The geometric framework provides a way for understanding the multi-dimensional nutritional relationships between consumers and their food. We use this approach to further our understanding of the feeding and nutritional ecology of a ubiquitous mixed-feeding insect herbivore that consumes a variety of host plants spanning a wide range of nutritional composition. Our overall objective was to examine feeding decisions, resulting performance, and post-ingestive consequences in a common mixed-feeding insect herbivore, Melanoplus bivittatus (Say) (Orthoptera: Acrididae), when presented with paired diets differing in protein:carbohydrate (p:c) ratio. Intake p:c of M. bivittatus differed among all but two treatments and in many cases was farther than expected from the previously identified p:c intake target for this species. Despite this variability in intake of protein and carbohydrate, we found few effects of the diet treatments on performance or post-ingestive processing. However, our results suggest that when feeding on high-quality diets, nutrients consumed in excess may be stored rather than excreted.