So much has been said about insects since the War began that it is, I think, advisable that some attempt should be made to summarise our knowledge of the more important insect-borne diseases and their vectors. While insects have long been suspected of being responsible for the transmission of serious diseases, it may be said that practically the whole of our knowledge of insects in this rôle has been acquired within the past twenty years. So rapidly, however, has the charge on this offence been made out against them that, although it is common knowledge that they have been proved guilty, it is not generally realised upon how many counts the verdict rests.