Hypostome

Definition

A ventral mouthpart structure that varies in form and function across arthropod and cnidarian groups. In ticks and some other parasitic arachnids, the hypostome is a barbed, blade-like structure that projects from the basis capituli and anchors the mouthparts in host tissue during blood-feeding; it contains the salivary canal and often the food canal. In trilobites, the hypostome is a ventral plate of variable shape—free-floating, attached to the doublure, or fused to the cephalon—that covers the mouth and may have served in feeding or enrollment. The term also applies to the raised oral cone surrounded by tentacles in hydrozoan cnidarians.

Etymology

Greek hypo- (under) + stoma (mouth)

Example

In Ixodes scapularis, the black-legged tick, the hypostome bears multiple rows of posteriorly directed teeth that mechanically lock the tick in place during the extended blood meal, requiring traction force for removal.

Related Terms

• basis capituli
• capitulum
• Chelicera
• palp
• rostrum
• gnathosoma
• mouthparts
• doublure
• cephalon

Usage Notes

The term is polysemous across phyla; context is essential. In acarology, 'hypostome' specifically denotes the toothed feeding anchor, not to be confused with the chelicerae (cutting organs) or palps (sensory appendages). In trilobite paleontology, hypostome morphology and attachment style are taxonomically informative. Do not confuse with 'hypostomate' (a fish mouth position) or medical 'hypostome' variants.