Imagine that you live in a dense forest of rough, dark trunks rising to an outside world you cannot see. You travel endlessly through this monotonous jungle, feeding when hungry on nourishing fluid readily available beneath your feet. When you do so, the scene shakes violently as huge pointed objects crash through the tangled canopy overhead, a bludgeoning directed at you as you scurry away between the thrashing stalks.
You are a flea living in the thick coat of a dog and, when you pierce his skin for your bloodsupper, he sits down and concentrates on relieving the irritation with claws and teeth.
Small as it is, a flea has a crush-resistant outer body. Even if a dog or cat succeeds in biting one, the insect’s tiny, skinny shape usually protects it from damage. If we pick a flea off a pet, no harm is done to it unless we put an end to its life by crushing it between our fingernails. Ugh.
Fleas and mammals are tied together. Everything about a flea’s highly developed specialty points to a hairy host. Fleas did not evolve their unique shape and habits until “modern” furred mammals came along, about 55 million years ago. Bird fleas appeared later, having evolved from those on mammals.
Where did fleas come from in the first place? Some primitive insects, like silverfish and springtails, never evolved wings, but complex wingless fleas don’t belong to that category. From embryonic and larval clues, and from a sophisticated jumping mechanism inherited from flying insects, we can tell that their distant ancestors probably were flies.
Fleas are among the outstanding successes of the insect world. With almost 1,500 different kinds worldwide, it is likely that not a single mammalian species is free of them.
Adult fleas are not full-time parasites. Remaining on a host only long enough to get a few good meals, they soon hop off into the world at large.
Behold the happy, bounding flea,
You cannot tell the he from she;
The sexes are alike, you see,
But he can tell and so can she.
This anonymous doggerel is incorrect, for “he” is easily distinguished from “she.” In fact, biologists have described the genitalia of a male flea as the most elaborate such device in the entire animal kingdom. Mating takes place on the host or on the ground, but when eggs are laid on Fido, they quickly drop off.
Larval fleas are not parasitic but devour decaying plant and animal matter. Blood in their intestinal tracks is “secondhand” blood, having already passed through the alimentary tract of adult fleas before being deposited where the youngsters can get it. Flea larvae grow rapidly, molting two or three times before entering a cocoon stage from which they emerge full-grown.
A flea is exquisitely equipped for the life it leads. Its body - slender side to side, but tall up and down - allows it to slip through a forest of hair so easily that it is almost impossible to track. Rear-facing spines allow it to move forward, but they catch on the host’s hair to prevent the insect being pulled backward or from falling out of its furry world. It is so streamlined in forward mode that even its antennae retract into grooves. A flea scoots through the tangled, hairy thicket by pulling with its front pair of clawed legs and pushing with its hind two pairs.
When necessary, either to escape its host’s attention, or during a feeding frenzy, a flea’s light weight and powerful hind legs allow it to jump a distance of over a hundred times its body length. If we humans could perform a standing high jump equivalent to that of a flea, we would leap 800 feet. A flea does this in a millisecond, using muscle power to release a previously cocked jumping mechanism in its hind legs. A substance known as resilin remains under tension in a cocked leg, immune to fatigue and low temperature, so a flea can wait for days and weeks under adverse conditions before instantly leaping toward a furry meal as it comes by.
When a dog- or cat-flea population has been confined to a vacant house or unused room, human visitors are in for an unpleasant surprise. Hungry adult fleas waiting there have yet to taste their first meal and, justifiably anxious, leap in swarms onto bare legs where they immediately begin to dine.
It may come as a surprise to the fastidious that we humans have our very own flea species, Pulex irritans, which probably evolved in northern Europe, where it lived happily and prolifically among our early, unwashed, fur-clad ancestors. This flea does not survive in tropical temperatures. Fortunately it is a rare species and, true to its name, is more of an irritant than a transmitter of disease.