We are now about to refer to certain parasitical insects whose name is usually associated with those which have preceded; they are well known by all, and attack both men and the mammalia with no less ferocity; we allude to fleas, which differ from gnats in this respect, that the male is as eager for blood as the female, and that both of them, like leeches, live by sucking it; besides, the larvæ of fleas live only on what
the full-grown insects bring them, whereas the larvæ of gnats get their own living; the mother flea sucks for herself first, and then divides the spoil with her larvæ which as yet have no feet. For a long time it was thought that the fleas of different animals belonged only to a single species, and consequently that the flea of man was not different from that of a cat or a dog.
Daniel Scholten, of Amsterdam, in 1815, showed by his microscopical observations, that fleas differ from each other; and in 1832, Dugès of Montpellier, investigated the distinctive marks of the various species. The observations of Scholten may be found in “Les Materiaux pour une faune de la Néerlande,” by R. T. Maitland.
The ordinary flea is called Pulex irritans, and especially attacks man in Europe and in North America; it may be called a fly without wings, and, together with its congeners, it forms a distinct family under the name of Pulicidæ. Van Helmont treated of these insects, and gave directions for making them, just as though he were describing a recipe for pomade. At that time, naturalists supposed that certain fish could be formed spontaneously, and that nothing but fermentation was necessary in order to bring forth a crowd of living creatures from this molecular disaggregation. Fleas may, perhaps, some day find a place in the chemist’s shop as well as leeches. We see no reason why homœopathic bleedings should not be resorted to, as well as homœopathic medicines; we should certainly have more confidence in the effects of the bites of fleas, than in the efficacy of remedies subdivided into the millionth part of a grain.
Fleas differ much in size, according to the places which they inhabit. Dugès, of Montpellier, gives us a curious instance of this. He devoted himself to researches on the zoological characters of this genus, studying the four species which are the best known, the Pulex irritans of man, Pulex canis of the dog, Pulex musculus of the mouse, and Pulex vespertilionis of the bat.
Fig. 20.—Human Flea (Pulex irritans), after Blanchard.
Fleas of a brown colour, almost black, and of enormous size, are commonly met with on the sandy shores of the Mediterranean, at least, in the neighbourhood of Cette and Montpellier; they are more than half as large as a common fly. These are human fleas, and their presence on the sea-shore during the heats of summer is due solely to the great number of bathers of both sexes and of all classes, which lay their clothes down there. If at some future day these insects were to be placed in the rank of surgical species, it would be necessary to resort to those shores in order to procure them; and we might suppose that, by judicious crossing, we might soon produce races that would be of real service; as yet, however, the therapeutic art has had
recourse only to leeches. Since we have seen these insects harnessed and performing their exercises in public, we cannot say that the future may not reserve for us a still greater surprise.