Complex sentence: In incipient colonies, polymorphism does not exist, but instead they are occupied by monomorphic workers called "minims" or "nanitics"; the average head-width in tested colonies increases during the first six months of development.
Split into simpler sentences: In incipient colonies, polymorphism does not exist, but instead they are occupied by monomorphic workers called "minims" or "nanitics". The average head-width in tested colonies increases during the first six months of development.