The importance of certain species of fleas in relation to the epidemiology of plague has been so amply demonstrated (see Journal of Hygiene, vols. vi. 1906, p. 426 and vii. 1907, p. 395) that an explanation appears scareely necessary for our publishing a paper dealing with a revision of the non-pectinated eyed Siphonaptera. With the demonstration that fleas are capable of transmitting plague, their study at once ceased to be the mere hobby of a small group of entomological specialists. An accurate knowledge of fleas, both with regard to their structure and biology, is at present a matter of prime importance to those who concern htemselves with the prophylaxis of plague.