I didn't realise that there were bat-flies in the Pacific. I had never actually thought about them until I came across an old paper by one B. Jobling in the library on the bat fly family
Streblidae. Turns out, there's a good number of species in the region: 15 according to the
Australasian and Oceanian Diptera Catalog. Some of them have rather large ranges--
Brachytarsina amboinensis is found from India and Okinawa, through the Philippines and Indonesia to Australia and New Caledonia.
While it seems that for the most part, bat flies tend to move from host to host with abandon, there are some which are only found on a single bat species.
Brachytarsina buxtoni from Fiji and Samoa is only found on
Emballonura semicaudata, while
B. rouxi from New Caledonia is only found on
Notopteris neocaledonica. Jobling reckons this is because these bats roost separately, preventing these species from colonising other bat species.
References:Jobling B. 1951. A record of the Streblidae from the Philippines and other Pacific Islands, including morphology of the abdomen, host-parasite relationship and geographical distribution, and and with descriptions of five new species (Diptera). Transactions of the Royal Entomological Society of London 102(4): 211-246