ADVANTAGE OF COMPLEX LIFE STYLE. “This increase in life cycle complexity is thought to be adaptive for two reasons: (i) it facilitates stage-specific resource utilisation and structural specialisation, such as adaptation of different sets of mouthparts for alternative food sources; and (ii) it encourages population growth by reducing intraspecific competition between stages.”
http://onlinelibrary.wiley.com/doi/10.1111/een.12313/full