The cuticular structures on the surface of the proboscis of a butterfly form a sealed coil against the head of the insect by interlocking in various directions.

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The tubular feeding structure (i.e., proboscis) of a butterfly remains tightly coiled against the head of the insect when it is not feeding. The proboscis is made up of two long, tubular structures known as galea. The coil is able to remain unfurled through the interlocking structures located on both the dorsal and ventral sides of the galea tubes. These scale-like structures, referred to as legulae, are varied in size and directional orientation so that they overlap and fit together tightly (much in the same way puzzle pieces lock together). This interlocking of structures creates a sealed cylinder that is unaffected by flight or other movements. These structures are unique to insects and help conserve energy by allowing the insects to use no muscle tension and exertion to keep the proboscis coiled (it was previously thought that the insects had to use their galeal muscles to hold the proboscis in a coiled shape).

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References

“Originating from the tubular stipes, the coilable galeae are linked to each other by pointed cuticular structures on the dorsal and ventral sides of the median food canal. The outer surface of each galea bears microtrichia, few uniporous sensilla basiconica, and bristleshaped aporous sensilla trichodea…The short double-tube proboscis is held together by special scales that extend from the ventral side of each galea.” (Krenn 2010:310)

“The galeae are held together by rows of interlocking cuticular processes, termed legulae, that exhibit different shapes on the dorsal and ventral sides of the food canal…The dorsal linkage consists of flat horizontally extending lancet-shaped plates. They overlap those of the opposite galea to form the roof of the food tube. Their principle variation in Glossata concerns the number of rows…Single-celled glands are situated in the galeal epidermis and empty toward the dorsal legulae; their secretion serves to lubricate and seal the dorsal linkage in butterflies…Two rows of ventral legulae are plesiomorphic in Eulepidoptera; in certain lineages of Ditrysia the lower legulae are modified into hooks that interlock with their counterparts, like a zipper, providing firmness to the proboscis linkage…”(Krenn 2010:312)

Journal article
Feeding Mechanisms of Adult Lepidoptera: Structure, Function, and Evolution of the MouthpartsAnnual Review of Entomology, 55: 307-327January 1, 2010
Krenn HW

“The proboscis is coiled in a tight spiral beneath the head…The number of coils varies from 3.5 to 7 coils in the investigated species…The dorsal and the ventral galeal walls of the consecutive coils are in close contract with each other throughout the entire length. The outermost coil rests against the labium on the ventral side of the head.” (Krenn 1990:106)

Journal article
Functional morphology and movements of the proboscis of Lepidoptera (Insecta)Zoomorphology, 110(2): 105–114March 1, 1990
Krenn HW

“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)

Book
The grand design: Form and colour in animalsJanuary 1, 1970
Sally Foy

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Living System/s

Organism
Morgan's Sphinx MothXanthopan morganiiSpecies

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