The behavior, development, morphology, and ecology of parasitic insects are known to be influenced by various aspects of their hosts. These aspects include chemical cues emanating from the potential host that influence host location and acceptance by the ovipositing parasite female, as well as the age, size, and general physiological state of the host in which the immature parasite develops. Some parasitic insects develop optimally in the host egg, others in early to mid-larval instar hosts, and still others in late-larval to pupal stage hosts. This niche partitioning among parasites likely reflects parasite strategies to minimize competition. However, important biochemical conditions mediated by endocrine and nutritional factors within the hosts themselves dictate the extent to which these strategies are successful. In this paper I suggest that endocrine-mediated metabolic events in late-larval through pupal stage insects render these stages to be a nutritionally distinct resource from the early through mid-larval stage. Consequently, members of these two groups are exploited by insect parasites which themselves have different nutritional requirements. To support this hypothesis, selected aspects of insect nutrition and endocrinology that may influence the suitability of early to mid-stage larval hosts vs. late-larval to pupal hosts for parasite development are reviewed. The possible impact of host suitability on aspects of parasite ecology, e.g., sex ratios and voltinism, i s also briefly addressed.