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Why is the extendable lower lip of an odonate larva called a mask?
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Odonata
Male jewelwing damselfly (Calopteryx splendens).
The extendable lower lip of an odonate larva is called a "mask" because, when the structure is in its resting position, it is folded beneath the larva's head and thorax, where it often obscures the front of the head.12
This specialized, prehensile labium is a fusion of the larva's third pair of mouthparts. When prey is detected, the mask can be extended almost instantaneously—in under 3/100 of a second—via a localized increase in blood pressure controlled by abdominal muscles. At the end of this structure are fanglike pincers used to seize prey, which is then drawn back to the mandibles to be consumed.12