mystax

Mystax is a term used in entomology to describe a moustache-like patch of bristles or setae on the face of certain insects, most commonly Diptera (true flies). The mystax is typically located on the lower face, between the frons and the mouthparts, and its size and density can range from a small tuft to a broad band that may partially cover the clypeus. This facial setal arrangement varies among species and is used as a taxonomic character to help identify and distinguish certain genera and species, particularly within the robber flies (family Asilidae) and related dipteran groups.

The functional significance of the mystax is not fully understood and appears to vary by lineage. Hypotheses suggest that the bristles could play a sensory role, aid in protection of the mouthparts from debris during feeding or in predation, or contribute to prey handling by providing tactile feedback. However, concrete functional evidence remains limited, and in some taxa the mystax is reduced or absent, indicating that its presence is not universally essential.

In descriptive works, researchers note the presence or absence of the mystax, its extent, and the arrangement of the bristles as part of the morphological toolkit used for classification. While most commonly discussed in Diptera, the term is rooted in the Latin mystax, meaning “mustache,” and reflects a traditional emphasis on facial bristles as a distinguishing feature in insect taxonomy.