Reading User-Agent field for spiders

The error is in your pool statement, you are missing the connector between spiders/www and the output of [HTTP::header Host]. I'm not sure how your pool is defined, but following the other rule it would look like this:

when HTTP_REQUEST {
  if { [matchclass [HTTP::header User-Agent] contains $::useragents] } {
  pool spiders_[HTTP::header Host]
  } else {
    pool www_[HTTP::header Host]
  }
}