Maybe there's a second act twist where the coroner is trying to figure out why the giant spider bites don't seem to match the wounds on their paralyzed victims. Then a spider explodes and hundreds of giant parasitic wasps burst forth from its swollen abdomen. And then you realize that the wasps can sting and paralyze humans, who forget that it happened and several characters have already been "stung" and infected. Wasps start exploding from all the victims, and the humans team up with the giant spiders to spin a giant web and catch all the thousands of baby wasps.