"I have a 20 month old son and an 8 month old daughter. My son runs and hides behind me and screams frantically whenever my daughter crawls towards him. What’s up with this?"
Heh. I'm imagining your daughter as a pint-sized Godzilla, storming through Tokyo as your son cowers behind Century Tower.
I think he's probably scared because she's crawling, and that just freaks him out. It's the same reason some adults freak out when we see a mouse running across the floor or a large spider crawling on the wall. Even though we're way bigger than the mouse or the spider, it's the crawling aspect and the "otherness" that scares us.
I have no idea what to do about it, really until she starts to walk, at which point I'm pretty sure he'll stop being afraid of her. Maybe some of the readers have a suggestion about what to do to stop the screaming and cowering? I'll just tell you to make sure you videotape his reaction at least once, because it'll be priceless when they're older.