Although silk is produced by some insects, centipedes, and millipedes and a similar substance is produced by mites, pseudoscorpions, and some crustaceans (ostracods and amphipods), only the spiders are true silk specialists. Spider silks that have been studied are proteins called fibroin, which has chemical characteristics similar to those of insect silk. The silk is produced by different types of glands in the abdomen. Ducts from the glands traverse structures called spinnerets, which open to the outside through spigots. Abdominal pressure forces the silk to flow outward, although the rate of flow is controlled by muscular valves in the ducts. Primitive spiders (suborder Mesothelae) have only two types of silk glands, but orb weavers have at least seven, each of which produces a different kind of silk; e.g., aciniform glands produce silk for wrapping prey, ampullate glands produce the draglines and frame threads, and cylindrical glands produce parts of the egg sac. Epigastric silk glands of male spiders produce silk that emerges through spigots in the abdomen between the book lung covers and provides a surface for the sperm to be deposited upon during sperm induction. Silk may have evolved from an excretory product.
Threads of silk from the orb weaver Nephila have a high tensile strength and great elasticity. Silk probably changes to a solid in the spigot or as a result of tension forces. Strands usually are flat or cylindrical as they emerge and are of surprisingly uniform diameter. The glob of silk that binds or anchors strands emerges from the spigot as a liquid.
The movable spinnerets, which consist of telescoping projections, are modified appendages. Two pairs are from the 10th body segment and two pairs from the 11th. Liphistius, of the suborder Mesothelae, is the only spider with a full complement of four pairs of spinnerets in the adult. Most spiders have three pairs, the forward central pair having been either lost or reduced to a nonfunctional cone (colulus) or flat plate (cribellum), through which open thousands of minute spigots. Spiders with a cribellum also have a comb (calamistrum) on the metatarsus of the fourth leg. The black widow is one such comb-footed spider (family Theridiidae). The calamistrum combs the silk that flows from the cribellum, producing a characteristically woolly (cribellate) silk.
Spider silk is incredibly tough and is stronger by weight than steel. Quantitatively, spider silk is five times stronger than steel of the same diameter. It has been suggested that a Boeing 747 could be stopped in flight by a single pencil-width strand and spider silk is almost as strong as Kevlar, the toughest man-made polymer. It is finer than the human hair (most threads are a few microns in diameter) and is able to keep its strength below -40°C. The toughest silk is the dragline silk from the Golden Orb-Weaving spider (Nephilia clavipes), so-called because it uses silk of a golden hue to make orb webs.
Spider silk is also very elastic and capture silk (sticky silk for catching prey) remains unbroken after being stretched 2-4 times its original length. Spider silk is tougher, more elastic and more waterproof than silkworm silk so it could have a much wider range of applications. It is simple to see why spider silk is of such interest to materials chemists since new ultra-strong fibres based on the silk could be developed.
Spiders use silk for a variety of functions:
- Swathing silk for the wrapping and immobilisation of prey.
- Webs for catching prey using sticky silk – it is elastic to prevent the prey from rebounding off the web.
- Draglines which are used to connect the spider to the web, as safety lines in case a spider should fall and as the non-sticky spokes of the web. Dragline silk is the strongest kind of silk because it must support the weight of the spider.
- Parachuting or ballooning which is used to aid the dispersal of young and to find new areas as a food source. Silk is released and is caught by the wind to lift the spider up into the air – flying spiders!
- Shelters such as burrows or nests
- Mating: male spiders weave sperm webs on which they deposit sperm and subsequently transfer it to their front palps, ready for placing on a females genital organs. Some species make a web and coat it with sex pheromones to attract a mate.
There are seven types of silk produced by seven silk glands. A single spider does not possess all seven glands but has at least three if it is male (dragline, attachment and swathing silk) or four if it is female. The additional one is for egg sac silk. The seven types of gland are:
- Achniform gland: swathing silk.
- Cylindriform gland: egg sac silk.
- Ampullate glands (major and minor): non-sticky dragline silk. Silk from the minor ampullate gland is only half as strong as that from the major gland.
- Pyriform gland: attaching threads – attachment discs are made which anchor a thread to a surface or another thread.
- Flagelliform gland: core fibres of sticky silk.
- Aggregate gland: outer part of sticky silk – droplets of an adhesive substance are deposited along the threads.
The glands are located on the lower side of the abdomen (see diagram below) and contain a watery fluid known as ‘dope’. This fluid passes through to the spinneret via a multitude of microscopic tubes where water recovery and solidification begins. Fluid from different glands can lead to the same spinneret so silk with specific properties required for a particular function can be produced. There are usually three pairs of spinnerets but this can vary between 1 and 4 pairs depending on the species. The substance exits through the spiggots which are mobile, finger-like protrusions and the resulting silk emerges as a solid. There are many spigots so many fibres are bound together like a cable. The diameter of a single fibre is controlled by the muscular action of a valve. The faster and tighter the strand is drawn, the stronger the silk.
