True spiders, belonging to the order Araneae, are overwhelmingly oviparous, meaning they lay eggs. This reproductive strategy is standard among the 50,000-plus known species of spiders globally. The entire process of reproduction, from mating to the emergence of the young, involves external egg deposition and specialized care that differs significantly from the live birth seen in mammals.
The Standard Method of Spider Reproduction
Spider reproduction begins with a complex mating ritual, as the male must carefully approach the typically larger female to avoid being mistaken for prey. Males do not transfer sperm directly; instead, they first deposit a droplet of sperm onto a small silk web, called a sperm web. The male then draws the sperm into specialized, syringe-like structures located on the tips of his pedipalps, which are small, leg-like appendages near his mouth.
The male subsequently inserts one or both charged pedipalps into the female’s genital opening, known as the epigynum, transferring the sperm into her internal storage sacs. Females can store this sperm for extended periods, sometimes for months, until conditions are optimal for egg-laying. The female then lays her eggs, which are fertilized by the stored sperm as they pass out of her body through the epigastric furrow.
The number of eggs laid varies dramatically by species, ranging from just a few to several hundred in a single clutch. The eggs are laid onto a pre-constructed silk pad, which marks the beginning of the protective structure known as the egg sac.
Understanding the Egg Sac and Spiderling Emergence
The female immediately begins enveloping her freshly laid eggs in a silk covering, forming an egg sac. This sac is constructed from multiple layers of dense silk designed to protect the developing embryos from physical damage, dehydration, and predators. The appearance of the egg sac varies widely between species, with some being spherical, others teardrop-shaped, and colors ranging from white to brown.
The mother’s involvement with the sac also varies, contributing to the “live birth” confusion. Many species simply attach the sac to a surface or hide it in a retreat, while others exhibit intense maternal care. Wolf spiders carry their spherical egg sac attached to their spinnerets until the young are ready to hatch. The spiderlings often remain inside the protective sac, undergoing their first molt there.
The moment of emergence often creates the illusion of live birth, as dozens or even hundreds of spiderlings suddenly burst forth. In species like the wolf spider, the mother instinctively helps the young emerge by biting open the tough silken sac. Once outside, the young of many species immediately climb onto the mother’s back and ride there for several days before dispersing.
Addressing True Viviparity in Arachnids
The biological distinction between egg-laying and live birth is defined by whether the young develop outside or inside the mother’s body. Spiders are oviparous because the eggs are laid and development occurs externally. True viviparity involves the young developing internally and receiving nourishment directly from the mother.
In the broader class of arachnids, which includes spiders, scorpions, and mites, true viviparity is not found in the order Araneae. It is characteristic of other arachnid groups, however. For example, all known species of scorpions exhibit either viviparity or ovoviviparity, where the eggs hatch internally and the young are then born live. This difference may contribute to the general public’s confusion about spiders. While some mites are also ovoviviparous, true spiders are fundamentally egg-layers, and any appearance of live birth is merely the synchronized emergence from the protective silk sac.