The National Park Service has turned off the lights surrounding the Lincoln Memorial in the early evening in hopes this will prevent millions of Potomac River midges from swarning to the memorial's lighted colonnade and covering its marble walls with greasy eggs.

Park Service officials also hope this will reduce the huge colonies of spiders attracted by the midges, which are tiny, gnat-like insects. The spiders have festooned the walls -- particularly the mortar between the marble blocks -- with little gray webs, giving the 59-year-old memorial a slightly moldy look. The midges and spiders do no permanent damage to the building.

The Park Service this week erected scaffolding at the memorial to steam clean the walls of midges, eggs and spiders, a messy, expensive and annoying process during which the spiders fall on and bite the cleaners.

The steaming, which costs about $3,000 to $5,000 in labor, will be done only once this year. The process used to be done twice a year, but budget have forced the Park Service to cut back the number of cleanings.

Several years ago, the mystery of the ring-around-the-colonnade was solved by University of Maryland entomologist acting as consultants to the Park Service, according to Park Service architect-historian Paul Goeldner. The Park Service previously thought the greasy spots were droplets of airplane jet fuel from the 1,000 planes that use National Airport daily.

But a simple, inexpensive solution that would eliminate the need for steam cleaning was not thought of until last year, when Park Service officials learned more about the habits of midges. The solution involves turning off the bright, warm lights in the early evening, when the midges swarm from the nearby Potomac to the sheltered colonnade the high-ceilinged porch that surrounds the memorial.

"Entomologists have informed us that midges swarm only at twilight between early April and the first frost," which usually is in November, Goeldner said. So waiting until one hour after sunset before turning on the memorial's outside lights should end the midge and spider "unpleasantness," Goeldner added, "and it also will save money on the Park Service electric bill."