Entomophagy in the riodinid butterfly tribe Eurybiini is demonstrated for the first time. Alesa amesis caterpillars and adults possess behavioral and morphological traits for feeding on Homoptera nymphs, and oviposition by A. amesis females is mediated by the combined presence of Camponotus femoratus ants and homopteran nymphs. Caterpillars are entirely entomophagous, and do not eat plant tissues. Alesa amesis caterpillars have distinct behaviors for feeding on their prey, and for soliciting and drinking honeydew secretions from homopteran nymphs. The leg lengths of entomophagous Alesa caterpillars are shown to be longer than phytophagous relatives. The legs of Alesa are used for prey handling and soliciting honeydew secretions. We suggest that elongation of the thoracic legs has been a general consequence of entomophagy in butterfly caterpillars. This study clarifies our understanding of A. amesis and its interactions with multiple species, and points to behavioral and morphological traits important to interpreting the evolution of entomophagy among caterpillars. Our observations establish the likelihood that other members of Eurybiini may be entomophagous, and suggest that entomophagy may have evolved independently among the Nymphidiini and Eurybiini.