Sternum (arthropod anatomy) in the context of Sclerites

Arthropods are covered with a tough, resilient integument, cuticle or exoskeleton of chitin. Generally the exoskeleton will have thickened areas in which the chitin is reinforced or stiffened by materials such as minerals or hardened proteins. This happens in parts of the body where there is a need for rigidity or elasticity. Typically the mineral crystals, mainly calcium carbonate, are deposited among the chitin and protein molecules in a process called biomineralization. The crystals and fibres interpenetrate and reinforce each other, the minerals supplying the hardness and resistance to compression, while the chitin supplies the tensile strength. Biomineralization occurs mainly in crustaceans. In insects and arachnids, the main reinforcing materials are various proteins hardened by linking the fibres in processes called sclerotisation and the hardened proteins are called sclerotin. The dorsal tergum, ventral sternum, and the lateral pleura form the hardened plates or sclerites of a typical body segment.

In either case, in contrast to the carapace of a tortoise or the cranium of a vertebrate, the exoskeleton has little ability to grow or change its form once it has matured. Except in special cases, whenever the animal needs to grow, it moults, shedding the old skin after growing a new skin from beneath.

The pleuron (pl. pleura, from Greek side, rib) is a lateral sclerite of thoracic segment of an insect between the tergum and the sternum. While the tergum is positioned on the top (dorsal), and the sternum on the bottom (ventral), the pleuron is positioned to the side (lateral). The terms pro-, meso- and metapleuron are used respectively for the pleura of the first, second and third thoracic segments.

A pleuron usually consists of a epimeron and an episternum. An epimeron is the posterior (back) sclerite of the pleuron, and an episternum is the anterior (front) sclerite.