Frons

Definition

The sclerotized, often shield-like anterior region of the insect head capsule, bounded by the vertex posteriorly, the clypeus anteriorly, and the compound eyes and genae laterally. In most insects the frons bears the median ocellus (when present) and the origins of the inner margins of the compound eyes; it may be flat, convex, or carinate and frequently carries diagnostic color patterns, setal arrangements, or sulci used in identification. The frons is homologous with the frontal plate of other arthropods and corresponds topologically to the forehead region.

Etymology

Latin frons, forehead, front

Example

In many Diptera, the frons is sexually dimorphic: males of certain hoverflies (Syrphidae) possess holoptic eyes that meet broadly across the frons, whereas females retain a distinct, setose frons separating the eyes.

Synonyms

• frontal plate
• forehead (vernacular)

Related Terms

• vertex
• Clypeus
• gena
• ocellus
• Sclerite
• epistomal suture
• frontoclypeal suture
• Tentorium

Usage Notes

The precise boundaries of the frons vary among insect orders and are defined by sutures (especially the epistomal suture anteriorly and the frontogenal sutures laterally) rather than by external landmarks alone. In some taxa—notably Odonata and certain Coleoptera—the frons may be reduced or fused with adjacent plates, requiring careful morphological interpretation. The term is not typically applied to arachnids, where the homologous region is generally called the propeltidium or simply the anterior carapace.