Spiders are air-breathing arthropods that have eight legs and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other ordersof organisms. Spiders are found worldwide on every continent except for Antarctica, and have become established in nearly every habitat with the exceptions of air and sea colonization. As of September 2015, at least 45,709 spider species, and 114 families have been recorded by taxonomists.A herbivorous species, Bagheera kiplingi, was described in 2008,but all other known species are predators, mostly preying on insects and on other spiders, although a few large species also take birds and lizards. Spiders use a wide range of strategies to capture prey: trapping it in sticky webs, lassoing it with sticky bolas, mimicking the prey to avoid detection, or running it down. Most detect prey mainly by sensing vibrations, but the active hunters have acute vision, and hunters of the genus Portia show signs of intelligence in their choice of tactics and ability to develop new ones. Spiders' guts are too narrow to take solids, and they liquidize their food by flooding it with digestive enzymes and grinding it with the bases of their pedipalps, as they do not have true jaws.
Male spiders identify themselves by a variety of complex courtship rituals to avoid being eaten by the females. Males of most species survive a few matings, limited mainly by their short life spans. Females weave silk egg-cases, each of which may contain hundreds of eggs. Females of many species care for their young, for example by carrying them around or by sharing food with them. A minority of species are social, building communal webs that may house anywhere from a few to 50,000 individuals. Social behavior ranges from precarious toleration, as in the widow spiders, to co-operative hunting and food-sharing. Although most spiders live for at most two years,tarantulas and other mygalomorph spiders can live up to 25 years in captivity.
Benefits to humans
The Ch'ol Maya use a beverage created with the tarantula species Brachypelma vagans for the treatment of a condition they term 'tarantula wind', the symptoms of which include chest pain, asthma and coughing.Spider venoms may be a less polluting alternative to conventional pesticides as they are deadly to insects but the great majority are harmless to vertebrates. Australian funnel web spiders are a promising source as most of the world's insect pests have had no opportunity to develop any immunity to their venom, and funnel web spiders thrive in captivity and are easy to "milk". It may be possible to target specific pests by engineering genes for the production of spider toxinsinto viruses that infect species such as cotton bollworms.
Possible medical uses for spider venoms are being investigated, for the treatment of cardiac arrhythmia,Alzheimer's disease, strokes,and erectile dysfunction. The peptide GsMtx-4, found in the venom of Brachypelma vagans; is being researched to determine whether or not it could effectively be used for the treatment of cardiac arrhythmia,muscular dystrophy or glioma. Because spider silk is both light and very strong, attempts are being made to produce it in goats' milk and in the leaves of plants, by means of genetic engineering.
Spiders can also be used as food. Cooked tarantula spiders are considered a delicacy in Cambodia, and by the Piaroa Indians of southern Venezuela – provided the highly irritant hairs, the spiders' main defense system, are removed first.
A human sized spider's web can even stop a Jumbo Jet
Arachnophobia
Arachnophobia is a specific phobia—it is the abnormal fear of spiders or anything reminiscent of spiders, such as webs or spider-like shapes. It is one of the most common specific phobias, and some statistics show that 50% of women and 10% of men show symptoms. It may be an exaggerated form of an instinctive response that helped early humans to survive,or a cultural phenomenon that is most common in predominantly European societies.
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