The importance of aquatic arthropods in the processing of organic matter in fluvial systems is well known, but this topic has been poorly studied in Neotropical rivers. In this research, we studied the composition of functional feeding groups (FFGs) associated with differences in elevation in a tropical river in northern Colombia during the wet and dry seasons. Between 2008 and 2013, we collected benthic arthropods at 3 sites located in the upper (San Lorenzo), intermediate (La Victoria) and lower (Puerto Mosquito) sections of the Gaira River. We found some differences in the gut contents and FFGs of the animals from different sites and between the climatic seasons. The dominant food source at all the sites and during both seasons was fine particulate organic matter (FPOM). At La Victoria, the genera Leptonema, Smicridea and Phylloicus (all belonging to Trichoptera) presented significant differences in the consumption of coarse particulate organic matter (CPOM) between the rainy and dry seasons (p < 0.05). At San Lorenzo, Leptonema had the highest animal tissue consumption value (p < 0.05). A discriminant function analysis based on gut contents suggested that some taxa may have been assigned to the wrong FFGs. We concluded that the diets of the aquatic arthropods in our study tended to present high trophic plasticity. Consequently, our results suggest that Neotropical rivers need to be re-evaluated in terms of traditionally established FFGs, which heretofore have been based on information from other regions of the world, producing incorrect assessments of aquatic systems.

A new riffle beetle, Zunielmis pax gen. et sp. nov., is described from Colombia (Vichada, Puerto Carreño, Bita River basin). It can be distinguished from all the other elmids by having large areas of a microgranular mesh (plastron) on the pronotum and elytra, crenulate lateral margin of the latter two, the shorter interocular distance than diameter of each eye, as well as by the characters of the structure of mandibles, the shape and sculpture of pronotum, and the structure of mesosternum, mesotibiae and elytra. The imagoes were collected on freshwater sponges. The habitat, where the new species was found, is described, and the adaptive meaning of the characteristics of plastron are discussed. Drawings and photographs of the adult habitus, distinctive morphological characters, and male and female genitalia are provided.