Spiders do not produce a true cocoon, which is a silk casing built by the larvae of some insects, like moths, for their pupal stage. Instead, the structures spiders weave are dedicated solely to reproduction and protecting their offspring. These silken spheres, disks, or bundles represent a significant investment in maternal care and are a testament to the versatility of spider silk.
Clarifying Terminology: Egg Sacs and Oothecae
The correct term for the silken structure containing a spider’s eggs is an egg sac, though the term ootheca is sometimes used in a broader sense for egg cases produced by insects. An egg sac is a specialized, multi-layered silk container spun by the female immediately after laying her eggs. Its primary function is to provide a comprehensive, climate-controlled barrier for the eggs. This structure protects the developing embryos from external threats, including predators and parasites. The silk layers also serve to shield the eggs from environmental fluctuations like excessive drying, wetting, or temperature changes. The number of eggs inside an egg sac can vary dramatically, ranging from just one to over a thousand, depending on the spider species.
Construction, Materials, and Contents
The creation of an egg sac is a precise, multi-step process utilizing different types of silk produced by the spider’s specialized abdominal glands. The female begins by spinning a foundational mat of silk onto which she will deposit her eggs, which are often glutinous and stick together in a cluster. Once the eggs are in place, the female quickly wraps and seals the cluster with additional layers of silk. The inner layer consists of a softer, cushion-like silk, while the outer layer is typically made of a dense, tougher silk for durability and protection. This outermost silk layer may be colored white, brown, or green, helping the sac blend in with its surroundings. The young spiderlings remain inside until they undergo their first molt, emerging by cutting a hole with their fangs when ready.
Diversity in Egg Sac Design and Placement
The design and placement of egg sacs are incredibly diverse, reflecting a wide range of maternal care strategies across different spider families. Orb-weaving spiders, such as the Argiope genus, attach their spherical or disk-shaped sacs to nearby vegetation or incorporate them into their webs. These attached sacs are often camouflaged with debris, leaves, or dull-colored silk.
Other species carry the protective case until the spiderlings hatch. Female wolf spiders attach their distinctive spherical egg sacs to their spinnerets and drag them along. Nursery-web spiders carry their sac using their jaws until the eggs are close to hatching, then construct a “nursery tent” of silk to guard the sac. Ground-dwelling species, like trapdoor spiders, hide or bury their egg sacs inside silk-lined burrows or under bark.