Rectum

Definition

The terminal, relatively straight section of the hindgut that receives fecal material from the ileum or colon, stores it temporarily, and regulates its expulsion through the anus or cloaca. In arthropods, the rectum is typically lined with cuticle (in insects and arachnids) or a modified epithelium and often functions in osmoregulation and water reabsorption prior to defecation.

Etymology

From Latin rēctum intestīnum, 'straight intestine', referring to its relatively straight course compared to the convoluted midgut.

Example

In desert beetles such as Tenebrio molitor, the rectal pads—specialized cuticular invaginations lined with cells that transport ions—enable the extraction of nearly all remaining water from feces, producing dry pellets and conserving body water in arid environments.

Synonyms

• anal canal (in some contexts)
• terminal intestine

Related Terms

• Hindgut
• Ileum
• colon
• anal canal
• Anus
• cloaca
• Malpighian tubules
• rectal pads
• cryptonephridial system
• defecation
• Osmoregulation

Usage Notes

In vertebrate anatomy, 'rectum' and 'anal canal' are distinct sequential regions; in many arthropods, the boundary is less sharply demarcated, and 'rectum' often encompasses the entire post-ileal hindgut terminating at the anus. The term is sometimes used loosely for any straight terminal gut section, but specialists reserve it for the defecation-regulating region. Contrast with 'colon' or 'ileum' in species with more differentiated hindgut regions. In insects, the presence of six longitudinal rectal pads is a common but not universal feature.