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Spider webs have existed for at least 100 million years, as witnessed in a rare find of Early Cretaceous amber from Sussex, in southern England. Many spiders build webs specifically to trap and catch insects to eat. However, not all spiders catch their prey in webs, and some do not build webs at all. The term "spider web" is typically used to refer to a web that is apparently still in use (i.e., clean), whereas "cobweb" refers to a seemingly abandoned (i.e., dusty) web. Theridiidae. While this large family is known as the cobweb spiders, they actually have a huge range of web architectures; other names for this spider family include tangle-web spiders and comb-footed spiders. When spiders moved from the water to the land in the Early Devonian period, they started making silk to protect their bodies and their eggs. Spiders gradually started using silk for hunting purposes, first as guide lines and signal lines, then as ground or bush webs, and eventually as the aerial webs that are familiar today.
Spiders produce silk from their spinneret glands located at the tip of their abdomen. Each gland produces a thread for a special purpose - for example a trailed safety line, sticky silk for trapping prey or fine silk for wrapping it. Spiders use different gland types to produce different silks, and some spiders are capable of producing up to eight different silks during their lifetime. Most spiders have three pairs of spinnerets, each having its own function - there are also spiders with just one pair and others with as many as four pairs. Webs allow a spider to catch prey without having to expend energy by running it down, making it an efficient method of gathering food. However these energy savings are somewhat offset by the fact that constructing the web is in itself energetically costly, 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 common for spiders to eat their own web daily to recoup some of the energy used in spinning. Through ingestion and digestion, the silk proteins are thus recycled. There are a few types of spider webs found in the wild, and many spiders are classified by the webs they weave. 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. It is hypothesized that these types of aerial webs co-evolved with the evolution of winged insects. As insects are spiders' main prey, it is likely that they would impose strong selectional forces on the foraging behavior of spiders. 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. It is reported that several Nephila pilipes individuals can collectively construct an aggregated web system to counter bird predation from all directions. Most orb weavers construct webs in a vertical plane, although there are exceptions, such as Uloborus diversus, which builds a horizontal web. During the process of making an orb web, the spider will use its own body for measurements. There is variation in web construction among orb-weaving spiders, in particular, the species Zygiella x-notata is known for its characteristic missing sector web crossed by a single signal thread. Many webs span gaps between objects which the spider could not cross by crawling. This is done by first producing a fine adhesive thread to drift on a faint breeze across a gap. When it sticks to a surface at the far end, the spider feels the change in the vibration.
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