exocuticle

Exocuticle is a layer of the arthropod cuticle, residing in the procuticle between the epicuticle and the endocuticle. It is formed during molting and often undergoes sclerotization, contributing to the rigidity and protective function of the exoskeleton. The thickness and degree of hardening of the exocuticle vary across body regions and developmental stages, reflecting functional needs.

The typical composition of the exocuticle is a chitin–protein matrix. Proteins rich in cuticle structural motifs combine with cross-linking chemistry, such as tanning through phenolic compounds, to create a more rigid, durable layer. Microscopic organization of the exocuticle supports mechanical strength and resistance to wear, and in many species it may be pigmented.

Functionally, the exocuticle provides structural support, abrasion resistance, and protection for tissues beneath the exoskeleton. Its properties are adapted to locomotion, habitat, and lifestyle, and different regions may exhibit varying degrees of sclerotization or thickness to meet specific mechanical demands. In some arthropods, regions of the exocuticle also contribute to the formation of joint surfaces and articulations.

Developmentally, the exocuticle is secreted by epidermal cells during the molt as part of the process that renews the cuticle. The exocuticle is deposited as the new procuticle forms, with sclerotization and pigmentation contributing to the mature hardness of the adult exoskeleton.