Chorion

Definition

The tough, protective outer membrane or shell of an insect or other arthropod egg, secreted by the maternal follicle cells during oogenesis. In insects, the chorion typically comprises multiple layers—often including an inner proteinaceous lamellate layer and an outer waxy or crystalline layer that limits water loss while permitting gas exchange. It may bear surface sculpturing, aeropyles (respiratory pores), or attachment structures that vary taxonomically and functionally. The chorion is distinct from the inner vitelline membrane and is shed or ruptured during eclosion.

Etymology

Greek chōrion, diminutive of chōros (enclosure, skin)

Example

The eggs of the vapourer moth (Orgyia antiqua) possess a ribbed, air-filled chorion that protects the developing embryo from desiccation; in many parasitoid wasps, the chorion is reduced or absent to allow direct nutrient uptake from the host.

Related Terms

• Vitelline membrane
• Serosa
• amnion
• Ootheca
• aeropyle
• Micropyle
• Eclosion
• exochorion
• endochorion

Usage Notes

In entomology, 'chorion' refers specifically to the eggshell, whereas in vertebrate embryology it denotes the outermost extraembryonic membrane (homologous concept, different structure). The term is sometimes loosely applied to the hardened egg coverings of some arachnids and crustaceans, though these may be structurally distinct. Chorion structure is taxonomically informative: eggs laid in dry environments typically have thicker, more elaborate chorions with complex respiratory systems, while aquatic or parasitic eggs often show reduction or modification of chorionic layers.