ovisac

An ovisac is a protective sac or casing that contains eggs (ova) in many oviparous animals. The term is applied across diverse groups, including arachnids, crustaceans, mollusks, and some insects, to describe an enclosure that surrounds and protects developing eggs rather than the eggs alone. Ovisacs may be produced from silk, wax, gelatinous secretions, or calcareous material, and they may be attached to vegetation, substrate, or carried by a parent.

In spiders, ovisacs are silk capsules that hold a clutch of eggs and are often guarded or carried by the female until hatching. In many insects, ovisacs may form as waxy or parchment-like coverings around eggs, sometimes as part of an egg case known in common usage as an ootheca. In crustaceans and mollusks, external sacs or capsules can house eggs during development, sometimes providing a controlled microenvironment.

The primary functions are protection from desiccation and predation, maintenance of an appropriate moisture and temperature regime, and, in some species, facilitation of dispersal or parental care. The specific structure and materials of an ovisac vary by lineage and ecological niche.

Etymology: from Latin ovis meaning egg and sac. The term is used mainly in descriptive biology and taxonomy; some groups use different terms such as egg case or ootheca for similar structures.

See also: egg case, ootheca, egg capsule.