Exoskeleton

Definition

The external, hardened integument that provides structural support, protection, and muscle attachment sites in arthropods and certain other invertebrates. Composed primarily of cuticle—a composite of chitin and sclerotized proteins or, in crustaceans, calcium carbonate—the exoskeleton forms a rigid outer armor that defines body shape and limits water loss. Because it is non-living and cannot expand, growth requires periodic shedding (ecdysis) and replacement.

Etymology

From Greek exo- (outside) + skeletos (dried body, skeleton)

Example

A beetle's elytra are hardened forewings that form part of the exoskeleton, protecting the folded hindwings and abdomen beneath; a spider's exoskeleton includes both the rigid prosoma and the more flexible opisthosoma, with muscle attachments visible as internal apodemes.

Synonyms

• cuticular skeleton
• external skeleton

Related Terms

• Cuticle
• Ecdysis
• exuvia
• Sclerite
• apodeme
• Endoskeleton
• Integument
• arthropod

Usage Notes

Distinguished from endoskeleton (internal, as in vertebrates) and hydrostatic skeleton (fluid-filled cavity, as in caterpillars and many soft-bodied larvae). In entomology, 'exoskeleton' typically refers to the entire cuticular covering, though specialists may distinguish between the thin, flexible cuticle of intersegmental membranes and the heavily sclerotized plates (sclerites) of the body wall. The term is sometimes applied more broadly to mollusk shells or other external hard parts, but in arthropod contexts it specifically denotes the chitinous cuticular system.