Scaling exponents (, , ) for the different species.

According to the values of the scaling exponents, the seven species listed are grouped into two categories: scale-free fractal networks and exponential (non-scale-free) fractal networks. The scale-free networks have a power-law degree distribution with exponent , and the non-scale-free fractal networks have an exponential degree distribution with . Notice that none of the networks are small-world. Instead, they are characterized by fractal/modular structures.