The cats do seem to have an innate sense of what can hurt them. One evening our older cat was raptly watching something on the floor and mewling loudly. He wasn't approaching whatever it was, but badly wanted to attract our attention to it. Closer inspection showed a very large black widow wandering across the hallway carpet. A heavy magazine quickly dispatched the trespasser.
Black widows are just a bit too dangerous to have wandering around the house.
They also are rather unpredictable if you try to capture them. While a bite probably won't kill you, unless you are very allergic, you will feel pretty lousy for a few days and have a nasty sore for a while. Update To my surprise, all the passion vines, transplanted a month ago, are throwing off new leaves. Plants can certainly amaze you sometimes. I thought all were goners.