Environmental pressures in caves can act on species, selecting functional attributes that allow their best performance in such habitats. Caves are simplified habitats, which makes them ideal for testing ecological models more accurately. From this perspective, we examined whether competition limits the morphological similarity among individuals of the species Heterelmis sp. and Hexacylloepus sp. (Coleoptera—Elmidae), along a surface-subterranean gradient. We used a Mann–Whitney test to assess morphological differences among individuals. Furthermore, we used the outlying mean index to explore the extent of niche overlap between the two species in surface and subterranean environments. The results indicate that in a cave environment, organisms avoid niche overlap by differing their body size and environmental preferences, and such avoidance is likely related to the increase of competitive interactions. Hence, it is plausible to assume that morphological modifications, such as increased body size, may occur in response to evolution within cave environments, partly due to the reduction of similarity between co-occurring species.