There are about 35,000 known spider species, although scientists estimate that there may be about 180,000 species in the world. About 2,000 of these inhabit the United States, but only a few of those spider species build elaborate webs.
There are two spiders venomous to humans commonly found in parts of the United States—the black widow and the brown recluse.
Spider webs are built from silk, which is produced within the body of the spider and pulled out of two openings—spinnerets—with the spider's hind legs.
All spiders have two claws on their feet, but web-spinning spiders have three. They are used not only to pull the silk but also to grip and release the web’s threads and provide traction as they move along the web.
Spiders spin two kinds of silk:
- Sticky silk or viscid silk. Used to capture prey, this stretchy, wet silk makes up the spiraling threads of the web.
- Non-sticky silk or dragline silk. Used to strengthen and provide structural support of the web onto which the viscid silk is woven, this silk is stiff and dry.
The strength of a web is dependent not only on the strength of the spun silk but also on the web's design itself.
The web is constructed so that if any part of it is disturbed, a reaction is felt across the entire web, alerting the spider to its trapped and struggling insect food.
In addition, it is constructed so that failure in one area will not cause the entire web to fail, but will forfeit only the failing section.
Spiders rebuild their webs by eating the silk then "recycling" it to make new webs.
Webs have multiple purposes, but the most important one is to trap insects for food.
Webs are also used for:
- Travel or "balloon" from one place to another.
- Protection at the entrances of their dwellings.
- To encase egg sacs.
It is the female spider who builds the webs.
Spider webs can prove to be useful indicators of environmental chemistry. For example, researchers were able to deduce levels of lead and zinc in limestone arches in caves in Australia based on the analysis of water-soluble ions in spiderwebs at the sites.
Types of Spider Webs
There are three main types of webs: orb webs, funnel or sheet webs, and the irregular webs of house spiders.
- Orb Webs: Orb-weaver spiders build their webs almost completely through touch, because, although they have eight eyes, they actually have poor eyesight. According to a Newton Nature Bulletin from the Argonne National Laboratory, "Beneath her abdomen are six spinnerets that can be extended or compressed and used like the fingers of a human hand. Each spinneret has 'faucets' and 'spools' connected by tiny tubes to several types of glands." The silk begins as a liquid then solidifies and becomes stronger exposed to the air. To build the web, the orb-weaver sets the foundation with radial and framework threads using dragline silk. The viscid silk is then applied on top of this foundation, made sticky by "dots" of a glue-like substance. The web generally takes about an hour to build.
- Funnel Webs: These sheet-like webs are generally spun outside by funnel-web or grass spiders. As explained in the Newton Nature Bulletin, "Each sheet extends outward from a funnel-like opening beneath which the spider lurks in waiting for grasshoppers and other insects that may alight upon It. She constantly enlarges the web and, if undisturbed, it may become a square yard in the area." Generally set horizontally, the web "funnels" down into a shelter, such as a crack in the ground or a rock crevice or thick vegetation. The funnel opening can be over a foot long, with the sheet extending up to three feet.
- Irregular Webs: House spiders usually build their webs at night, and is much more haphazard than that of the orb weaver. This web "has a central sheet of densely woven silk which serves as a hiding place and is anchored by numerous guy lines that are long and strong," the Nature Bulletin states. This spider uses its back comb-like legs, with their row of curved bristles, to capture prey by tossing the silken strands of the web over an insect that flies or wanders into it.
Sources for this article include the following government publications, some of which have been directly cited:
"Venomous Spiders." National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, 2018.