Understanding the ultimate and proximate mechanisms of spadefoot tadpole developmental polyphenism is the first step in gaining a mechanistic and evolutionary understanding of the factors responsible for the control and evolution of polyphenism and the connection between the environment, genotype, and phenotype. Tadpoles of the spadefoot toad Spea multiplicata and Spea bombifrons can display either a “typical” omnivorous phenotype or a carnivorous phenotype in ephemeral ponds in the deserts of the American Southwest. In chapter 1 the ultimate (environmental cues and behavior) mechanisms of carnivore development were investigated. Out of over one thousand tadpoles used in the chapter 1 study, only 44 total carnivore phenotypes were identified, which were spread out among different temperature, food-type, density, and substrate treatments. Additionally, no evidence was found that muscle use differences, for the two behaviors quantified, are associated with myoenlargement in the carnivore phenotype. In chapters 2 and 3 the proximate (allometry, histology, and cellular proliferation) mechanisms were investigated. In chapter 2, trait allometry was investigated to determine how traits are growing in carnivores relative to omnivores and relative to other traits. It was found that the omnivore developmental program was modified to produce the carnivore phenotype by trait-specific heterochronic changes, and although most traits in omnivores were negatively allometric, carnivores showed a mixture of negative allometry, positive allometry, and isometry. Also striking is that these traits are modified, at least statistically, as two distinct groups, suggesting trait modularity. Finally, in chapter 3 histological and cellular regulation of carnivore myoenlargement was investigated, relative to omnivores. It was found that myoenlargement of the orbitohyoideus jaw and tail muscles in the carnivore phenotype is due to both hyperplasia and hypertrophy, relative to the omnivore phenotype, and the timing of each process and regulation differences between and within the phenotypes, suggest there may be at least three different developmental events regulating carnivore myoenlargement. Finally, the BrdU analysis showed that even though it was predicted that carnivores would show more labeled daughter nuclei, because of their myoenlargement, omnivores, in fact, showed significantly more daughter nuclei or no significant difference from carnivores.