What does the word 'arachnid' mean to you? You probably think 'spider'. This is understandable, considering spiders are by far the most common arachnids. Arachnophobia means fear of spiders, and the word 'arachnid' comes from Arachne, a girl in Greek mythology who got turned into a spider after displeasing the goddess Athene. However, if spiders are arachnids, then why the separate words? The answer to this is that the arachnid class includes not only spiders, but also scorpions, ticks, mites, harvestmans, vinegaroons, windscorpions, and pseudoscorpions. With all these diverse organisms in one group, what is it that defines an arachnid? Read on to find out.

Anatomical Features

Arachnids are highly adaptable and are found all over the world: in the most well-kept European lawns, the Sahara desert, the jungles of South America, the African rain forests, the human scalp, the Australian outback, the frozen wastes of Siberia, and close to the top of Mount Everest. In fact, the only place they are not found naturally is Antarctica.

Arachnids are members of the animal kingdom. They are invertebrates, which technically mean that they have no backbones; in practice, however, most invertebrates have no bones at all. Arachnids are members of the phylum Arthropoda, which means that like insects, crustaceans, and polypedes, they have exoskeletons made of a tough protein called chitin to provide support.

All arachnids have two primary body parts: the cephalothorax and the abdomen. The cephalothorax comes from the Latin words cephalus (head) and thorax (chest). Thus, the cephalothorax is the arachnid's head and chest. It is to this that the legs and pedipalps join, and where the eyes, mouth, and other sense organs are located. The abdomen is the rear part of the body, and contains most of the internal organs, such as the book lungs, heart, and gonads.

All arachnids have eight jointed legs, four on either side of the body. These legs often end in tiny hairs that help them climb up vertical surfaces. They have two pedipalps in front of their bodies, which may have various uses. Spiders, ticks, mites, harvestmans, and windscorpions use their pedipalps as feelers, though they do not work as well as insects' antennae. Scorpions and pseudoscorpions have pedipalps that end in pincers, and in vinegaroons, they terminate in claws; these arachnids use their pedipalps for seizing prey.

In most cases, arachnids breath through 'book lungs,' which are so-called because they resemble the pages of a book. Found one on either side of the arachnid's abdomen, they take the form of a number of thin folds of thin tissue which are open to the air. Air flows into the spaces between the folds, and then passes through to blood vessels on the other side. A few very active arachnids also have a system of holes and tubes (technically known as spiracles and trachea respectively) in the sides of their bodies.

Arachnids have between zero and eight eyes inclusive. The eyes tend to be spread in a circle around the cephalothorax to allow for 360° vision. However, these eyes rarely give good vision. An arachnid would generally be able to make out very little detail and see only blurs of colour, but many can see movement very well. Those that hunt tend to have two large, powerful eyes that face forward, and several weak ones that see all around. Those that live in caves often have no eyes, as there is not enough light to see, and eyes here would be superfluous. This is why there is an arachnid called the no-eyed big-eyed wolf spider - it has no eyes, but belongs to the big-eyed wolf spider family.

The blood systems of arachnids are based on diffusion rather than circulation. The heart pumps blood, which flows out through a number of open-ended blood vessels and floods the body with blood. For very small animals, this works perfectly, as the blood travels quickly over a short distance, and the animal is not weighed down by a large, complex heart. However, larger animals such as mammals and reptiles require a closed system, as beyond a certain size, diffusion becomes inefficient, which is why humans have powerful, multi-chambered hearts.

All arachnids have special mouthparts called chelicerae, which look like they could be used for stabbing. Their purpose is to get saliva onto the arachnid's dead prey. Arachnids cannot digest solid matter, and so they dribble saliva onto it via the chelicerae to dissolve it. What is left is often referred to as a 'soup', which the arachnid then sucks up.

Arachnids never have wings. The top of the cephalothorax is covered with a tough carapace, and the abdomen often is as well. However, many arachnids have soft spots in the abdomen which allows them to stretch, thus facilitating the ingestion of larger meals. They grow by shedding their skin, passing through the stages of egg, larva, nymph, and adult. The nymph is what the larva becomes after a single moult, and resembles a miniature version of the adult. Moulting in this way allows arachnids to regrow lost legs and eyes. When they shed a skin, they are initially weak and vulnerable; the skin is soft in order that the arachnid may stretch within it, but hardens after a few hours.

