Biology of spiders

Spiders are not insects. They are close relatives of ticks, mites and scorpions, which all belong in the group called arachnids. Unlike insects, which have three main body sections and six legs, spiders have two body sections and eight legs. The eyes, mouthparts and legs are found on the front section of the body, known as the cephalothorax. The second section, the abdomen, bears the parts of the respiratory system (spiracles and/or book lungs depending on the type of spider), the digestive and reproductive systems, and the external organs used for spinning silk or webbing. Most spiders are identified by size, color, markings on the body and the number (usually six or eight) and arrangement of eyes. Female spiders wrap their eggs in a silken spun sac. Some spiders carry this egg sac, while others deposit it somewhere within their nest. Hatchling spiders (spiderlings) often produce a silk thread that allows them to disperse by "ballooning", i.e., being blown by wind currents to other areas.

How dangerous are spiders?

Encounters between people and spiders are usually accidental and bites are a response by the spider when its web or nest is disturbed. Most spiders produce venom therefore, they could be considered "poisonous". The venom is stored in glands that empty into the spider's fangs or chelicerae. For the most part, spider bites are insignificant. However, just as bee and wasp stings may trigger allergic reactions in some people, the same can be true for spider bites. Young children, the elderly and hypersensitive individuals are more likely to react more strongly to a spider bite. In North Carolina, there are few spiders that can inflect serious and painful injury. The two best-known poisonous spiders found here are the black widow spider and the brown recluse.

Control of Spiders Outdoors

Spiders are beneficial and control many insects that feed on the flowers, shrubs and other plants in our gardens and natural areas. Spraying for spiders in these areas may actually increase the number of pests by killing off other natural enemies as well. Web-building spiders are most likely to show up in areas where insects are abundant, e.g., wood piles, around porch lights, windows or water sources (such as water spigots). Knocking down these webs with a broom or burst of water from a garden hose is adequate for "control". Outdoor pesticide applications for spiders are largely unnecessary and should be avoided. On occasion, you will find spiders on objects or in areas that have been left undisturbed; this can include sandboxes or even children's toys. Check these items periodically for signs of spiders. Spraying pesticides around sandboxes or other play areas should be avoided whenever possible.

Finding a large number of spiders indoors usually means that there is an ample supply of insects and other "spider food" in the area. Any real attempts to get rid of spiders should focus on eliminating these insects. The long term solution includes non-chemical measures:

• Sanitation - reducing or eliminating conditions that attract insects, e.g., high moisture and ready access to food of some sort.
• Exclusion -find the entry points used by both insects and spiders and seal or close these areas.

There are a number of short-term solutions to spider infestations, as well. Knocking down and removing webbing, or mechanically removing/killing the spiders should be sufficient. Vacuum the areas along baseboards, in corners and under and behind furniture. Clean bookshelves periodically. If you're concerned that more spiders will show up (or hatch from an unseen egg sac), then you could resort to applying an insecticide along baseboards, in corners, and inside storage closets. Select a pesticide that is labeled for use against spiders indoors. Always read the insecticide label for complete instructions on how and where to use the product. Crawlspace are often attractive environments for spiders (but not necessarily the source of spiders entering your living area). Treating your crawlspace is an option, but simply setting off foggers ("bug bombs") is not likely to be effective and can be hazardous particularly if you contaminate your heating/AC ventilation system. Crawlspace treatments, if needed, are often best left to pest control professionals. The availability of particular insecticides may change from year to year, so consult the latest edition of the NC Agricultural Chemicals Manual or your local county Cooperative Extension office for advice on selecting insecticides.

Tips on Avoiding Spider Bites

Follow these suggestions to reduce the chances of being bitten:

1. Always check for spiders before sticking your bare hand(s) into dark corners or areas where you can't see your fingertips.
2. Always wear work gloves when handling boxes, firewood, lumber and other items that have been left in storage undisturbed for signficant periods of time.
3. Vigously shake clothing and shoes that have been left undisturbed for some time to dislodge any spiders and inspect them before wearing.
4. If you think you have been bitten by either a black widow spider or brown recluse, carefully apply ice or a cold-pack to the bite and seek medical assistance. If you can catch the suspect spider, bring it along for positive identification or take it to your local county Cooperative Extension office for identification.

For additional information about spiders in your garden in your garden, read Common Spiders in the Landscape. Additional informational about spiders and spider myths is available at the Burke Museum Website

Lycosid spider image courtesy of Dept. of Entomology, University of Nebraska-Lincoln

References used in preparing this publication:
Goddard, J. 1993. Physicians Guide to Arthropods of Medical Importance. CRC Press, Boca Raton, FL.
Kaston, B. J. 1953. How to Know the Spiders. Wm. C. Brown Co., Dubuque, IA.