pleuron

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of the two lateral portions of a body segment of an arthropod.

from The Century Dictionary.

• noun A lateral piece, part, or aspect of the body; especially, the side of the thorax: chiefly used of invertebrates.

from the GNU version of the Collaborative International Dictionary of English.

• noun One of the sides of an animal.
• noun One of the lateral pieces of a somite of an insect.
• noun One of lateral processes of a somite of a crustacean.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A lateral sclerite of a thoracic segment of an arthropod between the tergum and the sternum