The terrestrial slug Deroceras laeve is abundant in diverse habitats spanning an enormous geographic range. This paper explores the hypothesis that much of this success may be attributed to phenotypic flexibility. Breeding experiments demonstrated that food quality had an enormous impact on growth and maturation rates, maturation sizes, reproduction, and longevity. Maturation rates and sizes covaried along a plastic development trajectory in response to dietary constraints on growth rate. The strategy for a particular dietary regime agreed well with known optimal control models. Published models developed for particular genotypes conformed to single cross sections along the plastic trajectory. Adaptive plasticity was augmented by resilient compensatory responses to food shortages, and robustness with respect to locomotor abilities, an omnivorous diet, and resistance to extreme temperatures. This combination of plasticity, resilience, and robustness could offset the low rate of sexual reproduction and associated low genetic variance documented in this species.