The two-part proboscis of a butterfly unfurls to form a flexible feeding tube via two half-cylinders that curl over and interlock.
"The butterfly's proboscis, for example, is formed from the two soft 'lips' on the maxillae, which have become enormously elongated. When in use the two parts, semicircular in section, curl over and interlock to form a stiff but flexible cylindrical tube…" (Foy and Oxford Scientific Films 1982:163)
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