‚ÄčIf you pick a random species of insect and look inside it's cells, there's a 40 percent chance that you'll find bacteria called Wolbachia. And if you look at Wolbachia carefully, you'll almost certainly find a virus called WO, lying in wait within its DNA. And if you look at WO carefully, as Seth and Sarah Bordenstein, from Vanderbilt University, have done, you'll find parts of genes that look like they come from animals—including a toxin gene that makes the bite of the black widow spider so deadly.How on earth did this nested set-up evolve? How did a spider gene end up in a virus that lives inside bacteria that live inside the cells of insects?