Explanation: Spiders catch flies in their webs; flies (fly- singular) are small insects with wings. When spiders build spiderwebs the flies get stuck in them and the spiders eat them. When a baseball player hits a baseball and it goes up in the air to the outfield we say, “the baseball player has hit a fly ball,” that is, the player hit the ball and it flew through the air. The outfielders, the players standing far away from the batter, are waiting to catch the fly balls that have been hit. An outfielder who is good at this is good at catching flies… just like a spider.
Thanks “Tree” for the joke!