The research for this thesis was carried out in the Entomology department of the Animal Virus Research Institute, Pirbright. Membrane feeding and intrathoracic inoculation techniques were developed and used successfully for the first time to infect sandflies with viruses. These infection techniques were used to investigate the susceptibility of a laboratory colony of the sandfly Lutzomyia longipalpis to infection with three viruses of the Phlebotomus fever group, Sicilian and Naples sandfly fever, and Pacui. The suitability of the colony for use as a laboratory model for studies on the development of such viruses in an insect host was then assessed. Sicilian and Naples sandfly fever viruses are Old World viruses transmitted in nature by the Old World sandfly Phlebotomus papatasi. Pacui virus is a New World virus isolated from the New World sandfly L.flaviscutellata. A limited number of P.papatasi were obtained and both infection techniques were applied equally successfully to this species. The development of the New and Old World viruses in New and Old World sandflies was then compared. The susceptibility of two Culicoides species to the Phlebotomus fever group viruses was tested as part of a general programme of research into the specificity of arboviruses with respect to the insect host. Several Culicoides species are important as vectors of the orbivirus Bluetongue. The vector potential of sandflies for this virus was assessed and the development of Bluetongue virus in Culicoides and sandflies was compared. The distribution of virus in infected insects on successive days after infection was determined by infectivity titrations of dissected heads, thoraces and abdomens. A fluorescent antibody test was developed and used to detect viral antigens in tissue smears from infected Culicoides and sandflies. The possibility of using such a test to detect viral antigens in paraffin wax sections of infected insect was investigated
