How Can I Identify Spiders by Their Egg Sacks?

Some spiders can be identified by their egg sacs.

Identifying spiders by their egg sacs requires a certain amount of knowledge and expertise, and can still be a difficult endeavor. You must learn the habits of each spider that lives in your area, including the physical characteristics (i.e., shape, size and color) of its egg sac. Often, even an expert cannot identify a spider simply by its egg sac. Fortunately, you can easily learn to identify the egg sacs of several of the venomous spiders found in the United States.

Things You'll Need

• Flashlight
• Camera
• Jar with lid (optional)

Instructions

1. Investigate the location where the egg sac was placed, using a flashlight. Watch out for spiders guarding the egg sac! Venomous hobo spiders (a.k.a. aggressive house spiders) in the Pacific Northwest place their whitish egg sacs near their funnel-shaped webs or under rocks, logs or other items near their webs. Venomous widow spiders place their egg sacs in sheltered areas under patio furniture, in window frames or other secluded areas. Venomous brown recluse spiders often carry their egg cases around with them, but might leave them in sheltered, indoor areas.

2. Examine the texture of the egg sac. Brown widow spiders lay uniquely textured egg sacs that look like spiky, off-white orbs. Black widow spiders produce somewhat smooth egg sacs, whereas brown recluse and hobo spiders create smooth egg sacs but cover them with films of rough webbing.

3. Consider the shape of the egg sac. Black widows create egg sacs that are not only somewhat smooth, but are also slightly pear-shaped. Brown recluse and hobo spiders produce more oval-shaped egg sacs, although the shape might be camouflaged by webbing.

4. Photograph the egg sac or collect it. Bring the photo or the egg sac to your local Extension Service office for help with identification and advice for dealing with possible health hazards.

Tips & Warnings

• Collect or remove egg cases by winding the attached webbing around a stick to prevent damaging the egg sac and accidentally releasing hundreds of tiny spiders.

• Make sure to use the flashlight to illuminate the area while you examine the egg sac -- some spiders guard their egg cases and can deliver a nasty bite.

References

• Washington State University Extension: Spiders
• Animal Diversity Web; Black Widow Spider; Matthew McCorkle; 2002
• University of Florida Extension; Brown Widow Spiders; Fred Santana
• Animal Corner: Hobo Spider
• University of Florida Entomology; Featured Creatures; Brown Recluse Spider; G.B. Edwards; August 2003

Resources

• United States Department of Agriculture: Cooperative Extension Service Offices