I don't know why she swallowed the fly.
Once upon a time, I was worried about spiders biting my unsuspecting children. How foolish of me. The older kids can spot a hairy invader from 20 yards away--no unsuspecting victims, here!
The baby, of course, senses no danger but is curious about everything. He has perfected his pincer grasp and he's on the move. I was once worried about spider venom being poisonous but now I wonder what will happen if a child ingests the entire creature.
I justify smashing the critters with my shoe not just as a means of protecting my children but also as a type of mercy killing for the unsuspecting bug. If I were a spider, I'd rather go quickly than have to wriggle and jiggle and tickle inside of anyone.