Nematomorph parasites manipulate terrestrial invertebrate hosts to seek out and enter streams, thereby deriving substantial energy subsidies to stream salmonids. Despite this potential ecological role of nematomorphs, knowledge of their diversity remains unclear. Using molecular (i.e., 18S rRNA and mitochondrial COI genes) and morphological approaches, we explored the species diversity of suspected nematomorph specimens, as well as their terrestrial orthopteran hosts, in 10 stream and riparian ecosystems around the Kii Peninsula, central Honshu, Japan. We distinguished seven species of nematomorphs belonging to three genera based on molecular and morphological data. The identifications by the two approaches were consistent with each other at the genus level but partly not at the species level. Furthermore, among the suspected nematomorph specimens, eight nematode species belonging to the orders Mermithida and Trichocephalida were found from two sites. Several orthopterans, mainly camel crickets, were infected by nematomorphs and by a nematode without obvious species specificity. These results suggest that diverse parasites and their orthopteran hosts drive the parasite-mediated energy flow across the stream and riparian ecosystems.