"And here's the happy bounding flea --

You cannot tell the he from she, The sexes look alike, you see; But she can tell, and so can he."

Roland Young

And therein lies the problem.

Fleas those tiny, wingless creatures which cause more headaches and misery for pet oners than about anything else, have been plaguing man and his pets for, well, as long as there have been men and pets.

And the plague part is no pun: The Oriental rat flea probably holds the dubious distinction of having singlehandedly spread the bubonic plague that devastated Europe in the 14th century.

The havoc fleas wreak today in the homes of pet owners is seldom fatal, but try using that line to soothe the frazzled nerves of an owner who's been up for a week because his dog is scratching all night. For that matter, consider the plight of the poor dog, who's doing the scratching.

And if the scratching at your house has become particularly ferocious in the past few weeks, you're not alone. September probably is the height of the flea season here; the nasty critters have been breeding and hatching indoors and out since April. It doesn't get any better until the first killing frost. Just be thankful you don't live in Florida.

The question, then, is how to eliminate fleas from our pets and our homes. The answer unfortunately, is that we can't. We can control them, maybe, but no eliminate them.

"We're talking here about a parasite and its host," says flea expert Dr. James Sargent, assistant professor of Entomology at Ohio state University. "as long as we're here, fleas will be here, too."

Sargent, however, is optimistic about a new weapon to fight them. A brand-new pesticide called Precor (manufactured by Zoecon Corporation) is showing "very exciting" results, he says, in controlling the flea problem in areas it's been used.

A growth inhibitor, the chemical prevents flea larvae from reaching maturity. Since only the adult flea bites and spreads disease, preventing fleas from reaching that stage is a major advance toward effective control. Also promising is the fact that Precor is significantly less dangerous to humans and pets than most of the chemicals used in flea control.

The bad news: Precor is still hard to find because it's so new (Sargent suggests contacting professional exterminators), and for it to be most effective, it should be used early in the flea season.

So we have next year's answer. What to do about this year?

It's advisable not to enter the flea fight without a proper understanding of your enemy. There are 1,600 flea species, each of whom livess off the blood of some unsuspecting bird or mammal. While most fleas are named for the creature they like best (dog flea, cat flea, human flea, etc.), most fleas don't care a whole lot whose blood they have for breakfast. Fleas usually prefer a dog or cat to a human because of the warmer temperature of canine and feline blood, which means you'll do just fine if Rover or Puff is outside.

The standard flea cycle goes something like this:

Rover goes outside to play. During his travels, a female flea decides he'll make a friendly host and hops aboard. (Fleas can't fly, but they can jump up to about 4 feet, which is roughly equal to a 6-foot man jumping five city blocks.) The flea lays her eggs (up to 500) in Rover's fur.

Rover goes inside, and soon begins to scratch. The eggs falll off onto carpets, bedding, floorboards. The tiny larvae that hatch from the eggs live happily in even the cleanest of homes, off dirt, dried blood, flea excrement. After a few molts, the larva spins a cocoon, where it will stay until conditions are right, which can be anywhere from a week to several months. (This is why you may still have fleas in January, and why it's a good idea to do a thorough fogging shortly after the first frost.)

The adult flea emerges from the cocoon, jumps on the nearest available warm-blooded creature, and the cycle begins again. You'll probably know if and when your pet has fleas, but if there's any doubt, try ruffling the fur on the back of its neck. Even if you don't see the fleas actually running around, those little black flecks are called flea dirt, and they indicate you have a problem.

Though fleas won't kill you -- or your pet -- they are not something to be ignored. For one thing, the problem, will simply get worse if you don't deal with it. Fleas also carry tapeworm, which can be a serious problem; flea infestation can cause anemia in puppies and kittens, and they can be more than just annoying to adult pets.

"Some pets are hypersensitive to fleas," says veterinarian Michael Fox, scientific director of The Humane Society of the United States.

"One flea can cause an allergic reaction that causes such terrible itching the animal will chew at itself."

The result is frequently a "hotspot," which appears most often on an animal's leg. If your pet develops areas of redness, hair loss, or raised, moist lesions, it's time to see your vet, who will probably prescribe cortisone to relieve the intense itching.