Spiders do not experience pregnancy in the same way mammals do, as they do not carry live young internally. Instead, a female spider carries fertilized eggs within her abdomen before laying them. Observable signs indicate when a female spider is carrying eggs, preparing for oviposition. These indicators help identify a spider soon to lay an egg sac.
Physical Signs
One of the most noticeable indicators that a female spider is carrying eggs is a significant enlargement of her abdomen. The abdomen, the posterior section of the spider’s body, becomes much larger, rounder, and often appears distended. This swelling is due to the internal development and accumulation of eggs, stretching the abdominal cuticle taut.
Increased internal pressure from developing eggs can also lead to subtle changes in the abdomen’s appearance, such as slight stretching or thinning of the cuticle. Some species may also exhibit minor color changes or a shinier appearance as the abdomen expands. This distinct swelling differs from a spider that has simply consumed a large meal; a fed spider’s abdomen typically appears less uniformly distended and returns to normal size more quickly. The sustained and pronounced enlargement of the abdomen is a primary visual cue.
Behavioral Changes
Female spiders carrying eggs often change their activity levels and habits. They may become less active, moving more slowly or remaining stationary due to the increased weight of their abdomen. This reduced mobility helps conserve energy and protects the developing eggs from potential damage.
As egg-laying approaches, the female spider seeks a secluded, protected location. This might be a hidden crevice, an undisturbed corner, or a secure spot within her web, offering safety from predators and environmental disturbances. Web-building spiders may modify their web structure, constructing a denser silken retreat or a specialized chamber for the upcoming egg sac.
The Egg Sac Stage
The most definitive sign that a female spider has laid eggs is the presence of an egg sac. An egg sac is a protective silken casing constructed by the female to house her eggs. These sacs vary in appearance depending on the spider species, but they are typically spherical, pear-shaped, or lenticular.
Egg sacs are often white or cream-colored, made from strong, woven silk that provides insulation and defense. Many spiders attach their egg sacs to a substrate, such as in their web, under leaves, beneath rocks, or in secluded crevices. Some species, like wolf spiders, actively carry their egg sacs attached to their spinnerets, transporting them wherever they go.
Maternal care extends beyond just laying the eggs. Many female spiders guard their egg sacs, protecting them until the spiderlings hatch and disperse. The discovery of an egg sac is the clearest indication that a female spider has successfully undergone the reproductive process, transitioning from internal egg carrying to external offspring protection.