Female spiders lay eggs inside a specialized protective structure known as an egg sac. Since the tiny, individual eggs are rarely seen outside of this cocoon, the sac itself is the primary feature used for identification. These silken enclosures shield the developing offspring from predators, parasites, and environmental changes. The appearance of the egg sac is highly variable, reflecting the wide diversity of spider species, and offers distinctive clues to the identity of the mother spider.
The General Appearance of Egg Sacs
Spider egg sacs are fabricated primarily from silk, though the density and composition differ significantly among species. The most common colors are neutral shades of white, cream, tan, or light brown, allowing the sac to blend into its surroundings. Some species, like the Green Lynx spider, produce sacs that are pale yellow or light green, adding camouflage.
The texture of the sac can be a clear identifier, ranging from smooth and papery to fuzzy and cotton-like, or even reinforced with spiky protrusions. While many sacs are spherical or simple oval shapes, others exhibit more complex forms, such as a flattened disc or a teardrop shape. The size of the sac is generally proportional to the size of the female spider. The number of individual eggs inside is highly variable, sometimes ranging from 50 to over a thousand eggs in a single sac.
Distinctive Egg Sacs of Common Spiders
Visual characteristics of the egg sac provide the most reliable method for identifying the species. The Black Widow spider creates a sac that is smooth, tough, and spherical, often resembling a small, off-white to tan ball about the size of a pea. This sac is typically suspended within the mother’s tangled web. In contrast, the closely related Brown Widow constructs a sac easily distinguished by its prominent, spiky texture, giving it the appearance of a tiny, spiked ball.
Garden Spiders, such as the Black-and-Yellow Argiope, produce a large, multi-layered sac that is often suspended prominently in the center of their large orb webs. This sac is typically pear-shaped or round, brown in color, and has a papery texture, designed to withstand the elements while hanging out in the open. A completely different strategy is employed by the Wolf Spider, which produces a pale, spherical, and tightly woven sac that is physically attached to her spinnerets. This attachment allows the mother to actively carry the sac everywhere she goes, dragging it along the ground until the spiderlings are ready to emerge.
Placement Strategies and Location
The location and method of placement are just as important as the sac’s appearance for accurate identification. Spider mothers employ three main strategies for protecting their eggs. The first is suspension, where the sac is hung freely within a web or from a sheltered structure, such as the eaves of a house or a branch in a tree.
A second strategy involves attachment, where the sac is securely glued to a surface. These sacs are often found in concealed, protected spots like under rocks, beneath loose tree bark, or tucked away in the dark crevices of a basement or attic.
The third method is carrying, exemplified by the Wolf Spider and some Fishing Spiders. These spiders physically move the sac, which remains attached to the mother’s body, ensuring constant protection.