Wherever we live, they live. They lurk in the crevices and cupboards of our houses, stow away on our ships, hang out in our subway tunnels, find snug homes inside our computers. They fearlessly snack on pastry crumbs in congressional offices and steal chow from research rats in the laboratories of scientists charged with protecting the country from disease.

They venture out at night, protected by darkness, to feast on whatever we have dropped or left out: stray cornflakes, cakes of soap, eye makeup, the glue that binds books. When they sense light or movement, they flee. They mark their paths through our homes to food or water so others of their kind will find the way. When no other food is available, they live off their own waste -- or they eat each other.

They are cockroaches, an alien society in our midst. Ever since humans lived in caves, these insects have been our despised but determined tenants, as domesticated as dogs or cats but far more expert at survival.

While sticking to the same basic body design that their ancestors used 350 million years ago, cockroaches have evolved into about 3,500 separate species, most of which spend their lives unnoticed and unidentified in wild habitats such as forests and caves, where they eat dead plants and animals, helping to recycle waste matter. "They're the vultures of the insect world," says Philip G. Koehler, a professor of urban entomology at the University of Florida.

But a few types, especially the German cockroach, specialize in living with -- and living off -- people. In fact, some of them can't live without us. And as long as our own species, Homo sapiens, remains on this planet, we probably won't live without them.

Many people assume that, because roaches were here before dinosaurs, they're living fossils, life forms that have outlived their time.

Not so, Koehler says: "The reality is they've had a longer time to evolve. They're very flexible organisms. They're highly specialized for survival."

Walk into one of the cockroach breeding rooms in Koehler's Laboratory of Urban Entomology, a small, one-story building on the University of Florida campus in Gainesville, and the first thing you notice is the smell. Greasy, musty, stale -- it's hard to describe but disturbingly familiar to anyone who's ever lived in an old city apartment building.

It's the aroma of hundreds of thousands of German cockroaches crawling over and under each other in dozens of plastic tubs, mixed with the distinctively different fragrances emitted by the other species that Koehler and his research team keep handy to show visitors and take to schools as visual aids. A faint sound of chewing comes from the tubs and jars that line the floor-to-ceiling shelves. The stink is overwhelming.

"Here in this building," Koehler estimates, "there are 3 to 4 million cockroaches."

There are inch-long, lime-green Cuban cockroaches. There are hefty, two-inch brown American cockroaches, the "palmetto bugs" that live by the thousands in sewers and steam tunnels in the South. There are scary hissing cockroaches from Madagascar -- hard-shelled, cigar-shaped, three-inch critters with froglike faces that make a noise like an electric door buzzer whenever you touch them but are basically harmless to humans. And there are giant cockroaches from the South American rain forest, monsters the size of a drink coaster with flat, brown, flightless wings.

So distinctive is the smell of each insect that Koehler and his students can identify the species just by taking a whiff. Sometimes, their trained noses tell them more than they really want to know about the insects' presence. Graduate student Deanna Branscome recalls the time she sat down to dine in a Gainesville restaurant and picked up the scent of German cockroaches on her water glass.

Koehler says that, whenever he is called to consult on cockroach problems at a school or apartment, "before you open the door, you can tell whether it has a heavy infestation or not -- from the odor."

UNSAVORY HABITS

Cockroaches have good reason to run for their lives when they see or feel humans coming. People hate them. Even insect-lovers hate them. When the university opened a new building for its department of entomology in 1990, Koehler recalled, "I was the only professor not to move into it. My department chairman doesn't like roaches."

Despite their unsavory habits -- including cannibalism, tracking dangerous germs through kitchens, triggering asthma attacks in allergic humans and recycling their own poop -- cockroaches are creepily intriguing. Cockroach researcher Dini Miller of Virginia Polytechnic Institute in Blacksburg says she gets a standard reaction whenever she tells people what she does for a living.

"They say, `Oh, that's so disgusting! I can't stand them!' After they come down, they want to ask me every question about them and tell me every experience they have had. It's sort of a fear-fascination thing."

Entomologists consider cockroaches "primitive" social insects -- they live in large groups but don't cooperate with one another or divide tasks within the group, as bees or ants do. They share a common ancestor with termites. A few cockroach species even have bacteria in their guts that can digest wood.

But scientists disagree about where they belong on the insect family tree. Most believe that cockroaches, which belong to the order Blattaria, are most closely related to praying mantises. But mantises are meat eaters who prey on other insects, whereas cockroaches will eat virtually anything.

That lack of fussiness is part of a lifestyle among the most flexible of any in the insect world. Unlike mosquitos that require blood or monarch butterflies dependent on milkweed, cockroaches "don't have any habits that are so particular that they need to do something special," Miller says. "From egg case to reproductive adult, they can eat whatever. They can pretty much live wherever . . . . They reproduce very well even on a poor diet."

