When a caterpillar or silkworm gets ready to transform from a chrysalis into a winged adult butterfly, the chrysalis must be in perfect condition. Any overlooked structural flaws or weaknesses could be fatal to the insect, since it can’t emerge until it’s fully mature. But the chrysalis’ ability to stay in place is just as important. And new research shows that when it comes to metamorphosis, a butterfly’s delicate silk threads are one of the butterfly’s most powerful and reliable tools.
Researchers from Southwestern University in China recently studied the life cycles of two species of butterflies.Danaus Chrysippus (Heitora) and Swallowtail When the butterfly and silkworm larvae reached the chrysalis and cocoon-making stages, respectively, the team watched as the insects spun silk into web-like pads, then used arm-like appendages called cremasters to attach the chrysalis or cocoon to a branch.
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“We observed that the connection between the cremaster muscle and the silk pad resembles the hook-and-loop connection of a Velcro fastener, except that the cremaster muscle hook is an anchor type rather than a single hook,” the authors explain in their paper published July 22. ACS Biomaterials Science and Engineering“These connections remain strong and keep the pupa safe even when exposed to the elements.”
As the insects began to metamorphose, the team took silk samples and measured their diameter and tensile strength. Noting that the silkworm silk was stronger and thicker, the researchers carried out chemical analysis and found that the butterfly silk lacked as many beta sheets (a molecular structure found within certain proteins) as the silkworm silk.
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For this reason, butterflies build an extra layer of protection into their chrysalis: the caterpillar twists 20 silk threads together to create a tiny seatbelt-like cord that’s eight times stronger than a single thread, and then wraps it around its thorax inside the chrysalis to secure itself in place.
“We found that butterfly pupae develop unique body features to establish a secure bond with the silk,” the team wrote in a study summary, “which allows the butterfly to metamorphose efficiently and survive harsh weather and environmental conditions.”
This new research highlights the incredibly complex processes and planning required for successful metamorphosis, so the next time you see a cocoon or pupa outside, take a moment to marvel at their incredible strength.
