Ultrastructural analysis of the ventral region of the head - rostrum, buccula and stridulatorium sulcus - of 1st, 3rd and 5th instars of Triatoma klugi Carcavallo et al, Triatoma vandae Carcavallo et al, and Triatoma williami Galvão et al, are described in here. Morphological differences in the analyzed structures for all three Triatoma species studied were detected under scanning electron microscopy, allowing their grouping by their morphological similarities. Species-specific differences at each nymphal development stage were analyzed as well.