Antennæ apparently enable fleas to find their bearings, to communicate with one another and to discover the whereabouts of the opposite sex. But it is especially as organs of smell that they play a most important part in the flea’s social life. They enable couples to find one another; and, when the sexes come together, the antennæ of the male are usually raised and exposed from the groove. Insects generally have some means of cleansing dirt from their antennæ. Some make use of their legs, others of their mouth-parts. In fleas there is often a row of short hairs at the hind margin of the groove which may serve as a kind of comb for cleaning these delicate organs of sense. But further observation on this point would be interesting, for no one appears to have seen the comb in actual use. Female fleas are said usually to carry their antennæ ensconced in the grooves, whilst the males more frequently protrude theirs. The antennæ of the males are generally longer than those of the females. There are certain noteworthy organs of sense which appear to exist on the upper surface of a flea’s head and body. They take the form of small convexities of the body surface, lentil-shaped and each surrounded at the base by a ring. Somewhat similar sense-organs are widely spread through the insect world. As to their function, divergent views are held. Some think that they are for the perception of sounds, some for the perception of light rays, some for the perception of rays of which we are unconscious. Since these organs are placed, at times, in unprominent parts of the body it seems more probable that they are affected by sound than by light. The preference which fleas show for certain animals, and the repulsion which they manifest on being allowed to suck blood from an unaccustomed host, lead one to believe that they have a sense of taste. This sense in other insects is apparently seated in certain microscopic pits and hairs which form the ends of nerves and are distributed round the mouth. Whether fleas can hear is not, it seems, definitely known. A large number of fleas possess what is called a frontal tubercle. It is a notch in the centre of the forehead but nearer to the mouth than to the antenna. Sometimes the tubercle projects from a groove. This is most marked in the genus of African fleas _Listropsylla_. The real nature of this organ is unknown. Some regard it as an organ of sense. Its homology is also uncertain. To some it suggests the egg-breaker of the larva and they regard it as a relic of the larval stage. To others it suggests an eye and they regard it as the remnant of an unpaired ocellus possessed by the ancestral flea.