Spider

**Spiders** (class Arachnida, order **Araneae**) are air-breathing arthropods. They have eight legs, and mouthparts (chelicerae) with fangs that inject venom. Most make silk. The spiders are seventh in number of species of all animal orders. About 53,000 spider species, and 136 families have been recorded by taxonomists. Over twenty different classifications have been proposed since 1900.p3 

Spiders live on every continent except for Antarctica, and in nearly every habitat with the exceptions of air and sea. 
Almost all spiders are predators, and most eat insects. They catch their prey in several ways. Some build a spider web, and some use a thread of silk that they throw at the insect. Some kinds of spiders hide in holes in the ground, then run out and grab an insect that walks by. Others will make web 'nets' to throw at passing insects. Or they go out and simply attack their prey. Some can jump quite well and hunt by sneaking close to an insect and then jumping on it.

## Anatomy 
Spiders have a two-part body, the front part (cephalothorax) and the abdomen. Unlike insects, spiders do not have antennae. The more advanced spiders have a centralized nervous system, with their ganglia fused into one mass in the cephalothorax. Unlike most arthropods, spiders have no extensor muscles in their limbs, and extend them by hydraulic pressure.

### Size range 
The smallest of full-grown spiders can be less than 4&nbsp;mm. (0.1 inch). The largest of spiders can have a body length of 10&nbsp;cm. (4 inches) or more. The largest can weigh 150 grams (5.3&nbsp;oz). The largest of spiders are the tarantulas and the huntsman spiders. Some huntsman spiders in South East Asia can have a leg span of around 250–300&nbsp;mm (9.8–11.8&nbsp;in).

### Spider eyes and other senses 
Most spiders have four pairs of eyes on the top-front area of the body, arranged in patterns that vary from one family to another. The main eyes at the front are capable of forming images. Jumping spiders have visual acuity which is ten times better than that of dragonflies, which have by far the best vision among insects. The spiders do this by a telephoto-like series of lenses, a four-layered retina. They can swivel their eyes and put together images from different stages in the scan. The downside is that the scanning and integrating processes are relatively slow.

Spiders and other arthropods have modified their cuticles into elaborate arrays of sensors. Various sensors, mostly bristles, respond to touch, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and smell.

Young spiders of several families feed on plant nectar. Studies have shown that they do this for long periods. They also clean themselves regularly while feeding. These spiders also prefer sugar solutions to plain water, which shows that they are looking for nutrients. Many spiders are nocturnal, they are most active during the night. The extent of nectar consumption by spiders may therefore have been under-estimated. Nectar contains amino acids, lipids, vitamins and minerals in addition to sugars. Studies have shown that other spider species live longer when nectar is available. Feeding on nectar also avoids the risks of struggles with prey, and the costs of producing venom and digestive enzymes. Since nectar is a flowering plant production, its consumption by spiders came quite late in their evolution.

Various species are known to feed on dead arthropods (scavenging), web silk, and their own shed exoskeletons. Pollen caught in webs may also be eaten, and studies have shown that young spiders have a better chance of survival if they have the opportunity to eat pollen. In captivity, several spider species are also known to feed on bananas, marmalade, milk, egg yolk and sausages. A few spiders use the surfaces of lakes and ponds as "webs", detecting trapped insects by the vibrations that these cause while struggling.

Mature female bolas spiders of the genus *Mastophora* build "webs" that consist of only a single "trapeze line", which they patrol. They also construct a bolas made of a single thread, tipped with a large ball of very wet sticky silk. They emit chemicals that resemble the pheromones of moths, and then swing the bolas at the moths. They catch about the same weight of insects per night as web-weaving spiders of similar size. The spiders eat the bolas if they have not made a kill in about 30 minutes, rest for a while, and then make new bolas. Juveniles and adult males are much smaller and do not make bolas. Instead they release different pheromones that attract moth flies, and catch them with their front pairs of legs.

#### Using trapdoors 
 (family: Ctenizidae), an ambush predator]]

The primitive Liphistiidae, the "trapdoor spiders" (family Ctenizidae) and many tarantulas are ambush predators. They lurk in burrows, often closed by trapdoors and surrounded by networks of silk threads that alert these spiders to the presence of prey. Other ambush predators do without such aids, including many crab spiders. Wolf spiders, jumping spiders, fishing spiders and some crab spiders capture prey by chasing it, and rely mainly on vision to get their prey. However, they seem to be relatively slow thinkers, which is not surprising as their brains are vastly smaller than those of mammalian predators.

## Reproduction and life cycle 
 (*Nephila clavipes*) (near the top of the leaf) is protected from the female by his producing the right vibrations in the web, and may be too small to be worth eating.]]

Spiders reproduce sexually and fertilization is internal but indirect. In other words, the sperm is not inserted into the female's body by the male's genitals but by an intermediate stage. Unlike many land-living arthropods, In over 60% of cases the female of the Australian redback spider kills and eats the male after it inserts its second palp into the female's genital opening; in fact the males co-operate by trying to impale themselves on the females' fangs. Observation shows that most male redbacks never get an opportunity to mate, and the 'lucky' ones increase the likely number of offspring by ensuring that the females are well-fed. However males of most species survive a few matings, limited mainly by their short life spans. Some even live for a while in their mates' webs.

The spider that kills the most people, the black widow and other spiders in the genus *Latrodectus* are around 1&nbsp;cm. in body length. The *Atrax* and *Phoneutria* spiders, which are also capable of killing people, both average around 2.5&nbsp;cm. or one inch, and even the Widow spiders are large enough to be easily noticed.

Even relatively small spiders like *Phidippus audax* can give a painful bite if you hurt them, but spiders are very helpful to human beings because they control insects that eat our crops.

Being afraid of spiders is a very common phobia (fear). The widow spiders (black widows and other members of their genus) never willingly leave their webs, so usually people get bitten when they touch the spider by mistake.

## Gallery 
**Click on a picture to see it larger:**

File:Platycryptus undatus2.jpg|Small: *Platycryptus undatus*8–13&nbsp;mm
File:AaurantiaQtr.jpg|Medium: *Argiope aurantia*14–25&nbsp;mm
File:Theraphosa_blondi_1.jpg|Large: *Theraphosa blondi*120–140&nbsp;mm
File:Wandering spider.jpg|*Phoneutria nigriventer*Brazilian wandering spider

## References 
## Other websites 
* Platnick N.I. 20013. *The world spider catalog*. Hosted by the American Museum of Natural History, and edited by Peter Merrett and H. Don Cameron. http://research.amnh.org/iz/spiders/catalog/INTRO1.html
* all spiders a-z https://www.lexikon-spinnen.de