Darwin in his book, The Descent of Man and Selection in Relation to Sex, described his fascination with a male Chiasognathus grantii Stephens (Coreptera, Lucanidae), a stag beetle species with enormous mandibles. Wallace in his book, The Malay Archipelago, recorded his enchantment with a male Euchirus longimanus L. (Coreptera, Scarabaeoidae), a ‘chafer’ species with ‘immense fore-arms’ (tibia or femora). These oversized traits have been interpreted as the results of allometry through sexual selection. Both Darwin and Wallace were aware that there was a large variation in these oversized traits intraspecifically, leading to the question of the role of variability in species-level evolution. To answer this question, I studied the variation in developmental plasticity (DP; the individual ability to react to external environment) of the stag beetles, which shows a great variation in external morphology. These observations indicated that 1) DP (measured in body length) is independent of allometry of the mandibles, 2) allometry of the mandibles had been ingrained in most members of the Family before the differentiation of DP took place, 3) DP is indirectly related to sexual selection through elongated mandibles that are used for fighting or securing the mate, as evidenced by the close correlation of DP with the mandible length, and 4) the enormous mandibles of stag beetles are the product of enlarged DP through allometry and sexual selection. From these results, I add enhanced DP as an independent element in the evolution of seemingly oversized traits seen in many beetle species.