Not sure, as best as I can figure it seems to end up being about 75, 80% of the viewers on any given night.
If you go to the "who's online" page and there are 8 pages of viewers, you can be rest assured 5 or 6 pages will be spiders.. On nights that there are actually a mess of us on here, like 4 or 5 pages worth, the spider pages seem to go up proportionately..
Maybe one of our local IT, confuser, gurus can explain...