Specialized predators must obtain all required nutrients from a single prey species. In some species nutrient balance may be possible by selecting various body parts. We tested how different ant body parts affect the fitness of a specialized ant predator and whether the predator possesses adaptations in its feeding behaviour that allow nutrient balancing while feeding on only a single prey species. We used a formicine-specialist spider, Zodarion rubidum, reared on three diet types: an entire ant, two ant gasters and two ant foreparts (heads, thoraces and legs) of Lasius ants. Spiders grew faster, survived longer and developed earlier on the diet consisting of two ant foreparts. Spiders fed on the two ant gasters had the slowest growth, highest mortality and slowest development while spiders fed entire ants showed intermediate performance. In preference experiments, we studied the spiders' consumption of three Formicinae ant species that differ markedly in size. With small Lasius ants, spiders equally exploited the gaster and the foreparts, but with larger Formica and Camponotus ants, spiders fed significantly more on the foreparts than on the gaster. Spiders almost always fed on the gaster, suggesting that it might include beneficial nutrients. Nutritional analysis of the ant bodies of the three species revealed that there were more lipids in the gaster, while the foreparts contained more proteins. Our results suggest that ant-eating spiders might balance their nutritional needs by selectively consuming various body parts of their exclusive prey.