Monthly Archives: November 2012
A spider web, or cobweb (from the obsolete word coppe, meaning “spider”) is a device built by a spider out of proteinaceous spider silk extruded from its spinnerets.
Spider webs have existed for at least 141 million years. Insects 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. “Spider web” is typically used to refer to a web that is apparently still in use (i.e. clean), whereas “cobweb” refers to abandoned (i.e. dusty) webs.
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. Thus it is an efficient method of gathering food. However, constructing the web is in itself an energetically costly process because of 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. 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.
There are a few types of spider webs 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 bases of trees or along the ground
- Sheet webs
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.
The stickiness of spiders’ webs is courtesy of droplets of glue suspended on the silk threads. This glue is multifunctional – that is, its behaviour depends on how quickly something touching it attempts to withdraw. At high velocities, they function as an elastic solid, resembling rubber; at lower velocities, they simply act as a sticky glue. This allows them to retain a grip on attached food particles.
Administering certain drugs to spiders affects 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 of various substances. Visit the following link for more information.
Soap is a curious substance, designed to solve an intriguing problem. Most dirt that will not simply wipe off or be shaken out is in fact some form of fat or grease. In most households the most common cleaning agent is tap water. The problem is that grease and water fall into two different and largely incompatible chemical groups. Drop oil into water, and it will tend to float or form discrete droplets. Pour water into oil and you will see the same effect. Additionally, substances such as salt and sugar that dissolve in water will not dissolve in oil, whereas something like petrol will only float on water but is quite capable of dissolving oil
The Chemistry of Oils
This difference in behaviour is due to the nature of the molecules involved. Water is largely polar, that is, water molecules tend to separate into fragments with opposite electrical charges, one positive and one negative. Chemicals such as table salt that happen to be made up of collections of charged fragments, or ions, find it easy to dissolve in water because the positive ions in the salt are attracted to the negative ions in the water, and vice versa. Similarly, the charged nature of water means that water is a good conductor of electricity.
Fats and oils, on the other hand, tend not to be polar. Their molecules have no particular electrical charge, and so are not attracted to polar substances such as salt. Instead, they prefer to bond with other non-polar substances. Fats and oils tend to be electrical insulators.
This, then, returns us to the washing-up. You have a greasy dish in a bowl of water, but the grease is showing no inclination to dissolve in the water because the water is polar and the grease is not. Attack the grease with a cloth and most of what you achieve is to move it around on the plate, because it is trying to flatten itself against the surface of the plate in a effort to get away from the water molecules.
The soap molecule is a halfway house. It consists of a long strand with an ionic water-loving, grease-repelling group on one end, and a non-polar grease-loving, water-repelling group on the other. If you drop soap into clean water, all the molecules gather on the surface with their water-loving (hydrophilic) ionic ends stuck in the water and their fat-loving (lipophilic) ends waving in the air. Slide a dirty dish in, however, and the lipophilic end of each molecule sticks to the grease as it slips past. As the dish sinks, it takes the soap molecules with it, attached by their heads to the grease but still waving their hydrophilic tails in the water like microscopic tadpoles.
All you have to do now is bash at the dirt with a sponge or cloth, and it can be persuaded to leave the plate, for as it lifts off the surface it becomes insulated from the water as new soap molecules rush in and try to bury their heads in it. The end result is a small blob of grease completely surrounded by a layer of soap molecules, all with their lipophilic heads pointing inwards and their hydrophilic tails pointing outwards. As far as the grease is concerned, all it can see are lipophilic molecules, and as far as the water is concerned, all it can see is a rather large hydrophilic lump.
Eventually, of course, all the soap molecules are used up, and you have to tip out the washing-up water and start again. Pass the tea-towel.
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Continuing on our weather theme from last week, we’re looking at Thunder & Lightning today.
Double rainbow and supernumerary rainbows on the inside of the primary arc. The shadow of the photographer’s head on the bottom marks the centre of the rainbow circle (antisolar point).
Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.