Some plants recognize herbivore attack and distinguish it from mechanical wounding because of the chemical components in caterpillar regurgitants. Volicitin and its related compounds, collectively called fatty acid amino acids conjugates (FACs), are such elicitors that function as key compounds to trigger volatile emissions in several plants and ironically help foraging parasitoids. But we do not know if the chemical structural differences of FACs in caterpillars can explain the complicated plant responses which sometimes vary depending on the caterpillar species. It is even unclear why caterpillars make a variety of FACs. Previously, we proposed a hypothesis that FACs might be an intermediate that enhance nitrogen assimilation in insects. Also we reported a broad range of lepidopteran species which synthesize different FACs in different compositions. Based on these works together with the recent publications about FACs in a plant-herbivore interaction, we discuss the variation in FAC patterns in lepidopteran species.