A dimorphic pattern of macrocephalic and stenocephalic body shapes is observed in snail-feeding carabid beetles globally. The macrocephalic body shape is the phenotype that beetle heads and thoraces are short and widely expanded, while stenocephalic body shape is the phenotype that those are narrowly elongated. The former wide-short body shape exhibits high performance in crushing snail shells with powerful jaws, but their wide and short heads do not function when they eat snails by inserting their heads into the shell aperture and directly bite the snail soft-tissue. In contrast, the latter narrow-long body shape is effective in the shell-entry feeding behavior, whereas does not have sufficient high power to crush snail shells in their jaws. That is, trade-off exists between shell-crushing and shell-entry, either of which is effective in snail feeding. And therefore, the dimorphic patterns of macrocephalic and stenocephalic might be observed in the snail feeding carabid beetles. We will show experimental research by using a representative snail-feeding carabid beetles, Damaster blaptoides, to support this hypothesis. Recent years, we crossed wide-short species and narrow-long species, and produced their F₁ and backcrossed hybrids. We will also report the results of quantitative genetic analysis conducted by using these hybrid lines.