Mosquitoes know a few things. At least the females do.

Female mosquitoes know that mammals and birds, whose blood they need to make eggs, can be located by following the scent of exhaled carbon dioxide.

Somehow they know where the full moon’s highest tides will flow, and lay their eggs in hollows that later become warm pools safe from larvae-eating fish.And, by the slant of light, they know when winter is coming, so they endow the season’s last eggs with a natural antifreeze to keep them alive till spring.

But as savvy as these gals are, they don’t know that certain mesh cylinders — hanging from trees or sitting in plastic basins around Rhode Island — are traps.

Each summer, thousands of mosquitoes fly into these traps, lured by carbon dioxide steaming from dry ice or by specially brewed “stinky water” in the basins. The traps are set every week from June to October, at 40-odd locations, by the state Department of Environmental Management.

With millions of mosquitoes propagating in swamps, ditches and backyards, such trapping obviously doesn’t aim to reduce the population. Rather the goal is to keep an eye on it — the population’s size, and especially which viruses lurk in the mosquitoes’ guts.

That’s why, a few days ago, Karel Dlugokinski, a mosquito technician with the mosquito abatement section of the DEM, cinched the end of a mesh trap hanging from a tree in South Kingstown, confirmed that it contained females — the males, distinguished by the feathery plumes on their antennae, are irrelevant because they don’t bite — and popped it into a freezer in the garage of a low, shingled building at the University of Rhode Island’s East Farm.

Dlugokinski heads upstairs to the single long room that serves as the laboratory and administrative offices for the state’s mosquito program, which has been in operation since 1983. That was the year a nine-month-old Hopkinton girl fell into a coma after a mosquito bite; she died 11 months later. The baby had contracted Eastern equine encephalitis, a rare but deadly virus that mosquitoes pick up when they bite infected birds, and then transmit to people and horses when they bite again.

Back then, only a few states had mosquito testing programs. Since West Nile virus, also mosquito-borne, arrived in the United States in 1999, nearly every state has started testing.

Rhode Island spends about $150,000 a year on this testing program, says its director, Alan D. Gettman, a talkative, rumpled, wiry guy who holds a Ph.D. in medical entomology and who appears to have absorbed humankind’s entire body of knowledge on mosquitoes.

The state’s system, Gettman says, is not foolproof: there’s an element of chance in how many mosquitoes the traps catch, and which mosquitoes. In rain or wind, you catch fewer. By bad luck, you can miss the infected ones. Two years ago, a horse came down with EEE at Lincoln Woods even though not a single infected mosquito had been trapped in the state.

But by trapping and testing week after week, the program has successfully provided an early warning system, to encourage the public to take precautions, and also to identify any “hot spots” with an unusual concentration of infected mosquitoes. EEE has always been around at low levels but, for mysterious reasons, in certain years the virus proliferates wildly.

The “shining example” of that, Gettman says, was 1996, when his testing program found that 1 percent of mosquitoes in Westerly — way above normal — were infected with EEE, and the state did aerial pesticide spraying of the entire town. There were no human cases that year.

While Gettman is explaining this history, Dlugokinski — one of four technicians who work with him over the summer — sets to work on a batch of mosquitoes. She takes out a plastic tray, drops some blocks of dry ice into it, and hammers them into smaller pieces. Then she puts an identical tray on top, spreads some paper towels, and dumps the contents of a trap that was set last Monday in Portsmouth. To preserve any virus they may harbor, the mosquitoes have to stay cold while she works.

Dlugokinski sorts the mosquitoes by species, a process that requires astonishingly detailed knowledge of family resemblances, such as hair on wings, stripes on legs, proboscises of a certain length. When she had the same job last summer, Dlugokinski studied an illustrated book and carried images of mosquito parts into her dreams. Now it’s second nature.

With pointed tweezers she picks up one mosquito from the dozens that lie like minuscule knots of thread on the paper towel. Dlugokinski is 23; her young eyes can see small things. Without resorting to the microscope at her elbow, she can tell by the white markings on the abdomen and the bands on legs and mouthpart that this is a specimen of Aedes taeniorhynchus.

She records her finding in a slender notebook, and drops the bug into a test tube. Each test tube will contain up to 50 individuals of the same species. Each test tube also has five BBs that will later help grind them up.

There are 46 mosquito species found in Rhode Island, but only about a dozen that are common. Some species bite only birds while others have more eclectic tastes. That’s why they have to be separated for testing; if a virus is found, the testers need to know which species is infected — a bird-only biter or one with a liking for human blood.

When Dlugokinski finishes sorting the mosquitoes, she packs them on dry ice in a cooler and brings them to the Health Department’s laboratories on Orms Street in Providence.

There, every Friday morning, lab scientists Christopher Harmon and Nancy Benoit start the search for viruses. They add a liquid to the BBs and mosquitoes in each test tube. They place each test tube on a vibrating machine that resembles a kitchen scale, a process that literally beats the mosquitoes to a pulp.

Then the test tubes are spun in a centrifuge, 2,000 times per minute for 20 minutes. Viruses, which are lightweight, will rise into the clear, pinkish liquid that separates on top of the pulverized mosquitoes.

The technicians place drops of this liquid into nickel-size plastic wells on a post-card-size tray. Since last Monday, on the bottom of those little wells, kidney cells from monkeys have been growing into a thin layer. If there are viruses in the pink liquid, they will infect and destroy the cells.

Every day for seven days, the technicians will look through the microscope for evidence of cell death. If it appears that any monkey cells have died, they will put the remainder in contact with antibodies that react to each of four viruses — EEE, West Nile, Highland J (which doesn’t affect people but often coexists with EEE) and Jamestown Canyon (which causes a mild illness). If one of those viruses is present, the antibodies will latch on, and a fluorescent “tag” will glow apple-green.

So far this year, says Shashi Mehta, the lab’s chief virologist, the Health Department has tested 5,968 mosquitoes. Not once has anyone seen that apple-green glow, meaning the four viruses have not been present in any of the samples. But he expects to find them soon, because at this time of year both the mosquitoes and the viruses have multiplied to high prevalence.

The viruses are out there. Any day now, mosquitoes carrying one of them will make the fatal and informative mistake of flying into a DEM trap.

ffreyer@projo.com