Spider silk is produced and manipulated into different forms by the
spinnerets located on the abdomen of almost all spiders.
There are as many as seven different types of spider silk, each being custom-made by the spider from liquid form for whatever function it is required. However, no known spider possesses the spinnerets to manufacture all seven types of silk. When excreted by the spider, it hardens (polymerizes) on contact with air into a long, thin fibre, which in the case of the smallest spiders can be as fine as 0.00002 mm in diameter.
Two of the main forms are gossamer (which is used by smaller spiders, known as ballooning spiders, as a means to essentially paraglide to other locations) and dragline silk which is a stronger, thicker form which is used in the construction of webs. Other types of spider silk have specialised functions for creating different parts of a web, or wrapping prey to be consumed later, or for the protection of the spider's eggs. One of the more amusing uses is that female spiders will sometimes create a scented web intended to attract potential mates. The first male spider to arrive will usually cut this web down and ball it up to indicate that the position has been filled!
Formed from almost pure proteins, the primary constituents of spider silk are created using the two simplest amino acids, glycine and alanine. The silk is composed of long strings of fibres, combined with more elastic elements, which is where the incredible tensile strength and other properties come from. Given that the spider has to produce all this material from their own body, it makes sense for spiders to recycle old web material by consuming it and recycling it in their body for use in the future.
Dragline spider silk is rated at having around five times the tensile strength of the equivalent diameter of steel (or twice that of Kevlar), but is not as elastic as capture silk. Capture silk is twice as elastic as Nylon, being highly stretchable - up to three times its original length. The two silks are different on a minute scale. Both forms of silk have regular occurrences of a twist in their molecular chains, which turns 180 degrees, before bending back on itself. In the case of capture silk, these twists occur every 9 blocks as opposed to every 43 blocks for the more rigid dragline variety in any given length of thread. All forms of silk are waterproof and highly resistant to cold even as down far as minus 40 degrees Centigrade.
I will leave you with a final impressive fact, just in case the previous ones haven't been sufficient to establish a sense of wonder in you at the work of this diminutive creature. For a strand of spider silk produced by the common garden spider to snap under its own weight, it would have to be around 80 kilometres long...
Want to know more?
The seven types of silk glands :
- Aciniform glands produce silk for wrapping prey, also known as swathing silk. Most families of spiders have this.
- Aggregate glands works like a glue gun, producing sticky droplets, used to regularly dot the silk to trap prey. Only three families have this gland - (Araneidae, Theridiidae and Linyphiidae).
- Ampullate glands produce non-sticky silk used as draglines or to form the frames of their webs. All spiders have this, to produce the safety lines they leave behind them as they move about.
- Cribellar glands produce many-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. The insect doesn't break the strand, but instead bounces back to the web almost like a horizontal bungee jump. The woolly silk also snares hairy insect legs. Only Cribellate spiders have these glands and combs.
- Cylindrical glands (also known as tubuliform glands) produce silk for wrapping eggs. For this reason, males usually don't have them, and some families of spider (Salticidae and Dysderidaei) don't have them at all.
- Lobed glands also produce wrapping silk. This is used to protect eggs, and is found only in Theriididae which have reduced aciniform glands.
- Pyriform glands produce silk made into attachment disks at the bottom of web suspension lines. All families have them.
Other Nodes of Interest
soy silk, spiderweb
Sources
http://www.sfgate.com/hypertek/9706/spider.shtml
http://www.xs4all.nl/~ednieuw/Spiders/Info/spindraad.htm
http://www.howstuffworks.com/question87.htm
http://www.szgdocent.org/ff/f-ssilk.htm
http://www.sciencenews.org/sn_arc98/2_21_98/fob2.htm
http://dsc.discovery.com/news/briefs/20050502/spider.html (Secrets of Spider Silk Unraveled)
Also posthumous thanks to "Sammy the Spider" who was invaluable in my research until one of my cats ate him.
Last Updated : 11th May, 2006