CRAFTING COMPOSITE COCOONS TO SUIT NEEDS

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When the three little pigs were building their homes to protect themselves from the big bad wolf, none of them thought of using silk. It's understandable – we associate silk with fine clothes and not with its role in protective cocoons. Amazingly, silk is just one of two building materials that make up these cocoons, the other is a glue-like protein called sericin. While each component on its own is not very durable, put the two together and you have a resilient composite material. All cocoons are constructed to the same design, with layered sheets of entangled silk fibres held together by sericin, where the inner layers become more and more compact. However, by using different amounts of glue, cocoons with different properties can be made. David Porter and his colleagues from the University of Oxford, UK, decided to compare the properties of two very different cocoons from the moths Antheraea pernyi and Opodiphthera eucalypti (p. 2648).To begin, the team cut tiny squares from each cocoon and measured how well they coped with being stretched. They found that O. eucalypti cocoons, which use the most glue and are the least porous, were the strongest, withstanding more force. However, because of the glue, the multiple layers acted as one thick layer and when it failed, the stress penetrated all layers, causing a clean break. Although A. pernyi cocoons started failing under lower stress, the breakdown was more gradual, with the fibres gradually disentangling.Next the team used a rod to mimic a predatory pecking beak and pressed down onto the side of each cocoon, measuring how each cocoon deformed. Using a mathematical model, the team worked out that stretching was greatest on the inside convex surface of the indentation, hence why the innermost layer is the least porous and strongest. The team also found that by using less inter-layer glue, the individual layers could detach in A. pernyi cocoons, which the team thinks might help prevent stress transmitting between layers. Their results suggest that if your nemesis is a peckish bird, then less is more when it comes to gluing your cocoon together. While O. eucalypti cocoons would not stand up to a bird's beak, they may be more suited to different threats such as water or bacteria, where a strong barrier is required.