They don't necessarily look for sugar, but they often find the sugar. Once the ants have established a sugar source, they will hurry back to their colony to alert all the other ants what they have discovered. During their rush to the colony, they leave a trail, which results in a swarm of ants congregating on the sugar source.
Ants use their powerful antennae to uncover particles in the air. It is very much like the smell. Ants, like other insects, use chemosensor to find sugar and other food. Ants also can detect chemical substances in their environment, and when these chemicals are present (even at low concentration), they can be identified as smells.
Sucrose is a special type of sugar, and ants get attracted to sugar very easily. It has been researched that the fact that ants are attracted to sugar does not mean they don't have their preferred sugar. You can put five different kinds of sugar on the floor; they will only go for the one they consider as best for them. Ants are able to locate their foods not actually because of their eyes, but because they can use their antennas to smell the food.
When worker ants move out in search of food, they use their antennas to locate and to spot what they are looking for. They can easily detect the chemical composition of any food they come in contact with. Because sucrose is one of their best sugar, they know how it smells. After this, they will carry the sugar back to their nest. There is a special substance known as pheromones that will inform the rest of the colony about the food.
