The front of a fiddler crab specifically refers to the section of the carapace in between the eyestalks. Traditionally, fiddler crabs are often split into two groups based on the width of the front, the “narrow-front” and “broad-front” groups. While these groups may not be monophyletic, the concept still serves as a simple way to start evaluating species.
The narrow-front species tend to have the eyes extremely close together, while the broad-front range from moderately broad to very broad. There is variation within species, and the breadth of the front changes as crabs get older and larger (within broad-front species, the front gradually gets wider as the crab ages, with smaller/younger crabs having narrower fronts than larger/older crabs).