When a spider appears “pregnant,” many wonder about the implications of disturbing it or what it carries. Understanding the biological realities behind this appearance clarifies what is truly at stake.
Spider Reproduction: Beyond “Pregnancy”
Spiders do not experience pregnancy in the way mammals do; they are oviparous, laying eggs rather than giving live birth. After mating, a male spider transfers sperm to the female’s epigynum using his pedipalps. The female stores this sperm internally until she is ready to fertilize her eggs.
As eggs develop inside her, the female’s abdomen may noticeably enlarge, leading to the perception of “pregnancy” or a “gravid” state. When ready, she lays her eggs, fertilizing them as they emerge, and encases them within a protective silk structure.
The Egg Sac: A Cradle of Life
The structure containing a spider’s developing offspring is an egg sac, a silk enclosure. The female spider produces this silk from spinnerets on her abdomen. Egg sacs vary widely by species, appearing round, oval, or teardrop-shaped, with textures from papery to fluffy, and colors from white, cream, yellow, or brown.
Inside, the protective sac holds numerous eggs, from a few dozen to over a thousand. These eggs develop within the sac. The mother spider cares for her offspring by constructing, sealing, and sometimes camouflaging the egg sac. Many species, like wolf spiders, guard or carry their egg sacs until the spiderlings emerge.
Consequences of Destroying the Egg Sac
Destroying a spider’s egg sac is fatal for the developing eggs and spiderlings. Physical disruption, like crushing, directly damages the fragile contents. Even if eggs are close to hatching, premature exposure without the sac’s protective microclimate reduces their survival chances.
If the sac ruptures, releasing immature spiderlings, their survival outside is low. These young spiderlings are vulnerable, lacking maternal care and the sac’s controlled conditions. They may also lack hardened exoskeletons or developed fangs, making independent survival difficult. Even naturally, many spiderlings don’t reach maturity, and destroying the sac further reduces their survival.
Common Myths and Realities
A common misconception is that crushing a “pregnant” spider causes hundreds of baby spiders to “explode” and scatter. This event is associated with wolf spiders, which carry hatched spiderlings on their backs, not within an egg sac. If a wolf spider carrying live spiderlings is crushed, they disperse, but were already external to an egg sac.
If an egg sac ruptures, any developed spiderlings inside will scatter. However, their indoor survival is limited due to lack of food and proper conditions. Most spiderlings hatching indoors will move outside where resources are plentiful. While the sudden appearance of many tiny spiders can be alarming, they are harmless to humans; their fangs are often undeveloped and too small to pose a threat. Many common house spider species are not considered dangerous.