labium, labia, labial

From Latin labium, meaning lip.

labium noun, plural labia, adjective labial - the "lower lip" of the arthropod mouth. It closes the orifice of the mouth from below. Sometimes called the "tongue", and is regarded as a "second maxilla" because its structure is derived from the fusion of two maxilla-like appendages. The labium consists of two parts, the mentum and the ligula. In some groups, the labium is modified for sucking liquids as a proboscis. In some flies, the tip is modified for sopping liquids as a labellum. There are often labial palpi (sensory organs) attached.

(The plural labia would not be used for insects, since there is only one labium. The "upper lip" is the labrum.)

Diagrams and photos showing the labium: