Spiders are automated search engine listing programs that wander the internet 24/7 looking for new websites to list in the search engines. If there are spiders listed in your tracking this is a very good thing. It means that you are being found by search engines. Some nice spiders to look out for are the "Googlebot" from the Google Search Engine, and "Yahoo Slurp" from Yahoo, MSNBot from the new search engine Bing.
Spiders return to websites regularly to look for new and updated content. We request spiders to return daily, but due to the amount of websites on the internet it can take 2-3 weeks to be revisited.