Triatominae species are considered the main vectors of Chagas disease or American Trypanosomiasis. In Venezuela, the principal vectors are Rhodnius prolixus (St?l, 1959) and Triatoma maculata (Erichson, 1848), which are belonged to the tribe Rhodniini and Triatomini, respectively. The head conformation and size development of these species can reflect ontogenetic changes which contribute with the vectors biology studies, as well to support of instars determination. The goal of the paper is to the application of geometric morphometric techniques for describing head conformation and size of instars of these species. We photographed 140 heads in R. prolixus: First instar (I: 16), second instar (II: 17), third instar (III: 18), fourth instar (IV: 21), fifth instar (V: 21), adult female (F: 26) and adult males (M: 21); in T. maculata heads of 136 specimens were photographed, I: 20, II: 17, III: 26, IV: 15, V: 19, F: 20 and M: 19. Landmark coordinate (x, y) configurations were registered and aligned by Generalized Procrustes Analysis. Covariance Analyses were implemented with proportions of re-classified groups and MANOVA. Statistical analyses of variance found not significant differences in head isometric size (Kruskal–Wallis) among IV and V instars in both species. The a posteriori re-classification was almost perfect in R. prolixus (82%) and T. maculata (86%); the main head differences occurs in antenniferous tubercles, postocular and preocular. Our study using quantitative tools for describing the shape differences contributes to explain the morphology variability and development of Chagas disease vectors.