Host-parasite relationships between larval Sialis lutaria (Megaloptera) and Neoechinorhynchus rutili (Acanthocephala).

The role of the larva of the alder fly Sialis lutaria as a host for Neoechinorhynchus rutili (Acanthocephala) has been investigated. Brown trout (Salmo trutta) infected with N. rutili were found to be feeding on S. lutaria larvae which contained juvenile acanthocephalans in their haemocoels. These acanthocephalans have been identified as N. rutili. A previous report of this host-parasite relationship indicated some form of host response to the larval acanthocephalan. Stained wax sections of S. lutaria larvae infected with N. rutili did not show any evidence of either a host response or encapsulation. Samples of S. lutaria larvae revealed an interesting pattern of infection. Only larvae of a particular size range were infected. An experiment to infect commercially obtained rainbow trout (Salmo gairdneri) with N. rutili by feeding them with infected S. lutaria larvae was successful. Since S. lutaria larvae are known to be prey items for trout it appears that this route of infection could commonly occur in the natural environment. Larval S. lutaria is considered as an additional intermediate host to several ostracod species which have been shown to play a role in the transmission of N. rutili.