Student or Learner
What does "If not bats" mean? "If they were not bats"? A repetitive event that happened in the past? What did he notice related to spiders? Did he just assume spiders were there?
ex)David Walker woke up in the basement of the house...He glanced around, sure to see bats hanging from from the ceiling. If not bats, then surely thick hairy spiders.
He expected to see bats. Or spiders.
From the text you cited, it's not clear if he actually saw anything.