The following paper is intended only as a preliminary revision of this large and exceedingly important group of parasites, and it is the outcome of some six months' work on a small portion of the large mass of material awaiting identification in the collections of the British Museum and of the Imperial Bureau of Entomology. The writer not only has had access to all the types which he required to see that had been deposited in the British Museum, but also was so fortunate as to have been able to arrange with the authorities of both the United States National Museum and the National Hungarian Museum for exchanges of cotypes where sufficiently long series were available, and consequently has been able to examine and include in his key all, or the large majority of, the species that have been described from the Indo-Australian region by Continental and American writers. It should be placed on record that, in addition, the National Hungarian Museum, when not in a position to exchange cotypes, was so good as to forward to the writer on loan the types themselves. With all this extremely kind co-operation, therefore, it has been possible to make this paper tolerably complete; and for purposes of comparison it has been thought desirable to include in the key numerous Palaearctic and Ethiopian species, the types and cotypes of which are to be found in the British Museum.