The Gerromorpha are adapted on the water surface film. These insects exhibit strong associations bet- ween their genetic component, some structural elements of their body and the habitat. The genus Rhagovelia (Veliidae) is found in lotic aquatic ecosystems in heterogeneous microhabitats, where show differences in the middle legs, which are involved in the hydrodynamic of insect. Goals. In order to compare the interpoblational variability of the middle leg of Rhagovelia gastrotricha. Methods. This study analyzed four populations of R. gastrotricha in the sub-Andean foothills of the pacific slope in the west Andes at altitudinal ranges that go from 1000 to 1220 m.a.s.l., between the towns of "Ricaurte" (forest streams Quebrada Palpis and Quebrada Amor) and "Altaquer" (Quebrada Ensillada and River Ñambí), in the department of Nariño. Samples were collected in November 2011. We analyzed by means of geometric morphometrics (Fourier's elliptic descriptors) the outline of femur, tibia and tarsus segments, of 40 individuals from four populations of southwestern Nariño. Results. The insects with middle legs that present bigger contour of the femur and tibia were of the water bodies from Quebradas Palpis and Amor; next precedes River Ñambí. The Ensillada's specimens presented minor contour so much of the femur as of the tibia. The variance analysis indicate significant differences (p<0.05) in the outline of the middle leg between populations. Conclusions. Differences are justified by relation with the topography and some physic-chemistry parameters of water bodies.