David and Carol Hughes, the tirelessly dedicated wildlife photographers, spent 17 months up to their waists in Costa Rican mud to make "Rain Forest," the eighth season premiere of the National Geographic specials, but who could doubt that it was worth it when seeing the result--a nature film that is both essay and poem, art and science.

"Rain Forest," to be broadcast at 8 tonight on channels 26 and 22 and other PBS stations, probably won't be as high-rated a crowd-pleaser as previous Geographic specials on subjects like sharks and polar bears, but it is as exquisitely atmospheric and teleporting as "Etosha: Place of Dry Water," an unsparing look at survival of the fittest also made by the Hughes team. Like that film, "Rain Forest" is one of the infrequent programs that make you very glad you have a color television set, and it also makes a good ad for home video recorders, especially for viewers who would like to see this show and part three of "Nicholas Nickleby" as well.

Richard Kiley, the aptly chosen narrator, says at the outset that the world's tropical rain forests, bunched around the equator, are "home to half of all the animal species on earth." Some of these species have yet even to be named and identified. Indeed, at the conclusion of the hour, Kiley warns that "many will become extinct even before they have been described by science."

The sights are privileged, fascinating, sometimes compellingly gruesome, even other-worldly, as if these creatures reside in a realm somewhere between here and the twilight zone. The Hugheses write, produce and direct, so they control the script and keep it terse; they let the pictures and natural sounds speak for themselves, and Kiley's voice is so sepulchral it becomes part of the aural montage.

In the rain forest, indolent monkeys laze around in trees, worker ants painstakingly dismantle great leaves and cart away the pieces as if they were looting a department store, plants elbow each other out of the way for access to scarce and coveted sunlight, a tarantula mugs a katydid, golden toads mate on the only mountaintop in the world where they are known to exist, bell birds twirl mustaches that resemble Salvador Dali's, a brilliantly red-breasted male quetzal and its dishwater-drab mate flutter out of a nest in a tree's hole, a bird beats the luminescent blue wings off a butterfly before swallowing it, wasps spit and Technicolored coral snakes eat their own shed skins.

The close-up views are, really, magnificent. The giant red eye of a brightly green frog fills the screen. To mate, the male climbs on the female's back--and stays there, until eggs have been laid. And for all that, the eggs are then eaten by a snake. A basilisk lizard, meanwhile, demonstrates why he was nicknamed "the Jesus Christ lizard"--he gingerly walks on water to escape a predator. In slow motion, he looks like he's running for the goal line at an extremely waterlogged football game.

It's just as well "Rain Forest" is airing in winter and not summer, because it's alive with creepy-crawly things you wouldn't want to find in the bedroom. In terms of ghastly sights, a lizard noshing a grasshopper is outdone by the dread army ants, who raid and completely overrun a wasp nest looking for, and finding, larvae.

The program is an ecological tract that lets the pictures do the preaching, until 54 minutes in, when MAN appears, chainsaws in hand, and we get the bad news, to be expected in programs like this, that rain forests are being destroyed, plowed away, bulldozed off the earth. And then the good news--that Costa Rica, whose government cooperated in the filming of this report, has taken steps to protect some of the territory.

National Geographic's specials, which have been underwritten since their inception by Gulf Oil, still set the standards in television nature programs. This one is positively mesmerizing. It gives you time to think for yourself--there isn't a lot of nudging--and one of the things one might think, watching all the ceaseless activity of the forest, the life-and-death pageant that goes on and on whether cameras are there or not, is that this is the real business of this planet. Should the human race make itself extinct, as well it might, all this would go on, and human history would have been only a brief, rude and messy interruption.

The National Geographic specials aren't just about animals; they're about everything.