As children, many of us heard about how butterflies change through metamorphosis: a caterpillar wriggles and sheds its skin to make a tough chrysalis, from which a stunning butterfly eventually emerges. But what keeps chrysalises secured while the butterfly develops inside? Research published in ACS Biomaterials Science & Engineering indicates that even though the silk produced by caterpillars is weak and thin individually, they can skillfully weave it into support structures for their chrysalises that function like hook-and-loop fasteners and multi-strand safety tethers.
Silk is a natural fiber made of protein, most often connected with silkworms, but it is also created by various other insects, including butterflies. During their caterpillar phase, butterflies utilize silk to create tethers that help prevent falls, to bundle leaves for camouflage, and to weave small silk carpets for stability during metamorphosis. When they form a chrysalis, caterpillars use a special part called a cremaster, which can grasp this silk carpet to fasten the chrysalis securely to a branch. Some butterfly species even go further by creating a kind of seatbelt around their thorax for added security. But just how strong is this silk, and what kinds of structures are in these seatbelts and silk carpets? Researchers Qingyou Xia, Zhaoming Dong, and their team set out to discover the answer.
The scientists reared two butterfly species — Danaus chrysippus (plain tiger) and Papilio polytes (Common mormon) — alongside two types of silkworms, allowing them to create chrysalises or cocoons. They collected the silk threads used to attach chrysalises to branches and those forming the cocoons, measuring the thickness and tensile strength of each. Findings revealed that butterfly silk tended to be thinner and less strong than that of silkworms. Upon analyzing the structural proteins within the silks, they noted that butterfly silk contains fewer beta sheet structures compared to silkworm silk, which likely explains its relative fragility.
Nevertheless, the researchers observed that both butterfly species employ distinctive techniques to make effective use of their silk. The cremaster grips the silk carpet akin to a hook-and-loop fastener, with the design of the cremaster’s hooks ensuring a reliable attachment. When crafting their silk seatbelts, caterpillars twist together about 20 different strands into a rope-like structure, which enhances the strength by a factor of eight. The researchers believe their findings shed light on the ingenious methods caterpillars employ to secure themselves until they finally emerge as butterflies.
The authors acknowledge support from the National Natural Science Foundation of China, the National Key Research and Development Program of China, the Natural Science Foundation of Chongqing, and the Fundamental Research Funds for the Central Universities.
