Posted on: Sunday, November 14, 2004

Spider adapts to Isle environs

By Jan TenBruggencate
Advertiser Science Writer

The web-making of a group of Hawaiian forest spiders suggests that environment might be more important than genes in certain kinds of animal behavior.

Researcher Rosemary Gillespie and fellow scientists have found that these spiders of the high forests repeatedly have evolved to fill unique niches in their environment as they have moved from island to island.

The tetragnatha spiders, also called long-jawed spiders, are an interesting group — if you're into spiders.

Researchers believe their web-spinning ancestors arrived in the Islands about 5 million years ago, and they have evolved into 50 or more species.

Some have given up webs entirely and prowl the forest to hunt their prey. These hunters tend to have long spikes on their legs.

Some of the spiders are brown to blend in with twigs and bark, some are a maroon that matches certain mosses where they live, and others are green to match the leaves where they make their homes.

The green ones, oddly, often have a distinct bright-red spot on their backs.

Gillespie, formerly of the University of Hawai'i and now at the University of California at Berkeley, studied the webs weaved by some of these spiders.

Tetragnatha spiders spin orb webs, with long spokes connected by spirals. They tend generally to be lightweight, fragile webs that are respun daily because a heavy wind, falling twig or a big insect can tear them.

Gillespie compared the kinds of webs the spiders make with their genetic characteristics, which helped track their actual relationships to one another.

You might think that a tight-web species on one island might be more closely related to the tight-web species on the other islands, but not so, she said.

"You get independent evolution of the same set of web types as they moved from one island to the next," she said.

Thus, if a tight-web spinner moved to a new location where loose webs work better, it would evolve into a loose-web spinner.

Or, more likely, a single spider arriving on a new island would evolve into several different forms to take advantage of different habitats on that island. But each of those forms might look more like its distant cousins in similar habitats on the original island than like its immediate ancestor that arrived on the new island.

Gillespie said the tetragnatha spiders have bodies from a quarter- to a half-inch long, and with their legs, can be as large as 2 inches long. If you grab one, it will try to bite you, but it can't.

They tend to live in high native forest, from about the 1,500-foot elevation on up on the windward side of islands and at the 3,500-foot level and up on the drier, leeward side.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com or (808) 245-3074.