There are spider webs in the highland rain forest of Madagascar that are so huge, they make you wonder if the creatures that built them are big enough to have a starring role in a horror movie.

How huge? Big enough to wrap around a VW Beetle, that's how huge. But instead of being the size of the giant spiders in the Harry Potter books, they're more like the spiders in Harry's cupboard under the stairs—a little on the big side, but nothing out of the ordinary.

"They are no bigger than some of the spiders you can find in gardens in America or Europe," said Dr. Ingi Agnarsson, a biologist. He and his research partner, Dr. Matjaz Kuntner, believe the webs are made by several closely related spider species. All these species are smaller than the thumb of an adult human being.

These are orb spiders, which make large, round webs that look a lot like the ones people decorate with at Halloween. First, the spider will spin a silk thread between two objects, like tree branches. This is called a bridge line. The spider makes the bridge line stronger by traveling back and forth along the line, adding more silk to it. Then it adds a line that will help anchor the web to the ground or another object. It continues adding anchor lines until the web looks like a bicycle wheel with spokes.

These spiders, all of which are in the genus Caerostris, have not yet been fully studied by scientists and exist only on the island of Madagascar. Dr. Kuntner and Dr. Agnarsson plan to travel there in February to study them extensively.

The biologists do know two more things—besides size—that make these spiders extraordinary: They build their webs over running water, such as streams and rivers, and the silk they use to make their webs is stronger than that of other spiders.

The two facts are probably related, Dr. Kuntner said. Web-building spiders tend to use a lot of energy to make their webs, and orb spiders have to make bigger webs than most. The silk would have to be strong to hold up for long enough for the spider to catch food.

An orb spider’s entire web is large, but the bridge line is especially enormous. Those can reach up to 14 meters (45 feet), stretching across flowing rivers and streams. The spiders have no obvious way to get their bridge lines across the rivers and streams.

"We have no idea how they make these webs," said Dr. Kuntner.

How might they get those silk threads across the river? Biologists have many theories.

Do they send out a thread to float across the river and entangle in a tree on the other side? Probably not, said Dr. Agnarsson. The distance is too far, and it is likely that the thread would break and fall into the river.

Do they crawl across another spider's bridge line? Probably not, because some spider would have had to make the first bridge line across the river.

Do they put on a teeny pair of goggles, grab a leaf, point it in the right direction and wait for the right wind? Probably not, no matter how much fun that may sound.

The scientists are going to Madagascar to study these spiders to find out. They will have a team of people to watch the spider webs day and night to see if the spiders show up to build or repair webs. They may even damage some of the webs to see if a spider comes out to fix them.

One of the big questions the scientists want to answer is if the spiders' prey is as extraordinary in size as their webs are. It seems reasonable to wonder if a big web is meant to catch a big dinner. If it is, what do they do with it once it is caught?

The biologists will also collect some spiders to study back home in their labs. Dr. Agnarsson works at the University of Puerto Rico. Dr. Kuntner works at the Slovenian Academy of Sciences and Arts.