"Look, the desert floor is moving," I told her. "Look, for real, the sand is....moving like it's liquid." I got out of the truck, and there were thousands, thousands, of tarantulas all headed west, northwest. Eerie.
As far as I could see the valley floor, there were tarantulas covering every square foot heeding the call.
I was riding with some friends from Benson, AZ to Bisbee one evening, and there was a tarantula crossing the highway every 50-100 yards. Not as many as what you describe, but when you looked out across the desert it was hard to imagine how many were on the move. They were all headed generally west, too. That night I made sure to sleep with my feet toward the east.