Ileum

Definition

The posterior (distal) section of the small intestine, located between the jejunum and the cecum or large intestine; in vertebrates, specialized for absorption of vitamin B12, bile salts, and residual nutrients. In invertebrate zoology, the term is sometimes applied to the corresponding posterior region of the midgut in certain arthropods, though gut regionalization in insects and arachnids typically follows different nomenclature (midgut, hindgut, ileum/colon/rectum distinctions vary by group).

Etymology

From Latin ileum, ilium, denoting the groin or lower abdomen; anatomical usage established in Renaissance-era human anatomy.

Example

In the desert locust Schistocerca gregaria, the posterior midgut and anterior hindgut together perform functions analogous to the vertebrate ileum, reabsorbing water and ions from the digesta before fecal pellet formation.

Synonyms

• posterior intestine
• distal intestine

Related Terms

• Jejunum
• Duodenum
• midgut
• hindgut
• Cecum
• Malpighian tubules
• Digestive tract
• Absorption
• Peritrophic matrix

Usage Notes

In entomology and arachnology, 'ileum' is not standard nomenclature; most specialists refer to 'posterior midgut' or 'anterior hindgut' instead. The term appears mainly in comparative physiology and when discussing evolutionary homology of gut regions across bilaterians. Some insect anatomical literature uses 'ileum' specifically for the anterior hindgut (preceding the colon/rectum), but this usage is inconsistent—check source context carefully.