Net fitness in spider mites, defined as the number of adult offspring per female, is a function of the number of eggs produced and the survival rates of eggs, larvae, protonymphs and deutonymphs. The relationships between fitness and its components were estimated in some laboratory environments. It was found that egg production overwhelmingly determines female fitness. Since the fitness components completely determine the net fitness, the relationship may be considered as a closed system of interacting components. A method of analyzing such a system is also introduced.