Spider web

Spider webs

A Damselfly (still alive) caught in a web

A spider web, spiderweb, spider's web or cobweb (from the obsolete word "coppe", meaning "spider" ^[1] sometimes with the connotation of being a dusty, abandoned web) is a device built by a spider out of proteinaceous spider silk extruded from its spinnerets. Insects can get trapped in spider webs, providing nutrition to the spider; however, not all spiders build webs to catch prey, and some do not build webs at all.

Types of spider web

There are a few types of spider web found in the wild; and many spiders are classified by the webs they weave. Different types of spider webs include:

• Spiral orb webs, associated primarily with the family Araneidae as well as Tetragnathidae and Uloboridae
• Tangle-webs or cobwebs, associated with the family Theridiidae
• Funnel-webs, with associations divided into primitive and modern
• Tubular webs which run up the base of trees
• Sheet webs
• Dome or tent webs

Zygiella orb-web

Several different types of silk may be used in web construction, including a "sticky" capture silk and "fluffy" capture silk, depending on the type of spider. Webs may be in a vertical plane (most orb webs), a horizontal plane (sheet webs), or at any angle in between. Most commonly found in the sheet-web spider families, some webs will have loose, irregular tangles of silk above them. These tangled obstacle courses serve to disorient and knock down flying insects, making them more vulnerable to being trapped on the web below. They may also help to protect the spider from predators such as birds and wasps.

How spiders make webs

A orb weaver web anchored in a fork of a peach tree in winter

An orb weaver producing silk from its spinnerets

An Okanogan orb weaver Silhouette

Spiders have several spinneret glands located at their abdomen which produce the silken thread. Each gland produces a thread for a special purpose. Seven different gland types have currently been identified, although each species of spider will possess only a few of these types, never all seven at once.

Normally a spider has three pairs of spinnerets, but there are spiders with just one pair or as many as four pairs of spinnerets, with each spinneret having its own function.

During the process of making a web the spider will use its own body for measurements, a very practical and ergonomic design feature of any web. This will allow the spider to move quickly around its own web with very few faults.

It will start with the most difficult part of construction, the first thread. The spider effectively uses the wind to carry its initial adhesive thread. With some luck the silk is released from its spinners and carried by the wind to a suitable adherable surface. When it sticks to a surface the spider will carefully walk over the thread and strengthen it with a second thread. This process is repeated until the primary thread is strong enough to support the rest of the netting.

After strengthening the first thread the spider will continue to make a Y shaped netting. The first three radials of the web are now constructed. More radials are added making sure that the distance between each radial is small enough to cross. This means that the number of radials in a web directly depends on the size of the spider plus the size of the web.

After the radials are complete the spider will fortify the center of the web with about five circular threads. Then a spiral of non-sticky, evenly spaced, circular threads are made for the spider to easily move around its own web during construction. The spider then, beginning from the outside in, will methodically create the adhesive spiral threads. It will utilize the initial radiating lines as well as the non-sticky spirals as guide lines. The spaces between each spiral will be directly proportional to the distance from the tip of its back legs to its spinners. This is one way the spider will use its own body as a measuring/spacing device. While the sticky spirals are formed the non-adhesive spirals are removed as there is no need for them anymore.

After the spider has completed its web it will chew off the initial three center spiral threads then sit and wait. If the web is broken without any structural damage during the construction the spider does not make any initial attempts to rectify the problem.

Webs allow a spider to catch prey without having to expend energy by running it down. Thus it is an efficient method of gathering food. However, constructing the web is in itself an energetically costly process due to the large amount of protein required, in the form of silk. In addition, after a time the silk will lose its stickiness and thus become inefficient at capturing prey. It is not uncommon for spiders to eat their own web daily to recoup some of the energy used in spinning. The silk proteins are thus 'recycled'.

The tensile strength of spider silk is greater than the same weight of steel and has much greater elasticity. Its microstructure is under investigation for potential applications in industry, including bullet-proof vests, and artificial tendons. Researchers have used genetically modified mammals to produce the proteins needed to make this material. ^{[2] [3] [4]}

Uses

Australian garden orb weaver spider, after having captured prey

The spider, after spinning its web, will then wait on, or near, the web for a prey animal to become trapped. The spider senses the impact and struggle of a prey animal by vibrations transmitted along the web lines.

Spiders do not usually adhere to their own webs. However, they are not immune to their own glue. Some of the strands of the web are sticky, and others are not. For example, if a spider has chosen to wait along the outer edges of its web, it may spin a non-sticky prey or signal line to the web hub to monitor web movement. Spiders have to be careful to only climb on the non-sticky strands of their webs.

A spider positioned in the middle of the web makes for a highly visible prey for birds and other predators, even without web decorations. Many day-hunting orb-web spinners reduce this risk by hiding at the edge of the web with one foot on a signal line from the hub, or by appearing to be inedible or unappetizing.

Some species of spiders do not use webs for capturing prey directly, instead pouncing from concealment (e.g. Trapdoor spiders) or running them down in open chase (e.g. Wolf spiders). The Net-casting spider balances the two methods of running and web-spinning in its feeding habits. This spider weaves a small net which it attaches to its front legs. It then lurks in wait for potential prey and, when such prey arrives, lunges forward to wrap its victim in the net, bite and paralyze it. Hence, this spider expends less energy catching prey than a primitive hunter such as the Wolf spider. It also avoids the energy loss of weaving a large orb-web.

Some spiders manage to use the 'signaling snare' technique of a web without spinning a web at all. Several types of water-dwelling spiders will rest their feet on the water's surface in much the same manner as an orb-web user. When an insect falls onto the water and is ensnared by surface tension, the spider can detect the vibrations and run out to capture the prey.

In traditional European medicine cobwebs are used on wounds and cuts and seem to help healing and reduce bleeding.^[5]

Communal spider webs

The communal spider web at Lake Tawakoni State Park

Occasionally, a group of spiders may build webs together in the same area. One such web reported in 2007 at Lake Tawakoni State Park in Texas measured 200 yards (180 m) across. Entomologists believe it may be the result of social cobweb spiders or of spiders building webs to spread out from one another. There is no consensus on how common this occurrence is.^[6]

Outside influences on spider web weaving

Certain drugs, including caffeine, affect the way spiders build webs.

Administering certain drugs to spiders has an effect on the structure of the webs they build. It has been proposed by some that this could be used as a method of documenting and measuring the toxicity or the effects on motor co-ordination of various substances.^[7][8]

It has also been observed that being in Earth's orbit has an effect on the structure of spider webs in space.^[9]

Gallery

See also

• Filter feeder - similar feeding mechanism, with water as the surrounding fluid rather than air

External links

• Spiders on drugs The effects of drugs on spider web building
• More spiders on drugs More pictures of webs constructed by spiders under the influence of varied drugs
• Web construction in Araneus diadematus Movie of the web construction in the cross spider Araneus diadematus
• Oldest known spider web A 110 million year old web preserved in amber
• Spider Webs Vast spider web covering several trees at Lake Tawakoni State Park, Texas, in August 2007

References

• Ed Nieuwenhuys, The Spider Web and Thread. March 2002.