only persist in the long-term by switching between host species. This appears to be a general phenomenon among vertically transmitted parasites, as similar patterns are seen among bacterial endosymbionts (see §1) [6 – 8], and genomic parasites such as transpo- sable elements [27] and homing endonucleases [28]. Although the transfer mechanism is unclear, we have previously suggested that parasitic mites could act as vectors of sigma viruses [11], and arthropod vectors may be responsible for other endosymbionts and genomic parasites switching between host lineages [27,29,30]. A sigma-like virus outside of the genus Drosophila suggests that these viruses may be widespread in Dip- terans, if not insects as a whole. Unlike bacterial endosymbionts, the rapid evolution of the sigma virus genome makes it impossible to design a single pair of diag- nostic PCR primers that can be used to test for new strains of the virus. In the course of this study, we encountered CO2-sensitive individuals of other species of ﬂies from which we were unable to amplify virus using our primers, and these may harbour other sigma-like viruses. CO2 sen- sitivity has also been reported in 13 other Drosophila species [10], and in Culex mosquitoes [23]. Additionally, rhabdovirus sequences have inserted into the genomes of various insect species [31] and rhabdovirus-like particles have been found in ﬁrebug testes [32]. The non-Droso- philid sigma virus we found is of particular interest, as the closely related Dimarhabdoviruses are vector-borne diseases of vertebrates (some of which are vectored by other dipterans) [22]. The discovery of other rhabdo- viruses in insects that do not blood-feed may make it possible to understand how viruses may have switched between being vector-borne pathogens of vertebrates and being purely entomopathogenic. 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