Random Question from a friend working for a restaurant.

One of my friends works for a restaurant down in the city, and just texted me asking "Why is the last pour in a bottle of alcohol called a spider?" I don't drink so I have no idea, and a quick google search didn't give me jack. Can someone help?

Thanks!

Maybe the thinking is that the last bit of liquor in the bottle contained all the dead spiders & assorted detritus that had made its way in and settled to the bottom of the bottle, which would explain why it's traditionally on the house.