First the puzzle: Is the spider’s sentence correct?
Okay, “all” here means he dreams about only one thing. That’s singular, right? So the verb should be “is,” right?
Then why does “is” sound wrong, and “are” sound right? Think about it before you read the next paragraph.
The reason is called attraction. It’s a legitimate rule in Latin, but not in English, though sometimes it happens. Attraction is when you base the form of a word to agree with the closest candidate (even if it’s incorrect). And “flies” is closer to the verb than the subject is.