Median

Definition

Situated at or along the central plane of bilateral symmetry; pertaining to the midline of the body that divides left and right sides. In arthropod anatomy, median describes structures positioned on or near this central axis, as opposed to lateral structures toward the sides. The term applies to both the sagittal plane itself and to structures located upon it, such as median ocelli, median caudal filaments, or the median nervure of insect wings.

Etymology

From Latin medianus, from medius ("middle").

Example

The median ocellus of a dragonfly lies on the dorsal midline of the head between the compound eyes, while the lateral ocelli (when present) are positioned more to the sides.

Synonyms

• medial
• mesal

Related Terms

• Lateral
• Sagittal
• Bilateral symmetry
• Midline
• Paramedian

Usage Notes

Median indicates position on the central plane itself; medial indicates toward the median from some lateral point. Contrast strictly with lateral (toward the side), not with distal or proximal. In wing venation, "median" often refers specifically to veins or cells near the longitudinal midline of the wing. Some authors use mesal as a synonym, though mesal sometimes carries connotations toward (but not necessarily on) the midline.