Black harvester ants are a highly polymorphic species of ant. Polymorphism in ants means that workers may be at different sizes, which are divided into classifications such as minors (small) and majors (big). In these pictures, you can see a supermajor worker ant behind a regular worker ant. (There are also many other species of ants which noticeable polymorphism is present. One of those are, extremely polymorphic, such as Carebara diversa or Atta cephalotes).
Both found near their colony, near grasslands.