SUMMARYThe nematode, Heterotylenehus autumnalis, generally failed to develop successfully in Musca domestica L., Orthellia caesarion (Meigen) and Ravinia l'herminieri(Robineau-Desvoidy). Failure is attributed mainly to host resistance in the form of haemocytic encapsulation of the small gamogenetic female nematode. In a few instances, the nematode was able to complete development successfully in housefly and Orthellia, but was not capable of leaving the host. These few instances of development in the unnatural, experimental hosts are attributed in part to general ’weakening’ of the host by superparasitism. In many cases superparasitism resulted in the death of the fly larvae or pupae.Tracheolar proliferation was usually associated with the encapsulated nematodes and there was evidence that many of them were expelled from the host during pupation. We also observed nematodes with cephalic caps of dark material, presumably melanin, and some that were tightly girdled with this material. A host reaction, apparently never before reported for a parasitized insect, was the darkening (presumably melanization) of the larval anal organ.The authors would like to express their appreciation to Dr A. J. Nappi, Biology Department, State University of New York, Oswego, New York, and Dr G. 0. Poinar, Jr., Department of Entomology and Parasitology, University of California, Berkeley, California, for reading an early draft of the manuscript and offering critical comments. The senior author would like to express his deep appreciation to Dr W. R. Nickle, Nematology Investigations, Crops Research Division, Agr. Res. Serv., USDA, Beltsville, Maryland, and to Dr H. Welch, Zoology Department, University of Manitoba, Winnipeg, Canada, for their help and encouragement during the course of this study. This study was supported in part by a grant from the University of Connecticut Research Foundation.
