5. _Ctenophthalmus agyrtes._ This flea is commonly found as a parasite of voles and field-mice. When farm-rats take to an open life in the fields they pick up this species from the rustic rodents. In Hertfordshire, Hampshire and Suffolk one half the fleas from rats, collected in farmyards and hedgerows, were found to belong to this species; but whether it is as common on rats all over England is unknown. It appears not to bite man. A closely allied flea (_Ct. assimilis_) is found in central Europe on field-mice and equally on rats which live under the same conditions. It has not been found in England. The principal occasional parasites of rats are dog-fleas, cat-fleas, fowl-fleas, and human fleas. The proportions in which they and rat-fleas are found vary greatly in different parts of the world. For instance, in San Francisco nine per cent. of the fleas collected from rats have sometimes been found to be human fleas; whilst in Italy as many as twenty-five per cent. have been identified as cat-and dog-fleas. It must be borne in mind that when new countries are opened up by man the rats, which follow in his rear, exterminate numbers of the weakly native small mammals and take on their fleas. A change of habitat may be followed by an exchange of fleas. Some interesting work has been done in testing the appetite of different kinds of flea for human blood. The oriental rat-flea (_X. cheopis_) has been kept alive for three weeks on that diet. Other species show repulsion for mankind and refuse to suck. The experiments confirm the popular belief that fleas have a marked preference for certain individuals. When the flea has refused to bite the human arm, it becomes necessary to check the experiment by trying whether the refusal is merely due to want of hunger. For this purpose a rat must be at hand. It can be secured on a board by two bandages fixed at each end by drawing pins. The rat lies, of course, on its back with its head comfortably supported by a little pillow of cotton wool. A portion of the rat’s abdominal wall is left exposed and shaved. The flea, in an inverted test-tube, can then be put on the hairless patch of the abdomen and given an opportunity of biting, which it may or may not accept. When fleas are being collected from rats it has been noticed that the true rat-fleas are usually on the hind-quarters of the host, whilst the mouse-flea prefers the region of the head and neck. As regards the tastes and habits of oriental rat-fleas in the matter of food a long series of experiments may be summarized in this way: