Anus

Definition

The terminal external opening of the digestive tract through which feces and other waste products are expelled. In arthropods, the anus is typically located on the posterior body terminus (pygidium or telson region) and may be surrounded by sclerotized plates or associated with defensive glands, silk-spinning structures, or respiratory modifications. In insects, the anus opens into the proctodeal chamber of the hindgut; in spiders and many arachnids, it is positioned ventrally on the opisthosoma, anterior to the spinnerets or pedicel base. The anus is a critical landmark in larval taxonomy, chaetotaxy, and the diagnosis of certain parasitic groups.

Etymology

Latin ānus, of uncertain origin; classical term for the posterior body opening.

Example

In caterpillars (Lepidoptera larvae), the anus is located on the terminal segment (A10) and is often surrounded by a sclerotized anal plate bearing setae used in species-level identification; the arrangement of these setae relative to the anus is a standard chaetotaxic character in taxonomic keys.

Synonyms

• anal opening
• anal orifice
• proctodeal opening

Related Terms

• Hindgut
• Proctodeum
• cloaca
• Pygidium
• Telson
• anal plate
• stercoral pocket
• coprodeum

Usage Notes

In many invertebrates, particularly some arthropods and most vertebrates, 'anus' is distinguished from 'cloaca,' which serves as a common chamber for digestive, urinary, and reproductive waste. Some entomological literature uses 'anal' adjectivally (anal valves, anal fork, anal cerci) with the anus as the reference point. The term is absolute (not relative) in position—always the terminal gut opening—though its precise location varies by body plan (terminal, subterminal, or ventral). In larval beetles and flies, the relative position of the anus to the terminal sclerites distinguishes supra-anal from sub-anal configurations useful in family-level diagnosis.