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.
Laurel
My three are fascinated by ants, they sit and watch them for hours, but don't try to catch them. like they do with flies or some crawling beetles. The problem with ants is how to get rid of them in the house without harming the cats, and I have had to shut the cats out of a room while I treated it, then washed the floor thoroughly. I have not found an ant killer that is harmless to cats.