An exceedingly remarkable organ of sense, which is found in all fleas, is called the pygidium. It is a sensory-plate plentifully supplied with hairs and nerves and always placed on the back of the ninth abdominal segment. Of all its uses we are still somewhat uncertain but some observers declare that at the season of love the male flea bestows caresses on the pygidium of the female. In many species the male flea is sufficiently different in outward appearance from the female to be easily distinguished. The male is usually smaller and the last segments of the abdomen are so shaped as to give the look of a tail tilted into the air. The frontispiece represents a male flea and shows this well. The internal organs of reproduction (testes and ovaries) in the male and female are placed near the end of the abdomen. The seminal outlet and common oviduct open to the rear of the sensory plate on the ninth segment of the abdomen. The external genital armature of the male flea is exceedingly complicated and quite unlike that of any other insect. When the sexes are united, the usual position is reversed, and the male is _beneath_ the female. It is well known to every entomologist that the hinder segments of insects are often modified for reproductive purposes. In male fleas it is the eighth and ninth abdominal segments which are altered. In the females the eighth, and also often a portion of the seventh, has assumed a peculiar shape. The clasping organs of the male flea are portions of the ninth segment and form together a kind of claw reminding one of the pinchers of a lobster. It is used by the male flea in the breeding season to detain and hold the female. Every entomologist also knows that the external sexual organs of insects, of both sexes, are of special importance to the systematist or classifying naturalist. They often enable him to recognise the species when other organs do not show sufficiently striking characters. A minute study of the genitalia of fleas is an absolute necessity to the systematic entomologist, the more so as fleas do not present nearly as many, or nearly as varied, external differences as do the species of most winged insects where colour and pattern of wings are both important. CHAPTER IV THE INTERNAL ORGANS OF A FLEA