Only spiders have such complex silk spinning equipment, although many other arthropods can spin silk.
What is spider silk made of? It is a fibrous protein secreted as a fluid which hardens as it oozes out of the spinnerets, which are mobile finger-like projections. As the fluid oozes out, the protein molecules are aligned in such a way that they form a solid, the process is not yet well understood. The spider hauls out the silk with its legs, stretching, fluffing it up or changing it in other ways to suit the purpose at hand.
Weight for weight, spider silk is up to 5 times stronger than steel of the same diameter. It is believed that the harder the spider pulls on the silk as it is produced, the stronger the silk gets. Spider silk is so elastic that it doesn't break even if stretched 2-4 times its length. Spider silk is also waterproof, and doesn't break at temperatures as low as -40C. There are 7 types of silk glands and "nozzles" but no spider has all 7 types.
Spiders that build webs have more of these glands. Males often have less glands and their glands are less developed because their role is mainly to find and mate with females. While females have to build the egg cocoon and may even have to look after their young.
The seven types of silk glands:
Aciniform glands produce silk for wrapping prey, called swathing silk. Most spiders have this. Cylindrical (tubuliform) glands produce silk for wrapping eggs. So males usually don't have them, and some families (Salticidae and Dysderidaei) don't have them at all. Ampullate glands produce non-sticky silk used as draglines or to form the frames of their webs (non-sticky foundations). All spiders have this, to produce the safety lines they leave behind them as they move about. Pyriform glands produce silk made into attachment disks at the bottom of web suspension lines. All families have them. Aggregate glands works like a glue gun, producing sticky droplets, which regularly dot the silk to trap prey. Only three families have them (Araneidae, Theridiidae and Linyphiidae). Lobed glands also produce wrapping silk and found only in Theriididae which have reduced aciniform glands. Cribellar glands produce multi-stranded, slightly sticky silk through many pores which are combed with special hairs on the hind legs as it emerges to form fluffy woolly silk. This is good for catching flying insects because it is very stretchy so the insect doesn't break the strand but bounces back to the web like on a horizontal bungy jump. The woolly silk also snares hairy insect legs. Only Cribellate spiders have these glands and combs.
More about spiders ...
Spiders in general
Golden Orb Web Spider
Besides these amazing silk glands and nozzles, the spider's uniquely mobile abdomen also helps it to create its marvellous webs.
WACKY WEBS: The circular orb web is the most typical shape associated with spiders. See Golden Orb-Web Spider for construction details. But there are other web shapes. Some webs are horizontal platforms or tents in grassy areas, perhaps to trap jumping insects. Others make untidy tangles.
How did spiders develop silken webs? It is believed that silk was first used by ancient spiders to line their burrows or wrap their eggs, as many other arthropods still do. Later on, silk was used to catch prey. Today, only one-third of spider species spin webs to catch their prey. The rest use other silk traps to catch prey or grab their prey directly.
The Tent Spider builds a silken funnel in its tangled web and hides there Some spiders decorate in their webs with prominent bars of thick white threads, like the St. Andrew's Cross Spider (Argiope spp, left). One reason could be to warn larger flying insects and birds which may clumsily fly through and destroy the web; kind of like the safety strips we have on glass doors!
Another reason is that the white threads reflect ultraviolet light, much like flowers do, thus luring nectar seeking insects.
Only very large spiders confidently sit in the middle of their webs. Many hide on the side with a leg on a signal/trip line attached to the web. Those that stay on the web may hide in a curled up leaf, hide among rubbishy debris, pretending to be one of the bits.
Snacking on silk: Web-spinning spiders usually regularly eat their own web. This is because the webs usually lose their stickiness after a while, usually within a day, due to exposure to the air and accumulation of dust. So spiders usually have to rebuild the sticky bits regularly. They eat up their old web then rebuild the web all over again. Some spiders may also gain nutrients from the pollen or other bits stuck on the web.
