What are the differences between indoor and outdoor spiders?


When I see spiders outdoors, they are usually in the middle of a web. Inside my apartment, however, they just stand on my ceiling. Why don't my spiders spin webs indoors? Do they know that my apartment is not a normal bug habitat? Why do they come in here at all? Finally, if they are feeding on (gulp) me, is there even a slim possibility that I will wake up one morning with a web over my bed?

-- Patrick M., Bay Park


I was watching three big night spiders (not their biological name) outside the windows all spinning or repairing webs I had probably disturbed by day and wondered how they were excreting the building materials. Two were going clockwise, the other counterclockwise, so I presumed no hemispherical relationship like weather or bathtub water. Is there some other determining factor like left or right (six) leggedness? A fourth web seen by day high up between two Torrey pines with closest branch tips some 20 feet apart supports a night spider at mid-span. How did he or she span the initial strand?

-- Jeff, Encinitas

Every year at this time the air gets nippy and the big ugly spiders come out. The fat red ones that build webs near your porch light. They're particular favorites of Grandma Alice, who's named the one by the back door Fernando. A big juicy guy who hangs out at night and hides out by day. Jeff's "night spiders" are really garden spiders, which mature in late summer, so we see the adults at this time of year. They're one of the 2000 types of orb weavers that make the common spoke-and-circle webs. And unless they've met with some sort of disaster, Jeff's spiders, of course, have eight legs, not six. Insects have six legs; spiders aren't insects.

Grandma Alice fills us in on how Fernando goes about building a web. First he finds a high spot over the back door and exudes a long piece of silk from one of five abdominal silk-producing orifices, and the wind blows it around until it sticks to something. (Patrick's Torrey pines spiders do the same.) With this in place, he goes about building a frame and three or four radial threads. The rest of the radii are filled in by finding a radial thread at the center of the web, then adding the next thread next to it. He'll add one off to the left, then to the right, then toward the bottom, eventually filling in the whole frame. If you measure the angles formed at the hub, each new radius is nearly uniformly 15 degrees from the one next to it. While Fernando's busy laying down radii, he's also strengthening the hub and the perimeter with circular but non-sticky threads. Spider studiers believe this method of building helps keep the proper tension on the structure.

Between the two non-sticky spirals, the spider lays down the "catching spiral," a thread coated with drops of gluey substance. It stands on a radial thread, feels for the next one with its front legs, then pulls out the sticky thread with its back legs and attaches it to the radius it's standing on. He may proceed to the left or to the right for a while, then reverse direction. Since there are more catching threads at the bottom of the web, spiders don't just go round and round in a circle. The whole process takes about half an hour. For the most part web forms are species-specific, but when the science guys fed spiders caffeine, LSD, mescaline, or amphetamines, the web forms went all to hell. That's one for the "Well, Whaddya Expect?" files, I think.

Patrick's spider housemates might be temporary lodgers, in out of the cold. Or they might be making messy cobwebs (not orb webs) in niches behind bookcases or the refrigerator or ceiling corners. Patrick's place is buggier than he thinks if he has a steady clientele. Spiders have no interest in biting people unless we get in their way.

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