The Flea Capades

Having heard that fleas are species specific, did you ever wonder how fleas recognize their intended victim? It is all explained in an excellent article in the July 1995 issue of Smithsonian magazine. It seems that the anatomy of flea species vary one from another. Unique differences in body structure equip each flea for life with its particular host.

More than 2,000 flea species are currently recognized. The majority of these board their host by hopping about and hooking on when contact is made. These species are champion vaulters. For a tiny flea with short little hairy legs to jump upon a passing animal the size of a dog or cat requires incredible acceleration. These fleas lift off at 140 times the force of gravity, which is amazing when you consider that we humans experience discomfort when we exceed 6 Gís. Furthermore, if the flea fails to land its tumbling flight by hooking unto its victim, it will continue to jump, up to 600 times an hour, for up to three consecutive days if necessary.

In order for random leaping behavior to be successful, an appropriate host must be likely to pass by. A nest area is ideal for this. Also, while the adult flea feeds, it excretes the half-digested blood often called "flea dirt". The larval and pupa stages require this flea dirt as food during their development. Since flea dirt naturally accumulates in an animalís nest, this becomes an ideal place for new generations of fleas to develop. Thus, fleas are far more likely to be found in a nesting site, i.e. a dog blanket, dog house, or other heavily frequented area, and not out in open areas of lawn.

Itís not surprising, then, that host species for fleas generally are those with permanent nest sites. Because primates generally lack permanent nest sites and humans have only recently, relative to the 100 million years of flea history, been domesticated, there isnít a flea species evolved exclusively to feed on Homo sapiens. The so-called human flea, Pulex irritans, actually evolved to feed on pigs. While we humans are not honored to have an exclusive flea, approximately 20 flea species will feed on us as a byproduct of our association with their primary hosts. The most likely flea to feed on us today is Ctenocephalides felis, the cat flea, which feeds indiscriminately on cats, dogs, civets, hedgehogs, and humans.

Just as where a flea jumps is not completely random, the height to which a fleaís flight path reaches is a matter of specialization. For instance, while a rat flea jumps high enough to land on rats, the cat flea must jump higher. The height of a fleaís leap is accomplished by the presence of an elastic protein, resilin, that is the most efficient rubber-like substance known. The fleaís back legs are latched in a "locked" position with a ball of resilin compressed between the cocked leg and the plate of the thorax. When the latch is released, the flea springs upwards with the height of the arc determined by variations in the amount of resilin. Some flea species, however, no longer jump at all and no longer have resilin.

Differences in amounts of resilin are only one adaptive feature of fleas for specific hosts. Fleas also differ in the structure of their spines and combs. The combs and spines are specialized for the characteristic hair coat and the mode of locomotion employed by the target species. In some cases the life cycle of a flea is attuned to changes in hormone levels of their blood source. An example is the rabbit flea. The hormone burst of the pregnant doe is required for this flea to reach sexual maturity.

Equally complicated for the flea is their mode of copulation. There are various paths for male penetration of the female, and not all of these paths lead to the proper place for sperm deposition. Penetration alone may take up to ten minutes and, from beginning to end, copulation can last three to nine hours.

Did you know that poets once rhapsodized about fleas on women? Or that a study of flea evolution predicted the tectonic plate theory? Ever seen a flea magnified 200 times and in full color, a "dressed flea", or a flea chariot? All this and more flea lore are explored in the Smithsonian article that contains the subtitle, "You donít really want to know how good this amiable little vampire is at what it does, do you? You do? Well then, read all about it."


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