A trick some beetles use to breathe underwater.

A number of species of diving beetles can keep a thin layer of air trapped under the dense hydrofuge hairs on their underside. This layer is called a plastron. As the insect breathes from this layer of air, the partial pressure of oxygen drops below the partial pressure of oxygen in the water, and the oxygen in the water diffuses into the layer of air. This gives the insect a renewable source of oxygen while submerged.

Any small bubble will act as a sort of artificial gill for a diving insect; many water insects do simply use bubbles. A plastron is better than a simple bubble because a bubble will eventually pop due to changing pressures as the insect dives deeper, or changing size as the nitrogen in the bubble is slowly absorbed into the surrounding water.

Plas"tron (?), n. [F. plastron breastplate, plastron, LL. plastra a thin plate of metal. See Plaster.]

1.

A piece of leather stuffed or padded, worn by fencers to protect the breast.

Dryden.

3. Anc. Armor

An iron breastplate, worn under the hauberk.

3. Anat.

The ventral shield or shell of tortoises and turtles. See Testudinata.

4.

A trimming for the front of a woman's dress, made of a different material, and narrowing from the shoulders to the waist.

© Webster 1913.