[Illustration: Fig. 5. The antenna of a flea. A, concealed in the groove. B, protruded from the head. The versatile _basal segments_ and the terminal _club_, in this case with segments on one side of it, should be noticed.] We pass now from the central nervous system to the sense-organs of the flea. The chief are the eyes, the antennæ and the pygidium. In regard to the eyes nothing more need be said. The antennæ are probably far more important organs to a flea than its eyes; but inasmuch as they are at ordinary times concealed in a groove they are not very conspicuous (Fig. 5). The first tolerably accurate plate of a flea by a naturalist will be found in Hooke’s _Micrographia_ (1664). Robert Hooke (1635-1703) was a somewhat eccentric and irritable man of science who acted as secretary to the Royal Society. His labours were too varied to be effective. He nearly discovered the laws of gravity and also studied fleas. To him belongs the credit of having detected the antennæ groove. Just as many of the older naturalists thought that the maxillary palpi were antennæ, so others thought that the antennæ of a flea were its ears. And when, with the help of their lenses, they saw the antennæ erected and protruded from their grooves, they imagined that the insect was cocking its ears and listening after the manner of a horse or ass. But the antennæ of fleas are much more to them than ears; though it may be that they are also auditory organs. They are certainly tactile and olfactory organs as well. In outward structure each antenna consists of two parts which may be called the stalk and the club. The club is divided into a number of segments and is plentifully supplied with hairs. In some species the cuts which divide the different segments appear to be confined to one side of the club. In others a sort of central core holds the segments of the club together. The antennæ, therefore, are undoubtedly exceedingly complex organs. Such an insect as a flea may well be far more sensitive to movements of the air, vibrations of the earth, smells, light rays and sound-waves than a human being. In their origin the antennæ, like the paired mouth-parts, are modified appendages of the fused segments which compose the head of the insect. The fact that there are four pairs of appendages on the insect’s head, viz. (1) antennæ, (2) maxillæ, (3) labial palpi and (4) mandibles has been put forward by some entomologists as evidence that the head is formed of four primary segments.