After it dries out you will. You won't want to keep replacing a damp paper towel in the bottom of the enclosure every day... too much work. Just change it when it gets dirty. Also, depending on the species you keep, your mantids may need a drink, which misting around their heads or where they can bend down to drink the droplets on the container surface will take care of. They won't drink from a moist paper towel in the bottom like crickets or some other insects might.