Geographic clines in body size have been described for many species, but relatively few investigations have tested hypotheses for the ontogenetic mechanisms maintaining geographic clines. We formalize and test the predictions for the role of ontogenetic mechanisms (e.g., hatching timing and size, juvenile developmental time, juvenile growth rate) in maintaining a longitudinal cline in adult body size of lubber grasshoppers [Romalea microptera (Beauvois)]. To obtain hatching timing and size, we collected eggs from wild females from several populations along the longitudinal gradient in 2 yr (2006 and 2007) and measured hatchling size and hatch date. To obtain juvenile developmental time and growth rates, we surveyed populations along the longitudinal gradient during 2 yr (2006 and 2007) and estimated developmental time and growth rates. We found the developmental time (hatching to fourth instar) and female growth rates increase from west to east along the cline. Patterns of hatching timing and hatching size were not consistent with the size cline. The size cline becomes evident in the third instar and is magnified in the fourth and fifth instars. Our data suggest that the size cline arises from some combination of clinal variation in developmental time and female growth rates; prolonged development and greater growth rates lead to larger mean size. Equally important, we found no evidence that differences in hatching time or size are ontogenetic causes of this cline. Our hypotheses for ontogenetic mechanisms producing an adult size cline should serve as a template for ecologists seeking to understand the ontogenetic basis of spatial variation in phenotypes.