Biology of the Ground Spiders (Family: Gnaphosidae)

1. Classification & Appearance

Feature Description

Family Gnaphosidae (Flat-bellied Ground Spiders)

Size Small to medium (typically 3 mm to 15 mm).

Colour Generally dark, often black, grey, or dark brown, sometimes with metallic, iridescent sheens (especially on the abdomen).

Body Shape Characteristically flat abdomen (or high but truncated) and sturdy legs, making them well-suited for crawling under objects and through crevices.

Spinnerets A key identifying feature: Their posterior spinnerets (silk-spinning organs) are usually large, cylindrical, and clearly visible as they stick out from the end of the abdomen.

Eyes Eight eyes arranged in two rows across the front of the head.

2. Hunting and Habitat

• Hunting Method: Ground spiders are active, wandering hunters. They do not spin webs to catch prey. They rely on speed and ambush tactics.

• Activity: Primarily nocturnal (active at night), which is why they are rarely seen during the day.

• Diet: Generalist predators, feeding on other ground-dwelling arthropods like ants, beetles, small crickets, and other spiders.

• Habitat in Alberta: They are highly abundant in the soil and on the ground in virtually every habitat type: under rocks, logs, leaf litter, debris, and along house foundations.

• Silk Use: They only use silk for two purposes:

  1. Safety Dragline: For stability and safety while running.

  2. Retreat Sac: To build small, silken retreats or sacs in sheltered locations (under objects, in cracks) where they rest during the day, molt, or lay eggs.

3. Key Species Example: The Black Sac Spider (Sergiolus)

One common example found in Alberta gardens and homes is the Black Sac Spider (Sergiolus species). These are often black with distinctive, white, or yellow bands across the abdomen. They are frequently misidentified as the much less common Western Black Widow due to their dark color, but are entirely harmless.

4. Life Cycle and Reproduction

• Mating: Courtship is minimal. The male finds the female's retreat and attempts to mate.

• Egg Sac: The female produces a round, silken egg sac that is usually attached to a surface (like a rock or bark) or hidden inside her day-time retreat. She often guards this egg sac aggressively.

• Lifespan: Most species complete their life cycle within one to two years.

5. Interaction with Humans

• Bite Risk: Low. Being nocturnal, they usually avoid humans. Bites are rare and defensive, typically occurring only when the spider is accidentally pressed against the skin (e.g., while working in the garden or rolling over on one in bed).

• Toxicity: Like most common house and garden spiders, their venom is not medically significant to humans. A bite would result in minor, localized pain and swelling, similar to a mosquito or bee sting.