has to hold for about a year, because that's how long it takes for the spider to regenerate its entire outer shell and catching claws on its next molt.

The secret behind the great stability of this natural hypodermic needle has been studied by Bar-On and his colleagues using the example of the hunting spider Cupiennius salei. Originally native to Central and South America, this spider was introduced to Germany in the 1960s. With a length of 3.5 centimeters and a leg span of 10 centimeters, the nocturnal Lauerjäger is rather large and hunts for larger insects and even small mammals.

Bite claw in stress test: Red areas indicate increased load Bar-On et al. / Nature Communications

Optimal shape and curvature

For their study, the scientists first use micro-tomographic images to determine the exact shape and structure of the hollow claws of this hunting spider, traversed by a venomous channel. It turned out that the spider claws are almost ideally shaped to withstand strong forces: on the one hand, they are tapered in shape and their diameter, but also their wall thickness increases significantly towards the base. This distributes the forces acting on the tip and prevents claws from breaking off at their base, the researchers report. display

On the other hand, the curvature of the claws corresponds exactly to the marginal line of a quarter circle, this also gives stability at least if the associating movement exactly follows this line. "But the attachment mechanism of the spiders is more complex and the jaws can also be moved in non-circular tracks, " says Bar-On and his colleagues. That alone is therefore not enough to explain the resistance of the claws.

Stable through stratified fibers

In the next step, the researchers therefore analyzed the microstructure of the spider claws. As they discovered, the claw wall consists of several superimposed layers of chitin nanofibers in a protein matrix in principle, as expected. However, these nanofibers are arranged differently in each layer: layers with parallel chitin fibers alternate with layers of mutually twisted fibers.

This layered architecture gives the spider claws additional stability, as the researchers explain. Because the parallel, aligned in the longitudinal direction of the claw layers protect them from bending and breaking. On the other hand, the layers with the twisted fibers have a high resistance to heavy forces at least between five and 100 times more than the parallel layers. This protects the spider claw from twisting and splintering.

Stimulation of artificial materials

"This shows that the claws of the spiders are inherently perfectly adapted, " say the researchers. They have a great resistance and, because of their structure, they are firm and stiff enough to pierce insect screens. At the same time, the spider claws are another example of how nature uses layered composite materials to obtain optimal mechanical properties.

Likewise, for example, the light but solid shell of turtles are constructed, the hard beaks of octopuses, and the extremely tear-resistant retaining fins of mussels. According to the researchers, these biological composite materials also provide valuable tips for the production of new, synthetic materials. Nature conservation is worthwhile. (Nature Communications, 2014; doi: 10.1038 / ncomms4894)

(Nature, 28.05.2014 - NPO)