Newly hatched German cockroaches survive the early days of life by eating the waste of nearby adults. Older cockroaches eat their neighbors' waste, too. That's the reason why modern pesticide baits, once they've been eaten by one insect in a colony, soon are spread around to kill others.

Cockroaches also eat younger colony members, especially those that have recently molted, losing their hard outer shell. That behavior puzzles Koehler, who can't see any potential evolutionary advantage for insects who eat their own offspring. "In the [breeding] tubs, we lose 75 percent of the progeny due to cannibalism," he says.

Cockroaches are highly efficient at storing fuel from food. Unlike other insects and most other animals, they don't excrete uric acid, a nitrogen-containing waste product of protein.

Instead, they harbor specialized bacteria within cells of an internal organ called the "fat body" (analogous to a mammal's liver) that can recycle nitrogen, thus greatly reducing the insects' need for protein in their diet. The bacteria, which can't live outside a cockroach, are passed on by mother insects to their offspring.

Koehler once kept a colony alive for three years without feeding the insects protein. A diet too rich in protein, in fact, can poison cockroaches by choking up the cells of the fat body with nitrogen compounds.

All in the Family

Female German cockroaches, the species that typically inhabits human dwellings, need to mate only once. From that encounter, they can store enough sperm to fertilize all eggs they will produce during their nine-month life span. Both sexes spend three months as nonreproducing immature "nymphs" and six as adults.

Some other species don't have to mate at all. If no males are available, the females can reproduce through parthenogenesis -- production of offspring from unfertilized eggs.

Roaches are unusual in other ways. Compared with most insects, they live a very long time -- two to three years in the case of American cockroaches, as long as seven years for Madagascar hissing cockroaches. Yet, compared with other insects, they have few progeny.

The German cockroach, the most prolific, makes only about seven egg cases containing about 30 eggs each -- roughly 200 young. "That's pretty sorry, compared to a housefly," which lays as many as 900 eggs in its lifetime, Koehler notes. Even so, cockroach colonies expand quickly. One laboratory population grew from 10 pairs of adults to a total of 51,000 insects in seven months.

In a survey conducted by Koehler of about 1,000 urban apartments for low-income tenants, 50 percent had more than 13,000 cockroaches each. Heavily infested dwellings can contain 30,000 or more.

Asian cockroaches, a flying outdoor species that first appeared in this country in 1985, live under leaf litter and can achieve densities of 250,000 per acre in suburban neighborhoods, where they swarm around lights and disrupt outdoor barbecues.

And American cockroaches, the sewer and tunnel dwellers, can attain massive populations. In 1951, when engineers at the University of Florida flooded a sewer filter, they triggered an unexpected mass migration of American cockroaches, which can fly when necessary. "They say the sky turned dark," Koehler says. "There were just roaches everywhere."

Going Places

The German cockroach, Blattella germanica, is an intrepid hitchhiker that has accompanied humans to every continent. Despite its name, it is thought to have originated in Asia, not Germany. "It's been in Europe a long time," Koehler says. "It was probably brought in with the Mongols or something."

Today's German cockroaches are frequent fliers, often stowing away in carts that ferry food trays onto jets and posing a pest-control challenge for airlines.

Similarly, the so-called American cockroach probably originated in Africa and was long thought to have been brought to this country by slave ships. However, recent excavations of a shipwreck near Pensacola that predated the slave trade have found the remains of American cockroaches on board, Koehler says, indicating that the "palmetto bug" reached Florida earlier.

Several members of that species even flew on the space shuttle with John Glenn last year as part of an experiment designed by students at DuVal High School in Lanham to see how the insects would fare during space travel.

Three adults, three juveniles or "nymphs" and three egg cases were launched in a sealed canister with plenty of food and water and were videotaped at frequent intervals during the flight, says Carolyn Harden, the DuVal biology teacher who supervised the project and who coordinates the school's aerospace instruction program.

Only two of the research subjects -- both nymphs -- survived the trip, but the main reason that the others died appears to have been a blockage in their water supply, not the rigors of space travel, Harden says. One of the spacefaring roaches, a male, is still alive at the school, and Harden says her students are hoping to mate him soon with a female roach from the school's "ground control" colony.

StayinG Alive

Except for humans, cockroaches have few natural enemies. Fire ants sometimes attack them, Koehler says. So, it seems, do centipedes. But few birds can stand to eat them because the roaches' foul smell and taste are a defense mechanism.

If a roach loses a leg or an antenna in an attack, it can regrow the part. Speed and camouflage probably are a cockroach's best protection, however.

The insects have excellent vision and dash for cover at the first sign of a threat. Tiny hairs on structures called cerci at the rear of the abdomen sense air currents and are connected by nerves that control the leg muscles. Thus, as soon as a roach senses movement, it runs reflexively without having to wait for a command from its brain.

How does a cockroach pass the time? During the day, it hides in the "harborage" -- the safe, dark place where it lives with other roaches of all ages. Such togetherness facilitates mating and probably helps the insects to maintain a "microhabitat" warmer and moister than the surrounding environment, Miller says.

