Egg sacs, spiderlings and dispersal

Find out how spiders protect their eggs and how the newly hatched spiders make their way into the world.

The egg sac silk protects the eggs against physical damage and excessive drying, wetting or heating, as well as providing a shield against predators like ants and birds. However, this protection is often breached by parasitic wasps, flies and mantispid lacewings that succeed in laying their eggs or infiltrating their larvae among or within the spider's eggs. Spiders like redbacks lay many eggs and make several egg sacs to ensure that enough eggs survive these seasonal onslaughts.

The eggs of many spiders are glutinous and stick together allowing them to be laid in a continuous stream into the partly built silk egg sac. They vary in colour from pearly white to green and in number from 4 to 600 in a single egg sac, depending on the species concerned.

Egg sacs come in all shapes, sizes and colours. They may be built inside a burrow (e.g, trapdoor spiders), under bark (e.g, huntsman spiders), in the web (e.g., black house spiders), in a curled leaf (e.g., leaf curling spiders), suspended on a long line (two-tailed spiders), or hidden among foliage (e,g., orb weaving spiders). Some spiders stay with the egg sac, guarding it until the spiderlings emerge (e.g, huntsman spiders, trapdoor spiders) or carry the egg sac about with them (wolf spiders, water spiders), sometimes in their jaws (daddy-long-legs spiders). Wolf Spiders carry their spherical egg sacs slung from the spinnerets. When the young hatch they climb onto the mother's back, clinging to special knob-shaped hairs. The mother carries them about until they moult and disperse.

In many species, like orb weaving spiders, the egg sacs are simply abandoned, sometimes protected among leaves or in silk barriers, or even shallowly buried in soil (Nephila pilipes). Exposed egg sacs usually have a surface silk layer of dull brown, green or russet coloured silk, often further camouflaged with leaf debris to help prevent eggs being eaten or parasitised.

After hatching from the eggs the spiderlings stay within the egg sac until they undergo their first moult - their small cast skins can be seen inside the old egg sac. After this they emerge, having cut a neat hole in the sac with the fangs (perhaps aided by a silk digesting fluid and sometimes helped by the female from outside). The spiderlings cluster together initially, still living largely upon the remnants of yolk sac in their abdomens.

After several days (or weeks in the case of some mygalomorph spiders) and sometimes another moult, the spiderlings begin to disperse gradually away. This is necessary to avoid competition for food and prevent cannibalism among the hungry siblings. Some species, especially ground and burrow dwellers, disperse by walking, often over only relatively short distances. Others, especially foliage dwellers and many web builders, but also wolf and mouse spiders, disperse by bridging and ballooning. Bridging is a means of travelling by repeated climbing up through foliage and then dropping down on a silk line to cross to adjacent branches, often with some breeze-assisted swinging. Ballooning involves ascending to a high point on foliage and letting out fine silk lines that catch the breeze and eventually gain enough lift to waft the spider up and away. While long distance flights can occur (Charles Darwin noted spiderlings landing on the rigging of the Beagle, 100 km out at sea), the more usual outcome is for spiders to be deposited anything from a few metres to a few kilometres from the start point.

Simultaneous ballooning by thousands of spiderlings can result in a remarkable carpet of silk, called gossamer, covering shrubs or fields.

Having survived the perils of wasp, fly and mantispid lacewing egg parasitism in the egg sac, the life of spiderlings remains beset with dangers. Only a few will avoid being eaten and find adequate shelter and food to ensure their survival to adulthood, so any help is useful. The first orb webs of St Andrew's Cross spiderlings have a 'doily'-like patch of white silk at the centre which may be both attractive to insect prey and provide a 'hide' for the spider to disappear behind when predators appear. Some spiderlings simply don't leave home and grow up in communal webs and dispersing just before maturing (e.g., Phryganoporus candidus). Sticky web building spiderlings can partly support themselves simply by eating their own webs. Sticky webs like orb webs pick up valuable nutrients such as pollen grains that simply get windblown onto them - and, because sticky silk absorbs moisture from the air, which also condenses as dew on silk lines, the spiderling gets a drink as well.