In the southern Mexican Cueva del Azufre, a cave ecosystem with high concentrations of toxic hydrogen sulfide, one species of hemipteran (Belostoma cf. bakeri) has adopted a unique ecological function in that it acts as one of the top predators in the subterranean food web, preying on the abundant cavefish Poecilia mexicana. To date, several aspects of this predator-prey interaction have been explored, but basic questions related to the ecology of the waterbugs remained unstudied. We tested whether there is evidence for reproduction of waterbugs within the cave, which would indicate a self-sustaining population. Furthermore, we investigated the habitat affinities of different size-classes of waterbugs. We infer that waterbugs reproduce inside the cave despite the toxic properties of the water, as evidenced by the presence of nymphs of all sizes and males carrying developing zygotes. We also found size-dependent differences in use of habitat, and particularly small nymphs occupy different microhabitats than larger individuals. Adult waterbugs and large nymphs were most common at sites with cavefish. Small nymphs were rare at these locations, possibly to avoid cannibalism or exploit different resources in other locations. Furthermore, stable-isotope-analysis indicated that waterbugs likely derive their carbon from a combination of fish, dipteran larvae, and chemoautotropic bacteria.