Spiracle

The spiracles, openings that allow gas exchange in insects, are visible on this large caterpillar.
The spiracles, openings that allow gas exchange in insects, are visible on this large caterpillar. Photo: Shawn Hanrahan, Wikimedia Commons (cc-by-sa)

Definition:

Breathing holes located along the sides of insects. The spiracles are connected to the air tubes, or trachea, and allow the insect to exchange gases. The insect can open or close the spiracles by contracting muscles along its side.

See also: How Do Insects Breathe? and The Insect Respiratory System