From ZooTerms (Dictionary of Invertebrate Zoology)
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labium (noun; plural labia; Latin labium, lip): 1. A lip.

2. (Arthropoda: Chelicerata) In Araneae, the lower lip, forming the floor of the mouth cavity.

3. (Arthropoda: Crustacea) The metastoma.

4. (Arthropoda: Insecta) One of the mouth-part structures, the lower lip, composed of fused second maxillae; has been referred to as the tongue at a certain phase of development.

5. (Mollusca) The inner lip of a univalve shell, the inner side of the aperture or columellar lip extending from the origin at the lip of the labrum and resting on the columella

See also: inner lip.