Butterflies anchor cocoons with silk Velcro and seat belts

A chrysalis must be perfect when a caterpillar or silkworm prepares to transform from pupae to winged adults. Because the insect cannot emerge until fully mature, overlooked structural defects or weaknesses can be fatal to them. But just as important is a chrysalis’ ability to stay anchored in place — and according to a new study, when it comes to metamorphosis, a butterfly’s delicate silk threads are some of its strongest and most reliable tools.

Researchers from China’s Southwestern University recently studied the life cycles of two butterfly species:Danaus chrysippus (common tiger) and Papilio polytes (common Mormon) – and two species of silkworms. When it came time for the butterfly and silkworm caterpillars to build their respective pupae and cocoons, the team watched as the insects spun each silk into net-like pads. Using an arm-like appendage called a cremaster, they then attached their chrysalis or cocoon to a branch.

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“It was observed that the connection between the cremaster and the silk pad was similar to that between the hook and loop of a Velcro closure, except that the cremaster hook is shaped like an anchor rather than a single hook,” the authors explain in their article published in July. 22 inches ACS Biomaterials Science and Engineering. “Such a connection remains secure and guarantees the safety of the dolls during exposure to wind and rain.”

As the insects began to undergo metamorphosis, the team collected silk samples to measure their diameter and tensile strength. After noticing that silkworm silk was stronger and thicker, researchers conducted a chemical analysis to determine that the butterfly silk lacked as many beta sheets (molecular structures in certain proteins) as silkworm silk.

[Related: This butterfly hybrid thrived despite evolutionary odds.]

This gives the butterflies extra protection in their pupae. The caterpillars twist about twenty individual silk strands into a small belt that resembles a seat belt and is eight times stronger than a single thread. They then wrap the rope around their chest inside the doll to hold them in place.

“We found that butterfly pupae develop unique body features to establish secure connections with silk,” the team wrote in the study’s summary. “This allows them to effectively metamorphose and withstand harsh weather conditions and environments.”

The new study highlights the intense complexity and planning required to successfully undergo a metamorphosis. So the next time you see a cocoon or chrysalis outside, pause for a moment to admire their incredible strength.