A spider egg sac is a specialized nursery meticulously constructed from silk to house and protect the developing eggs from environmental hazards and predators. Spiders are drawn to indoor environments primarily because human dwellings offer a consistent, stable microclimate favorable for incubation. The enclosed nature of a house provides a dependable refuge with less temperature fluctuation and reduced risk of disturbance compared to outdoor habitats.
Identifying the Egg Sac Structure
The appearance of a spider egg sac varies significantly by species, but all are composed of tightly woven silk. These sacs range in color from pure white or cream when freshly spun to a dull tan or brown as they age and collect dust. A typical sac is small, often measuring only a few millimeters up to about two centimeters in diameter.
Shapes can be spherical, resembling a small cotton ball, or exhibit more distinctive forms like a flattened disc, a teardrop, or a flask-like structure. The texture is also a clue; some sacs are smooth and papery, while others are loosely woven and fluffy. For instance, the brown widow spider creates a recognizable sac covered in tiny, spiky projections, unlike the smooth sacs of the black widow.
Prime Interior Locations for Egg Deposition
Spiders select specific microclimates within a house that maximize the survival chances of their offspring, prioritizing darkness, stability, and seclusion. The most commonly chosen areas are those that experience minimal human activity and temperature shifts. Undisturbed storage areas such as attics, basements, and crawl spaces are preferred due to their consistent humidity and quiet.
Within these areas, spiders often tuck their sacs into crevices or attach them to the underside of stored items and clutter, utilizing boxes and forgotten belongings as a shield. Heavy furniture or large appliances that are rarely moved, like refrigerators or shelving units, create deep, secluded voids where sacs can be safely suspended.
Spiders also frequently use the narrow gaps and corners of structural elements. This includes the upper corners of closets, the hidden recesses within window frames and door jambs, and the sheltered spaces inside ventilation grates. By choosing these deep niches, the female ensures the eggs are protected from accidental crushing, cleaning attempts, and the drying effects of circulating air.
Common Household Spiders That Lay Eggs Indoors
Several species frequently take advantage of indoor environments to lay their eggs, each producing a distinctly identifiable sac. The common house spider typically creates a flask-shaped or teardrop-shaped sac that is brown and papery in texture. These sacs, which can contain up to 250 eggs, are often suspended within the messy, tangled cobwebs the female spins in undisturbed corners.
Cellar spiders, often called “daddy long-legs,” produce a very different structure. Their sacs are loose, thin, and contain fewer eggs, often appearing as a small bundle held together by minimal silk. The female cellar spider does not abandon the sac but carries it loosely in her jaws until the young hatch. Wolf spiders, a ground-dwelling species, also carry their egg sacs, attaching the tightly woven, spherical sac to their spinnerets.
The black widow spider produces a robust, spherical sac that is typically yellowish or white and has a papery or silky texture. These sacs are usually suspended within the female’s irregular web and can hold several hundred eggs. Brown recluse spiders create small, off-white sacs that are loosely woven and often camouflaged with surrounding debris, typically containing around 40 to 50 eggs.
The Hatching Timeline
The time required for spider eggs to develop and hatch is not uniform, varying widely based on the species and the ambient temperature of the location. Generally, once an egg sac is deposited, the eggs may hatch in as little as two to three weeks under optimal, warm conditions. This period can extend considerably in cooler environments.
Temperature is the main factor influencing the duration of embryogenesis; warmer temperatures accelerate the metabolic rate and shorten the incubation period. Conversely, in cooler areas of a house, such as an unheated basement or garage, the process can take several months. Some species that lay eggs in the late fall have spiderlings that hatch quickly but then remain dormant inside the protective sac until the warmth of spring triggers their emergence.
Finding an intact sac indicates the potential for a sudden, large emergence, as a single sac can house dozens to hundreds of spiderlings. Once the spiderlings are ready, they chew their way out of the silk casing, often dispersing quickly via air currents in a process known as ballooning.