Anal tube

Definition

A tubular, often telescoping terminal abdominal structure in certain insects that houses the anus and associated openings, typically projecting posteriorly from the abdomen. The anal tube serves as a protective sleeve for the terminal segments and may facilitate defecation, oviposition, or respiratory exchange depending on the group. In some taxa it is retractable or bears specialized setae or sclerites.

Etymology

From Latin 'anus' (ring, anus) + 'tubus' (pipe, tube), referring to the cylindrical form of the terminal abdominal projection.

Example

In many larval Lepidoptera, the anal tube is a conspicuous, often pigmented projection at the posterior end of the abdomen, retractable within the preceding segment and bearing the anal opening at its apex; in some aquatic Hemiptera nymphs, the anal tube extends to the water surface for atmospheric respiration.

Synonyms

• anal siphon
• respiratory siphon

Related Terms

• pygopod
• cercus
• paraproct
• Epiproct
• terminalia
• abdominal apex
• anal proleg

Usage Notes

Not to be confused with the siphonal process of some aquatic larvae, though function may overlap; usage varies by taxonomic group—some authors restrict 'anal tube' to structures with explicit respiratory function, while others use it more broadly for any tubular terminal abdominal projection. Contrast with 'anal fork' (bifurcate) and 'anal comb' (pectinate). In arachnids, the homologous region is typically termed the anal tubercle or postabdomen, not 'anal tube'.