Nope, ants are not harmful to cats at all if they are eaten. Unless there was some sort of pesticide or something on them, but then the ant probably would have already been dead, and your cat would have to eat a LOT of them before ingesting enough to present any harm. The only insects your cat should not eat are ones that are poisonus (bees, wasps, poisonus spiders etc.) Because they can bite the mouth or throat and cause it to swell, which can stop them from eating and even obstruct their airways. As long as it was just an ant, I see no harm in it.