Animal growth depends on nutritional intake during development. In many animals, nutritional status is uncoupled from moderation of adult stature after adult size is achieved. However, some long-lived animals continue to regulate adult size and fertility in a nutrition-dependent manner. For example, the regenerating flatworm Schmidtea mediterranea becomes smaller, or degrows, during periods of starvation. These animals provide an opportunity to readily observe adult stem cell population dynamics in response to nutritional cues. We explored the role of insulin signaling in S. mediterranea. We disrupted insulin signaling via RNA interference and showed that animals, despite eating, degrew similarly to starved animals. Utilizing in situ hybridization and immunofluorescence, we assessed cellular changes in proliferative populations including the planarian adult stem cell population (neoblasts) and the germline. Both impaired insulin signaling and nutritional deprivation correlated with decreased neoblast proliferation. Additionally, insulin signaling played a role in supporting spermatogenesis that was distinct from the effects of starvation. In sum, we have demonstrated that insulin signaling is responsible for regulation of adult animal size and tissue homeostasis in an organism with plastic adult size. Importantly, insulin signaling continued to affect stem cell and germline populations in a mature organism. Furthermore, we have shown that adult organisms can differentially regulate specific cell populations as a result of environmental challenges.