The privet tree, Ligustrum obtusifolium (Oleaceae), defends leaves with lysine-decreasing activity caused by oleuropein that makes proteins non-nutritive. Privet-specialists caterpillars adapt by secreting glycine in the digestive juice to prevent the lysine-loss. Our survey into 42 lepidopteran and hymenopteran species showed that the average concentration of glycine in digestive juice for 11 privet-feeders (40.4 mM) is higher than that for 32 non-privet-feeders (2.2 mM). The glycine concentrations exceeded 10 mM in 7 out of 11 privet-feeding species. In Macrophya timida (Hymenoptera), it reached 164.8 mM. Three out of the four remaining privet-feeding species had other amino acids instead. Larvae of a butterfly, Artopoetes pryeri (Lycaenidae), had 60.8 mM of GABA. In two other specialists, b-alanine was found. The three amino acids were far more effective than other amino acids and amines in preventing the lysine-loss. Our results show a rare case of the convergent evolution of herbivores’ molecular strategies to feed on a plant with a chemical defense in a manner minimizing the cost of adaptation, and suggest the existence of a quantitative type of coevolutionary arms race between plants and herbivores.