Abstract Perithemis mooma, Kirby, 1889 and Perithemis icteroptera (Selys in Sagra, 1857) live in sympatry from southern Brazil to central Argentina. The taxonomy of the genus Perithemis Hagen, 1861 has been hampered by the use of characters that are highly variable or show slight differences among species. Our objective was to assess the efficiency of traditional morphometrics (TM) and geometric morphometrics (GM) to discriminate between these species using wing size and shape and vulvar lamina contour, and to analyze the presence of sexual dimorphism in wing size and shape in both species. The TM and landmark-GM methods were applied on the fore and hind wings, while the outline-based GM method was applied on the vulvar lamina. GM allowed species delimitation using shape variables of either wing. The wing and vulvar lamina shapes were confirmed to be good diagnostic characters to separate these species and appear to be promising tools for distinguishing among other species of this genus. Centroid size failed to achieve species separation. Both species exhibited sexual size dimorphism (SSD). In contrast to what would be expected for Perithemis whose males are strongly territorial, P. icteroptera and P. mooma showed female-biased SSD suggesting a common pattern in Perithemis.

The multivariate allometric growth patterns from longitudinal data of leg and rostral segments of all instars of Belostoma elegans (Mayr) and B. cummingsi De Carlo are presented for the first time, and the allometric coefficients are compared. The segments of the middle and hind legs present allometric coefficients with opposite polarity to those of forelegs. This discrepancy in the ontogenetic trajectories may be due to the different functions of the legs. The foreleg main function is to capture of the prey, while the middle and hind legs are adapted to swimming. The relative size of rostral segments is an important taxonomic character in Belostoma. In B. cummingsi, the proximal segment is longer than the middle one, while in B. elegans they are subequal. Our purpose is to explain these differences through the analysis of their ontogenetic trajectories in an attempt to illuminate the morphological differences among species from a developmental perspective.

Los patrones de crecimiento alométrico de los segmentos del rostro y de las patas de Belostoma elegans (Mayr) y B. cummingsi De Carlo son presentados y comparados por primera vez. Se emplearon datos longitudinales de todos los estadios ninfales en un contexto multivariado. Los segmentos de la pata media y posterior presentan coeficientes alométricos con polaridad opuesta a los de la pata anterior. Estas diferencias observadas pueden deberse al diferente rol que presentan las patas. La función principal de la pata anterior es la captura de las presas, mientras que las patas media y posterior están adaptadas para la natación. El tamaño relativo de los segmentos del rostro es una característica taxonómica importante en Belostoma. En B.cummingsi el segmento proximal es más largo que el medio, mientras que en B. elegans son subiguales. Nuestro propósito es explicar estas diferencias a través del análisis de las trayectorias ontogéneticas en un intento de aclarar diferencias morfológicas entre especies desde una perspectiva del desarrrollo.