Why doesn't a spider get stuck on its own web?
Find out in Ben Prins' article on Micscape's webpage.
Some less elaborate but ingenious silk traps include:
The tiny African Bola Spider (Cladomelea longipes) (left) perches on the end of a twig and hangs a single thread weighted by a drop of glue, scented with chemicals that resembles a female moth's pheromones. This attracts male moths from far and wide. As a moth comes near, the spider whirls the thread then releases it as the moth flies by to snag then haul it back. The spider can change the chemicals to attract the moth that is in breeding season (moth flavour of the month?!).
The Gladiator/Net-Casting Spider (Dinopidae) (right) holds a small square web between four front legs and hangs upside down on a thread over the ground. When prey passes underneath, the spider stretches out the net to trap it. The net is not sticky but the strands are very stretchy and made loopy by combing the silk with its back legs. These loops catch on the little hairs of insect prey, entangling them in the spider's net.
The Scaffold Web Spider rigs a sticky thread from the ground to a bent branch. Insects blundering into the trap are hoisted into the air until collected by the spider.
Spitting Spiders (Scytodidae) don't build webs but spray two jets of glue-venom through their fangs to immobilise their prey. Some hunting spiders (Hersiliidae) immobilise prey by spreading silk as they jump over and around it.
Other uses for silk:
As handy clingwrap for paralysed prey, to eat later; for eggs.
Parachutes to catch the wind; called "ballooning", the silk is gently released into a breeze and eventually, enough silk is produced to lift off the spider. To land, the spider simply climbs along its parachute and rolls it up. They can travel long distances this way, ships at sea have encountered ballooning spiders! This is how some baby spiders disperse, and some adults find new food.
For love: Most males make a little web to transfer semen to their front palps. Some males leave draglines marked with pheromones to attract the babes. In the Sierra Dome Spider of the US (Linyphia litigiosa), it is the female that advertises by scenting her web which broadcasts her readiness to mate. The first male to arrive will snip up and ball up her web to terminate the advertisement!
Make draglines in case they fall off; or release a line into the wind to cross from tree to tree.
Build shelters; line burrows and nests.
By surprise: The Crab Spider (Thomisidae) uses its excellent camouflage to lie in wait for prey. One looks and smells like bird shit complete with a white blobby web under it. This repels predators, while attracting the flies which it feeds on (Bird Dung Crab Spider, Phrynarachne). A few crab spiders that lurk on flowers to catch insects can even change colour like a chameleon to match the colour of the flower they are on, from yellow to white, or pink and yellow.
By eyeball: The Jumping Spider (Salticidae) actively hunt for prey with their excellent eyesight. They capture prey by jumping up to 20 times their body length. Some have adhesive pads so they can clamber over slippery surfaces to sneak up to their prey.
Stealing: The tiny Thief Spider (Argyrodes) live in the webs of larger spiders, stealing their prey. They are probably too small for the host spider to notice.
Hunting: The Wolf Spider is not a sit-and-wait predator but actively hunts for its prey, alone and not in packs. It hunts during the day, using its excellent eyesight which can see all around and are very sensitive to movement. It stalks its prey to creep up within range without alerting the prey, then leaps onto the prey.
Ingeneous uses of webs: People have also used spider silk. In the South Pacific, the web silk is used to make fishing lures, traps and nets. In the Solomon Islands, the spider web is collected by winding it around sticks to make large sticky balls which are suspended just above the water. Needle fish are lured to jump out and get entangled in the ball. In Southeast Asia, people make a net by scooping up the web between a stick bent into a loop. Apparently, until the early 1960's, spider web filaments were sealed in glass and used as cross wires in gun sights. This silk came from the highly venomous black widow spider (latrodectus mactans). Spider webs have been used as bandage to stop blood flow and used to make bird snares. There are even modern efforts to develop commercial applications of Golden Orb Web spider webs!
Further informations in german can be found here!