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Silk sack lunch, Soggy asteroids

Q: Why do spiders wrap their prey up in their web? Is the prey dead by that point or is it just paralyzed?

How come spiders don't get caught on their own web? Are only certain spots sticky and they know which spots to step on? (Meg, Indianapolis, Indiana)

A: Remember poor Frodo in the Lord of the Rings? How the monster spider Shelob wrapped him in spider silk and paralyzed him? Fiction borrowed from fact. That much is true. Fortunately for Frodo, though, the plot omitted the dissolving of his flesh.

A spider building a web, Indiana. [Courtesy Phyllis Cooper, US Fish and Wildlife Service]

A spider swathes her prey in silk to subdue him and prevent his escape.

"Spiders are soft-bodied creatures who need to protect themselves from injury as the prey thrashes about in the web," says Susan C. Jones, urban entomologist at Ohio State University.

First the spider detects web vibrations that alert her to a struggling insect (or even bat or small bird). She dashes out along safe silks and shoots silk strands around the insect to hold it and stop its wild flailing. She stabs her fangs into the victim and injects poison to paralyze it and digestive fluid to dissolve the content of the prey’s body. This takes an hour or two. The prey does not live long.

She can feed at her leisure. The spider plunges her fangs in, pumps with her muscular stomach, and sucks out the soup— using her hollow fangs as straws. Conserving energy spent on silk making, she also liquefies and eats the enshrouding silk. Done with dinner, she cuts away the hollow prey husk and it falls to the ground.

Spiders don’t get caught in their own web because, as you guessed, they deal with non-sticky webbing. An orb spider, for example, lays down a temporary scaffolding of non-sticky silk. She walks on the non-sticky part and fills in a spiral of sticky threads. As she builds the sticky web, she cuts away the temporary scaffolding. She also puts safety strands into the web that are non-sticky. These allow her to later wend her way safely to her prey.

By the way, not all spiders build webs and some build webs totally from non-sticky silk. Orb web design is unique to each species. Spiders also use silk to line a lair, create a lifeline (like astronauts spacewalking), and encase their eggs.

"Males of some species use silk to wrap the female prior to mating," says Jones. (Untrusting souls.)

Further Reading:

Ohio State University: Spiders in and around the house by Susan C. Jones

Maurice Burton and Robert Burton, ed. The International Wildlife Encyclopedia. New York: Marshall Cavendish Corporation, 1969.

Cool quiz: How spiders spin webs

Seattle Rose Society: How spiders build webs

Q: Where do asteroids get water? (Bob, New York City, New York)

Gaspra, a tiny asteroid about a tenth of a billionth the size of our Moon that rotates so fast its day lasts 7 hours. It orbits the Sun in 3.29 years. [NASA]

Asteroids got water from the same place that Earth did — originally from hydrogen and oxygen dispersed by supernova explosions of huge stars. Oxygen and hydrogen combine to form water. Most asteroids are made of the same flotsam floating in space as are the Sun and planets: dust, rocks, water, and other compounds, notably carbon. A typical asteroid contains about 10 to 20 percent water.

"The form of the water [for example, liquid, permafrost, or impregnated in minerals] depends on many things, such as the distance that the asteroid formed from the Sun (and therefore its temperature) and what internal heat the asteroid has since generated," says astronomer Peter Thomas of the Center for Radiophysics and Space Research at Cornell University in Ithaca, New York. Heat drives the chemical reactions that could combine water with minerals or compounds. Without sufficient heat, the reactions don’t occur.

Furthermore, we just learned that the largest known asteroid, Ceres, might contain large amounts of pure water ice beneath its surface.

A favorite science fiction theme is to mine the asteroids for water, among other things, and then use the water to generate steam for powering spacecraft to the stars.

"In reality, though, mining water from Ceres for long trips wouldn’t make much sense as you have to fight Cere’s gravity," says Thomas, "and the good water (if really there!) is probably buried, requiring a big effort to get it. It is actually easier to carry water from Earth if heading out than to stop at some place along the way, such as the Moon or Ceres."

Further Reading

Space.com: Riches in the rubble by Michael Paine

Newswise.com: Largest asteroid may be a mini planet with water ice

NASA: Asteroid fact sheet

NASA: Gaspra, astronomy picture of the day

(Answered Oct. 14, 2005)