For the reader who can accurately picture to himself the external structure of a flea and of the typical insects belonging to other orders, a few words may be said on the probable ancestry of fleas and their relationship to other living insects. This vexed and much debated question is still, as the older naturalists would have said, _tremendum mysterium_. Very little light has yet been thrown upon it, and the most divergent views have been expressed by learned and competent entomologists. A historic survey of the various opinions that have been held since the days of Linnæus would fill many pages; but a short summary of the different orders to which fleas have been referred by different zoologists will suffice. The older authors, Linnæus, Geoffroy, Cuvier and Duméril, and Gervais placed them among the _Aptera_ because they were wingless. Kircher regarded them as _Orthoptera_, an order which includes grasshoppers and crickets; but he has had few followers. By Fabricius and by Illiger they were treated as _Hemiptera_ or bugs. Lameere, a Belgian, has recently expressed a decided view that fleas are really a family of _Coleoptera_ or beetles. Those who have held the once orthodox opinion that they belonged to the _Diptera_ or flies are Roesel, Oken, Straus-Dürkheim, Burmeister, Newman, Walker, von Siebold and Wagner. The structure of an adult flea, however, differs from that of an adult fly in the following noteworthy respects: the mouth-parts are differently constructed, the head of the flea is closely joined to its thorax, the three divisions of the thorax are not joined and fused, the flea is wingless, the eyes of fleas are simple ocelli, and there are differences of lesser importance in the stigmata, which give access to the tracheal system by which all insects breathe. The number of those who have regarded fleas as belonging to a distinct order of insects is considerable: they are Lamarck, De Geer, Latreille, Kirby and Spence, MacLeay, Leach, Dugès, Bouché, van der Hoeven, Westwood, Landois, Brauer, Kraepelin, and Taschenberg. Modern opinion is all but unanimous on this point. There remains, however, a second question. Even if it be agreed that there must be a distinct order for _Suctoria, Aphaniptera, Siphonaptera_, or fleas; where ought that order to be placed? In which other order of insects must we look for the nearest relations of fleas? For a time after the acceptance of the fact that insect forms have been evolved, and not separately created, the ancestors of fleas were searched for among some species of fly.