A striking feature of Triatominae is that males, females, and nymphs of all instars can transmit T. cruzi if infected [35, 36]. Therefore, studies about nymphal instar have not only taxonomic and phylogenetic interest but also epidemiological importance. Specifically, about the genus Rhodnius, the following works can be mentioned: Mascarenhas , which studied the five instars of R. brethesi; Ponsoni et al.  and Marconato et al. , which carried out a biometric study of nymphs of Rhodnius neglectus Lent, 1954 and R. prolixus; Santos , which described nymphs of the five instars of Rhodnius colombiensis Meija, Galvão & Jurberg, 1999, Rhodnius ecuadoriensis Lent & León, 1958, R. milesi and Rhodnius stali Lent, Jurberg & Galvão, 1993.
Morphological characters are useful tools for taxonomic and systematic studies in Triatominae, in addition to being useful for epidemiological surveillance. The morphology analyses show the separation of the three species by characters was observed at head, thorax, and abdomen shape. This made it possible to separate them in all five nymphal instars and characterized for the first time the development stages of R. marabaensis. In the chapter on the nymphal instars, Lent & Wygodzinsky  mentioned that R. prolixus and R. robustus do not have sub median tubercles or aggregations of granules along midline, but such characters were noticed in all five nymphal instars of those species, as well as in R. marabaensis. Rosa et al. , studying 1st and 2nd instar nymphs of Triatoma arthurneivai Lent & Martins, 1940, distinguished the two instars by morphological characters of the thorax. Thus, by scanning electronic microscopy, they noticed the absence of collar, glabrous areas, and tubercles in the 1st instar of T. arthurneivai, which were present in the 2nd instar. Nevertheless, the differentiation among 3rd, 4th, and 5th instars of R. marabaensis, R. prolixus, and R. robustus was made using the same characters observed by Rosa et al.  in nymphs of the previously mentioned instars of T. arthurneivai, i.e., the formation and conformation of the two pairs of wing pads located on the thorax.
In this study, the results of the morphometry of characters from the abdomen, antenna, head, proboscides, and thorax show little discrimination between the three species. In general, the compared averages are little or no significant, the morphometric study is not suitable for identification. However, R. marabaensis had its nymphs characterized morphometrically and morphologically for the first time.
The relative length of the four antennal segments in R. marabaensis shows the same pattern for the first three instars, another for the 4th instar, and a third pattern for the 5th instar, whereas R. prolixus and R. robustus show the same pattern for the 1st and 2nd instars, another for the 3rd and 4th instar, and a third one for the 5th instar. Santos , measuring R. colombiensis, R. ecuadoriensis and R. stali, found two patterns of relative length for antennal segments of the five nymphal instars. For R. milesi the author found three patterns: one for the 1st and 2nd instars, another for the 4th and 5th instars, and a third one for the 3rd instar, hence different patterns from the ones observed in R. marabaensis, R. prolixus and R. robustus.
Rosa et al.  carried out a morphometric study of the four antennal segments of nymphs of the five instars and adults of Panstrongylus megistus (Burmeister, 1835), R. neglectus, R. prolixus and Triatoma vitticeps Stäl, 1859. The patterns identified in R. neglectus and R. prolixus were the same found for R. prolixus and R. robustus in this work. Rosa et al.  measured the antennal segments of T. rubrovaria (Blanchard, 1843) and found patterns different from R. marabaensis, R. prolixus and R. robustus, but similar to those observed in P. megistus by Rosa et al. . However, in relation to the relative length of the four antennal segments, it is not possible to differentiate the studied species. The different results were described for R. colombiensis, R. ecuadoriensis, R. milesi, R. stali  and R. neglectus , T. rubrovaria , P. megistus, and T. vitticeps . Furthermore, our data show that R. prolixus and R. robustus are like R. neglectus  and can be distinguished from R. colombiensis, R. ecuadoriensis, R. milesi, and R. stali  as well as T. rubrovaria , P. megistus, and T. vitticeps  for this characteristic.
Geometric morphometry allows evaluating the variation of shape in relation to causal effects . The technique allows us to quantify biological forms and discuss the evolution of phenetic patterns . The technique is used in paleontological, anthropological, ecological, zoological, and botanical studies [29, 33]. In triatomines, geometric morphometry is used to assess the shape and size variables of hemelytra [43, 44], heads [12, 45], and eggs . Also, for ontogenetic studies [47, 48, 49].
Recently two subcomplexes of the genus Triatoma were studied by means of geometric morphometrics, which indicated the potential of the technique to study specimens that are phylogenetically close [43, 45]. Geometric morphometrics allowed describing the differences in head shape and size of the five nymphal instars. In relation to the CS, all the values obtained were significant and allowed to differentiate the three species in the five nymphal instars. A variation among the instars is noticed but considering the general aspect R. robustus is easily characterized by the geometric profile of the heads of nymphs. The 2nd and 4th instar showed less discrimination potential, i.e., only approximated size means were recovered.
The metric estimator of Mahalanobis distance was used to recover NJ dendrograms, where it is possible to visualize that in all evaluated instars R. robustus is distant, whilst R. prolixus and R. marabaensis form a single clade. However, CVA ellipses show that in the 1st and 2nd instars R. marabaensis and R. robustus remain close, while groups are clearly separated in the 3rd, 4th, and 5th instars. Regarding the shape, the values of the Procrustes ANOVA test reveal differences among the cephalic capsules, allowing discrimination. We show that the multivariate morphometric technique is more efficient to discriminate against the studied species when confronted with linear morphometric data.