Head

Definition

The anterior-most tagma (body segment) of an insect, arachnid, or other arthropod, bearing the principal sensory organs (antennae, eyes, ocelli), the brain, and the mouthparts. In insects, the head is derived from six embryonic segments that fuse into a single hardened capsule; in arachnids, it is fused with the thorax to form the cephalothorax. The head is connected to the thorax by a flexible neck membrane and is controlled by the subesophageal ganglion, which coordinates mouthpart movements.

Etymology

Middle English hed, heved, from Old English hēafod, cognate with German Haupt; applied to anterior body region in arthropods by anatomical analogy.

Example

In a honey bee worker, the head houses the compound eyes with their ommatidia for vision, the antennae with sensory sensilla for olfaction, and the proboscis for nectar feeding—all enclosed in a heavily sclerotized exoskeletal capsule.

Synonyms

• cephalon (embryological)
• caput (anatomical, rare)

Related Terms

• thorax
• abdomen
• Tagma
• cephalothorax
• antenna
• compound eye
• Ocellus
• mandible
• maxilla
• labrum
• labium
• proboscis
• subesophageal ganglion
• clypeus
• frons
• Vertex
• Occiput
• sclerite

Usage Notes

In entomology, "head" refers specifically to the fused anterior tagma, not merely the front part of the body. Contrast with "cephalon," which is sometimes used for the embryonic precursor or in comparative embryology. In arachnids, crustaceans, and some other arthropods, the head is not a separate tagma but is incorporated into the cephalothorax (prosoma). The term is absolute (not relative) and unambiguous in insect morphology, though in some soft-bodied larvae the head capsule may be reduced or retracted into the thorax, requiring careful examination to locate.