These are the working ants. These are the source of the ant empire's power. Without these harmless ants the ants cannot survive, and you cannot complete the level :)
You have a limited amount of working ants (which can later be upgraded from 10 to 15, 20, 25 and max of 30). Protect these ants with everything you got, since they are the only ones capable of bringing the food to the ant's nest.