You will never find out the biggest clue and i won't tell
But you are right the code allows only 200 spiders do collision avoiding so some you are seeing wont do it but those are rather far away so you cant tell exacly what they are doing.
Maybe someone can guess what the main trick is until i finshed the wall walking stuff and then i post a demo which shows the trick which is i think a very good way to reduce cpu usage xD (BTW: do you have a link where the walk-walk-con. is?)