Applications of Spider Silk
Humans have been making use of spider silk for thousands of years. The ancient Greeks used cobwebs to stop wounds from bleeding and the Aborigines used silk as fishing lines for small fish. More recently, silk was used as the crosshairs in optical targeting devices such as guns and telescopes until World War II and people of the Solomon Islands still use silk as fish nets.
Current research in spider silk involves its potential use as an incredibly strong and versatile material. The interest in spider silk is mainly due to a combination of its mechanical properties and the non-polluting way in which it is made. The production of modern man-made super-fibres such as Kevlar involves petrochemical processing which contributes to pollution. Kevlar is also drawn from concentrated sulphuric acid. In contrast, the production of spider silk is completely environmentally friendly. It is made by spiders at ambient temperature and pressure and is drawn from water. In addition, silk is completely biodegradable. If the production of spider silk ever becomes industrially viable, it could replace Kevlar and be used to make a diverse range of items such as:
- Bullet-proof clothing
- Wear-resistant lightweight clothing
- Ropes, nets, seat belts, parachutes
- Rust-free panels on motor vehicles or boats
- Biodegradable bottles
- Bandages, surgical thread
- Artificial tendons or ligaments, supports for weak blood vessels.
However the production of spider silk is not simple and there are inherent problems. Firstly spiders cannot be farmed like silkworms since they are cannibals and will simply eat each other if in close proximity. The silk produced is very fine so 400 spiders would be needed to produce only one square yard of cloth. The silk also hardens when exposed to air which makes it difficult to work with.
The alternative approach is to learn how spiders spin silk and then copy them to make synthetic spider silk. The silk itself would also have to be artificially made. Chemical synthesis of spider silk is not viable at present due to the lack of knowledge about silk structure so the replication of silk is currently being achieved using genetic engineering. Randolph V. Lewis, Professor of Molecular Biology at the University of Wyoming in Laramie, has inserted silk genes into Escherichia coli bacteria to successfully produce the repeated segments of spidroin 1 and spidroin 2.
More recently, Nexia Biotechnologies Inc in Montreal, Canada have inserted silk genes into goats to produce silk proteins in their milk. This is hoped to be a better method because protein from bacteria is not as strong due to faulty crosslinking of the proteins and hard white lumps can form. Milk production in mammary glands is similar to silk protein production in spiders so it is thought that proper protein crosslinking could occur in goats.
It has been suggested that the whole gene sequence might not be needed to produce useful spider silk. Prospects include possible gene insertion into fungi and soya plants. It may also be possible to alter the silk genes for specific purposes. For example altering the genes responsible for camouflaging spider silk in nature could lead to a range of silk colours.
There are still problems with developing synthetic spider silk production. An artificial method of spinning silk remains a mystery. Spider spinning dope is approximately 50% protein but this is too high a concentration to use industrially since the fluid would be too viscous to allow efficient spinning. The silk is also insoluble in water but this can be overcome by attaching soluble amino acids such as histidine or arginine to the ends of the protein molecules. In addition, the silk coagulates if the fluid is stirred so it would have to be redissolved. Current research focuses around these problems and a possible solution would be to adapt the composition of silk proteins to alter its properties. Research is still in its early stages but unravelling the secrets of spider silk is underway.
Some interesting web facts:
Not all spiders weave webs.
- Spiders do not stick to their own web because only the central spiral part of the web is sticky, not the spokes. The spider knows where to tread!
Webs lose their stickiness after about a day due to factors such as dust accumulation and exposure to air. In order to save energy the spider eats its own web before making a new one so the protein used for the silk threads is recycled.
Nananana Makaki’i Black Widow
READING DAY-READING MONTH CELEBRATION
Reading day reading month was celebrated successfully in a grand manner. It was a month
long programme well planned and conducted to strengthen the reading & digital reading
among the students.
CLASS-WISE PROGRAMME SCHEDULE
- Administered READING PLEDGE in the morning assembly on 25.06.2018.
- REVERSE READING COMPETITION for classes VI-VIII on 03.07.18
- DRAWING &PAINTING COMPETITION for classes VI-VIII on 04.07.2018
- DISPLAY OF CHARTS ON READING for classes VI-VIII 05.07.2018
- QUIZ COMPETETION for classes IX to XII on 07.07.2018
- BLOCK AND TACKLE for classes IX TO XII on 10.07.2018
- ESSAY COMPETETION on the topic “WHY I READ” for classes IX TO XII ON 12.07.2018 – ONLINE COMPETITION
- CONDUCTED BOOK FAIR FOR LIBRARY IN-HOUSE BOOKS on 16.07.2018
- MASS READING IN THE MORNING ASSEMBLY on 18.07.2018
The celebratition of READING DAY-READING MONTH inculcated reading habit and
bookmindedness among students . Especially mass reading inspires all the students.We
planned to conduct mass reading in the assembly in all the Saturdays in coming future .