A complex arrangement of materials in the weevils' slender snouts make them strong and flexible.


The slender snout that protrudes from the acorn weevil’s head may be more fantastic than a unicorn horn. It’s almost as long as the weevil’s 0.4-inch (1-cm) body, and it extends straight outward, then gradually curves 90 degrees downward.

At the snout’s end are sharp mandibles that scissor into an acorn’s hard shell. The weevil raises its forelegs, tilts its head to apply downward pressure, and rotates around the hole, using the snout as a drill. While the weevil sucks up nutrients through the snout’s hollow center, it excavates a thin, straight chute into the acorn’s depths, into which the insect will later deposit eggs that hatch into larvae.

During the excavation process, the curved part of the snout straightens, bending to the brink under the strain, but it does not break. When the weevil withdraws its snout, it instantly flexes back into its curved shape, no worse for wear.

If the snout were to shatter under the repeated, extreme stress of drilling, it would mean death to the weevil and its future generations. How can this slim, seemingly fragile snout be so strong and yet so flexible?

Image: Pxhere / Public Domain - No restrictions

Acorn weevils have a long slender snout that they use to extract food from acorns while boring a thin tunnel to lay eggs into. The snout has to be both strong and flexible.

Image: Nik Chawla, Nico Franz, and Andrew Jansen/ASU / Copyright © - All rights reserved

A microscopic image of a cross-section of an acorn weevil’s snout reveals a complex composite material, with multiangled fibers embedded in a protein matrix, that gives the snout flexibility and strength.

The Strategy

The secret lies in the complex composite material that the snout is made of. It consists of tough fibers of chitin embedded in a matrix of s. This material is arranged in thin layers that align like steps in a spiral staircase. Each layer’s outer edge is rotated slightly forward from the one below it, so that they cumulatively form a .

In general, the long fibers best withstand breaking when pressure comes at them from the direction parallel to their length. Their strength diminishes, and the risk of breaking mounts, when force comes in from other angles. But scientists think that a confluence of factors work together to keep the material strong and flexible. The small angle differences between adjacent layers, their tightly packed spacing, and the broad spectrum of fiber directions encircling it combine to absorb and distribute incoming force through many layers, so that no one layer bears the brunt of an impact.

In addition, the multi-sided, multi-angled structure ensures that any cracks that do form between layers will hit a wall and have to change directions, unable to propagate far along a path of least resistance. In the same way, any multiple tiny cracks that form will be stymied from coalescing into long, wide ruptures.

The thickness of the chitin fibers also plays a role in changing the materials’ properties. The more brittle outer layer of the snout, called the exocuticle, has fibers measuring s in diameter, while the inner layer, called the endocuticle, has fibers that are 1,000 times thicker, making it much stronger. The endocuticle gets progressively thicker down the length of the snout to fortify the areas where the snout must be able to bend under extreme pressure without breaking.

The Potential

In general, the properties that make materials rigid and strong are the opposite of those that make materials flexible and fracture-resistant. Scientists are investigating the intricacies of weevil snout microstructure to design materials that optimally balance both properties in a variety of combinations that suit different uses.

Such materials would be a boon for a wide range of products, including ships, cars, airplanes, buildings, textiles, packaging, housewares, medical devices, and replacements for plastics—all of which must be strong, flexible, and unbreakable in the face of turbulence, pressure, earthquake shaking, and other forces that can bring them to a breaking point. They may also provide a less-polluting alternative for plastics.


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The acorn weevil's extraordinary life cycle is wholly dependent on its unique snout, featured in this excerpt from BBC Earth. 

Last Updated March 18, 2020