Do you mean an "aphid?" The answer is because both get milked. Humans milk cows (or use machines to cause the cows to be milked); ants 'milk' aphids. Learn more here: http://www.bigsiteofamazingfacts.com/how-do-ants-milk-aphids-for-food-like-cows-and-do-ladybugs-eat-aphids