The variation in instar number and the pattern of sequential instar development time of Tenebrio molitor L. (Coleoptera: Tenebrionidae) was studied under 4 different diet regimes. Addition of dietary supplements consisting of dry potato or a mix of dry potato and dry egg whites significantly reduced the number of larval instars and total development time. The pattern of sequential stadia length showed regularity and low variability in instars 5 - 9. Stadia length continuously increased between instar 10 and the last instar before pupation (P₋₁). Instar number significantly impacted total development time and was also significantly correlated with stadia length. The length of the 1^st, 2^nd, 3^rd, and 4^th stadia was not significantly affected by diet indicating that they may represent a stable part of the life cycle of T. molitor. The length of the last stadium (P₋₁) was regular within food treatments regardless of sequential instar number, but P₋₁ was also significantly affected by nutrition in a similar way as all the sequential stadia. The pattern of instar variation is discussed in regard to the insertion of new instars based on variation of stadia length.