The proboscis of the butterfly unwinds from a tightly coiled position via muscular contraction and a hydraulic, step-wise mechanism.

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During rest, the tube-like feeding structure of the butterfly (i.e., the proboscis, equated to a "tongue") remains coiled tightly against the head. However, when the butterfly moves to feed upon the nectar of a flower or something akin, the proboscis unfurls to extend downward into the flower's center. The uncoiling is initiated in the muscle closest to the head, the basal galeal muscle, which lifts the coils slightly to "unlock" it from its tightly held position. The coil begins to unwind via the contraction of other muscles throughout the proboscis, known as stipes muscles. This contraction places pressure on various areas throughout the feeding tube by compressing the stipial tube. This creates a step-wise reaction that further unfurls the tube through pressurized increases in the stipes' valve-like structures.

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"The uncoiling process relies on a hydraulic mechanism...The proboscis is ?rst unlocked from its resting position—it is elevated by the basal galeal muscle. Owing to its elasticity, the coil of the proboscis loosens somewhat. Further uncoiling is enabled by contraction of stipes muscles, whereby movements of the sclerotized part of the stipes compress the stipital tube. The repeated compressions of the stipes force hemolymph into the attached galea. Because the structures of the stipes form a valve, the proboscis is stepwise uncoiled when the hemolymph pressure inside the galeae increases. The increased internal pressure results in an outwardly arched dorsal wall in the uncoiled proboscis. The uncoiled proboscis normally assumes a bent position with a more or less distinct ?exion, termed the bend region or knee-bend. The bend region is probably caused by the elasticity of the proboscis. Increased hemolymph pressure can lead to a nearly straight position of the proboscis; in some cases, the distal proboscis bends slightly upward. During ?ower handling, the proboscis is elevated by contraction of the basal joint muscles, while forward or backward movements of the region distal to the bend region are caused either by an increase in pressure due to stipital pumping or by action of the intrinsic galeal musculature of the proboscis, respectively." (Krenn 2010: 315)

Journal article
Functional morphology and movements of the proboscis of Lepidoptera (Insecta)ZoomorphologyMay 6, 2005
Harald W. Krenn

Journal article
Feeding Mechanisms of Adult Lepidoptera: Structure, Function, and Evolution of the MouthpartsAnnual Review of EntomologyDecember 5, 2009
Harald W. Krenn

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