Sac Spiders

Details

Sac spiders are a broad group of spiders that build small silken retreats or “sacs” in which they rest, rather than relying on large capture webs to catch prey. The name most often refers to species in the familyCheiracanthiidae – especially the pale “yellow sac spiders” in the genus Cheiracanthium – but similar sac-like retreats are also made by some related spiders in other families.

Basic Biology

• Representative scientific scope: Family Cheiracanthiidae – sac spiders (notably the genus Cheiracanthium)

• Common English names: Sac Spiders, Yellow Sac Spiders (for pale species)

• Body size: Most species are 4–10 mm in body length, with a leg span of about 1–2 cm.

• Life span: Typically around 1–2 years for small species, with females producing several egg sacs during their lives.

Silken Sacs & Behaviour

Sac spiders are primarily nocturnal hunting spiders. Instead of constructing a large prey-catching web, they spin a small, flattened silken sac on a leaf, in a crevice or in the corner of a wall:

• By day they rest or moult inside the sac;

• Females often lay and guard egg sacs within these retreats;

• After dark they leave the sac and actively search for prey on foliage, walls and other surfaces.

Appearance

Typical sac spiders are small, agile spiders with slightly elongated bodies and relatively long legs, but they are not as extremely long-legged as cellar spiders. Common “yellow sac spiders” are usually:

• Pale yellow, cream or light green in overall colour;

• Often with a slightly darker head region and sometimes a faint dorsal stripe;

• With smooth, somewhat glossy abdomens and minimal patterning.

Their movements are quick and purposeful, and when disturbed they tend to run rapidly or retreat into their sac.

Distribution & Habitat

Sac spiders in the family Cheiracanthiidae are distributed worldwide, especially in temperate and tropical regions. They occupy a wide range of habitats:

• Leaves and branches in forests and scrub;

• Crops, grasslands and orchards, where they roam on foliage;

• Gardens, hedges, walls, fences and outdoor lights around houses;

• Sometimes indoors, where they may build small silken sacs in corners, window frames or behind furniture.

Diet & Ecological Role

Sac spiders are opportunistic nocturnal predators that feed on:

• Aphids, leafhoppers and other soft-bodied plant pests;

• Small flies, moths and various other insects;

• Occasionally other spiders or small arthropods.

Because they consume many herbivorous insects, sac spiders are often considered beneficial natural enemiesin agricultural and garden ecosystems.

Venom & Medical Significance

Like all spiders, sac spiders have venom glands to subdue prey. Some yellow sac spider species have been reported in the medical literature as occasionally biting humans, and there has been debate about whether their bites can cause significant skin reactions.

Current clinical evidence suggests that most sac spider bites cause local pain, redness and mild swelling, sometimes with a burning sensation that may last for several hours or days. Serious systemic effects are rare and they are generally not considered highly dangerous to healthy people.

Human Encounters & Precautions

Because sac spiders readily use human structures as daytime retreats, people may notice their small white silken sacs on leaves, under outdoor furniture or in corners of buildings. Simple precautions help reduce the chance of unwanted bites:

• Check and gently shake garden tools, pots and outdoor furniture before handling;

• Avoid squeezing or tearing open silken sacs with bare fingers;

• If a bite is suspected, wash the area with clean water, monitor symptoms and seek medical advice if pain or redness persists or worsens.