(1) When many rat-fleas are present some will attack man, even when a rat is available for their food-supply. (2) When the number of rat-fleas is small, and when their true host is present, they will not attack man. (3) When rat-fleas are starved they will readily attack all animals, not being particular in the choice of a host. (4) Rat-fleas deprived of their food for from 72 to 96 hours attack and feed on man more readily than at other times. (5) Rat-fleas, even when starved, prefer their true host to man. (6) Rat-fleas may be attracted to man, jump on him, but take some time to feed on him. Plague-infected fleas might in this way be carried from one place to another without infecting the man; but they would, when brought near a rat, attack it in preference to man. The fleas found on bats possess certain peculiarities which have led to their being grouped together. They form a family to which the name _Ceratopsyllidæ_ has been given. They are recognized by two flaps, one on each side of the head. What these are and what service, if any, they render to their possessors is unknown. Bat-fleas also, as a rule, have maxillæ shaped like dumb-bells; but in one genus (_Thaumopsylla_), found on fruit-bats, they are triangular as in other fleas. The maxillæ as the reader may remember, are parts of the insect’s mouth, and, though placed like jaws on each side of the aperture, they are not used in piercing the skin and sucking blood (Fig. 4). They bear feelers called the maxillary palpi. The flea (_Thaumopsylla breviceps_) which is found on South African fruit-bats and which has triangular maxillæ, seems to be a connecting link between this peculiar group of fleas and the main family _Pulicidæ_. Bat-fleas are commonly well supplied with combs. They usually have them on the abdomen, as well as the head, and the maximum number of eight combs is found in bat-fleas. Their structure and life-history agree generally with that of other fleas. They breed in hollow trees, caves, ruins, church-towers and lofts where bats hibernate or spend the hours of daylight. The larvæ feed on the droppings of the bats, and the mature insect, after emerging from the pupa case, takes the first opportunity that comes of getting on to its host. Bats are seldom found to be much infested with fleas; for this reason, bat-fleas are somewhat difficult to obtain and many of the species that are known are extremely rare.