Spiracle

Definition

An external respiratory opening in the exoskeleton of insects and some other arthropods, typically paired and segmentally arranged on the thorax and abdomen, through which air enters the tracheal system for gas exchange. Spiracles are usually surrounded by a peritreme and may possess closing mechanisms (spiracular valves) to regulate water loss and airflow. In insects, they are the primary portals for tracheal respiration; oxygen diffuses directly through the tracheae and tracheoles to tissues without involvement of the circulatory system.

Etymology

Latin spiraculum, from spirare 'to breathe'

Example

A caterpillar has eight pairs of abdominal spiracles, visible as small dark dots along each side of the body segments, which open and close rhythmically to ventilate the tracheal network during locomotion.

Synonyms

• spiraculum
• stigma (historical, now discouraged)

Related Terms

• Trachea
• Tracheole
• peritreme
• spiracular valve
• book lung
• haemolymph
• Cuticle
• respiration
• insect physiology

Usage Notes

The term 'stigma' was formerly used for insect spiracles but is now reserved for wing-cell markings to avoid confusion. Spiracles are absent or highly modified in some aquatic and endoparasitic insects; some larvae possess only one functional pair. In arachnids, book lungs and tracheae may open via spiracle-like structures, though homology varies. Not to be confused with vertebrate spiracles (persistent pharyngeal slits in sharks, rays, and some primitive fish).