Will you walk into my parlor, said a Cretaceous spider to an ancient fly.

The classic spider's web, like Charlotte would have woven, was invented just once, way back in the Cretaceous period some 136 million years ago, scientists report.

Called an orb web, it's the generally circular style spun by two major types of spiders, which had raised the possibility of the two groups evolving this form separately.

But a paper in Friday's issue of the journal Science says a comparison of the spider genes related to web making shows that the orb web developed just once.

Researchers led by Jessica Garb of the University of California, Riverside, compared orb-web building spiders in the genuses Deinopoidea and Araneoidea.

Both build orb webs to catch prey and the deinopoids also include net-casting spiders that throw a modified orb web over their prey.

Araneoids include the orb weavers such as golden silk spiders with their traditional spiraling web as well as those that weave sheet webs.

Garb said in a statement that the finding "does not support a double origin for the orb web," but indicates that the unique design evolved only once.

While the two groups probably developed orb-web spinning from a common ancestor, they came up with different ways of making the web catch prey.

Araneoid webs have glue droplets that make prey stick to the web, while deinopoids wrap their threads with a different type of silk fiber that "the spiders comb, until it almost has the appearance of Velcro under a microscope, and they snag insects that way," Garb reported.

Not all spiders make orb webs. The black widow, for example, weaves a web that is a tangle of silk without the circular pattern.

In a separate paper in the same issue, a team of researchers including David A. Grimaldi of the American Museum of Natural History reports the discovery of a Cretaceous-era spider web encased in amber along with some captured insects.

The amber, found in Spain, preserved 26 strands of silk, many of them connected to one another. Glue droplets are visible on the web and prey includes a fly, a mite, a beetle and a wasp.

The amber was dated to about 110 million years ago and is the oldest known example of a web with trapped insects, according to Grimaldi.

This finding confirms that spiders and complex, sticky webs date back early enough to have affected the evolution of the most diverse groups of flying insects, the researchers said.

Garb's research was funded by the National Science Foundation and the Army Research Office while that of Grimaldi was supported by the Spanish-French Scientific Research Program and the Spanish Ministry of Education and Science.