Roaches release a chemical called an "aggregation hormone" that makes them flock together.

"Cockroaches are very loyal to their particular crack or crevice," Miller says. "There may be a bunch that live behind the door, others behind the refrigerator."

Female German cockroaches spend much of their adult lives pregnant and remain inactive in the harborage most of the time. They eat voraciously after dropping an egg case, then return to the harborage to begin developing the next one. Males, however, tend to venture out nightly for water, sometimes visiting a nearby house plant or the condensation pan under the refrigerator, and to forage for food.

Getting in Touch

A few years ago, Miller, then a graduate student in Koehler's lab, began to wonder how tiny, newly hatched roaches in a colony knew where to go for food and water. "Do they have to search the entire room randomly and hope to heaven that they stumble on it before they die?" she asked. "Or can they simply follow in the footsteps of the adults?"

Miller decided to find out whether cockroaches use communication chemicals called pheromones to lay trails for each other. Because the insects see so well, she constructed a 4-by-4-foot arena with blank walls and without special visual cues. She painted chemical trails on filter paper, let the liquid dry and tested the trails' effect on cockroach behavior.

Miller found that roaches would repeatedly follow a trail made from an extract of cockroach fecal material, choosing it in preference to an alternative trail without such ingredients. Now she plans further studies to isolate the chemical compound in cockroach feces that serves as the trail marker.

She says adult roaches probably lay the trails automatically while going about their normal routine. "Since they have these regular routes of travel . . . they're pooping as they go along," she says. "Small, young cockroaches . . . can actually follow this trail of fecal material and use that to get back and forth."

Cockroach trails are surprisingly difficult to eradicate, Miller adds, and probably serve as signposts that help new arrivals to find what they need in a house or apartment.

"When [people] clean up apartments

. . . they'll tend to paint over it," she says. "That doesn't get rid of it. If you brought new cockroaches in, they'd go to the exact same place where the other cockroaches were before."

Female cockroaches also use a pheromone to attract mates, and males can detect it from many yards away.

Koehler recalls that University of Virginia chemists once synthesized 20 grams of such a pheromone, at a cost of $500,000, for a company to market as a pest-control agent. When it didn't sell, the remainder -- a small vial containing the equivalent of "billions of cockroaches' output" of the pheromone -- was donated to Koehler's lab.

Besides having powerful chemicals of their own, cockroaches are adept at inactivating chemical weapons that humans use against them. In the early 1950s, they were found to have become resistant to DDT, an insecticide once used widely but banned in the United States since 1972.

In the decades since, some populations have become resistant to multiple insecticides. Unlike other insects, cockroaches often seem to utilize several strategies simultaneously to resist being killed by pesticides.

They may increase their ability to break down a substance chemically, develop a tougher outer coat more difficult for pesticides to penetrate or become less sensitive to the pesticide's toxic effects on target organs such as the nerves or digestive system.

Battle Lines

Richard J. Brenner, a Gainesville entomologist with the Agricultural Research Service of the U.S. Department of Agriculture, remembers a call a few years ago asking him to deal with a serious cockroach infestation in the offices of the House of Representatives.

"They were highly resistant to most of the pesticides in use in the past 15 years," he says.

This fall, Koehler was asked to help solve a cockroach problem in the animal research facility at the federal Centers for Disease Control and Prevention in Atlanta.

Current methods of cockroach control rely on gel baits developed in the last decade. Some poison the nervous system, some interfere with digestion and some are cellular toxins that block energy production.

The baits can be placed in harborages, out of the way of pets and children. Many baits are highly effective, Koehler says, but roach populations are showing signs of resistance to some of them.

That ability to counter new threats in their environment is just part of what makes cockroaches such an evolutionary success story, Koehler says. Even studying their behavior is difficult because they don't behave predictably, like ants, moths and most other insects.

"Seventy-five to 80 percent do what's expected, and about 20 to 25 percent do something else -- which is tremendous variability," Koehler says. "I think that's part of the plastic nature of being able to survive. If they all did the same thing, they wouldn't have lived this long."

A Bug's Life

A cockroach's body is beautifully adapted to its lifestyle of eating indiscriminately and fleeing from danger. Legs are long and well-muscled for running. Hairs on the cerci sense air movements and are connected via nerve pathways to the leg muscles, allowing the insect to run by reflex. Many species fly, but the German cockroach (shown here) does not.

The digestive system extracts nutrients from whatever the insect eats. The fat body -- a yellowish-white tissue that fills much of the space within the body cavity -- has cells specialized to break down and store nitrogen compounds, allowing the roach to derive maximal energy from its diet. The fat body also helps to detoxify pesticides.

Nymphs, or immature cockroaches, molt -- losing their hard external skeleton and growing a new one -- about six times before becoming adults. They double their weight with each molt.

SOURCES: University of Florida, Institute of Food and Agricultural Sciences; Understanding and Controlling the German Cockroach, edited by Rust, et al.