Spiders

Ranging in size from less than a centimetre long to over 30cm, spiders are the most well-known arachnids, and with good reason. They are the most common arachnids and the most useful, as they eat insects that would otherwise cause disease. The abilities of the spider are the basis for the powers of a certain well-loved comic book hero. They are also rather misunderstood; millions of people are terrified of spiders, believing them to be unclean and lethally poisonous. While it is true that a few spiders can kill humans, this only applies to about 2% of all spider species. The others are totally harmless - to humans, anyway.

Spiders have no brains; instead, they have two ganglia (nerve clumps) which regulate metabolic activities and reactions to stimuli. They all have two fangs which contain venom, though as has already been mentioned, this is rarely harmful to humans. However, it is quite effective in bringing down smaller prey, such as insects, tadpoles, and occasionally small frogs, chicks, and rodents.

Silk

Not all spiders spin webs, but many do. They do this with two organs called spinnerets located at the base of their abdomens. The spinnerets, as their name suggests, spin out silk, which is produced inside the spider's body. When silk emerges, it is a liquid, but solidifies on contact with air. Though elastic, spider's silk is actually stronger than steel of the same thickness. All spiders have spinnerets and silk organs. The spider begins spinning the web by sitting on a branch or leaf and squirting out a thin stream of silk. This gets caught on a draught, which lifts it up until it latches onto another twig or leaf. The spider crawls along this strand, letting off another strand as it goes, which is attached to the original twig/leaf. When the spider reaches the other end, it lets off a little more silk, then attaches it to the other end of the original strand. The spider now goes down to the centre of the new strand and attaches another length of silk to the centre, then drops down and attaches the end of this new strand to a leaf or twig below, forming a Y shape. The spider attaches numerous other strands to the centre of the Y, like spokes on a bicycle wheel. It then spins out a number of concentric circles around the centre, forming a very effective net. Finally, the spider coats each strand of silk with a sticky substance to catch insects, purposefully leaving some portions of the web unsticky so that it does not get caught itself. An oily secretion from the spider's feet also helps prevent it becoming ensnared. When all this is done, the spider waits in the centre for something to fly into the web; when something does, it sets off vibrations which the spider can detect. When this happens, it quickly moves to the place from where the vibrations originate, kills the insect, and wraps it in silk for later.

Spiders spin many different kinds of webs, and have numerous different uses for silk. Signature spiders each spin a pattern in a part of their web, and this pattern is as unique as a fingerprint; hence the name. Rafter spiders and black widow spiders build messy webs in corners, which are nonetheless effective at trapping prey. Jumping spiders use silk as a bungee chord, allowing them to get back to where they were quickly after jumping on top of a victim. Many baby spiders spin small amounts of a fine silk called gossamer and use it to float away; this is known as ballooning. Nearly all types of spider seal clusters of eggs in non-sticky silk to protect them.

The trapdoor spider has a unique use for silk. First of all, it makes a burrow, and lines this with spider's silk. Next, it puts a cap on top attached to the ground with a simple hinge, which is how it gets its name. After this, it erects several tiny twigs in the ground around its burrow, and strings strands of silk between them, connecting each one to its burrow. When prey comes into contact with one of these trip wires, the trapdoor spider leaps out of its burrow, grabs its victim, and is back in in about 0.02 of a second.

Tarantulas

There is actually no such thing as a tarantula. Originally, it referred to a type of wolf spider in the town of Taranto in Italy, where it was blamed for causing a plague. The people believed that they could only cure themselves by sweating the poison out, and so they invented an energetic dance called the tarantella. When explorers reached South America, they encountered large spiders which were hairy like the wolf spiders. These looked quite fearsome, and the explorers, believing them to be poisonous, named them tarantulas as well. When most people talk about tarantulas, they in fact mean bird-eating spiders. These spiders only very rarely eat birds, and are covered in irritating hairs which they can shoot when they feel threatened. They may look scary, but their venom is only harmful to people who are allergic to them. To date, no-one has ever died from a bird-eating spider attack.

Dangerous Spiders

One of the most venomous animals in the world is the wandering spider. Found only in Brazil, the bite of this member of the wolf spider family can kill a human in just a few hours. As a matter of fact, only a few snakes, such as cobras and vipers, have more potent bites. However, humans rarely come into contact with it. Another type of poisonous spider, the recluse spider, is more common, but its name derives from the fact that it likes to hide away, and like any spider, will only bite when threatened. While harmful, it is only dangerous to children. The most common spider killer is the black widow, so called because it is black and the female tends to eat the male after mating. A closely-related species, the redback spider, can be identified by the distinctive red hourglass marking on its abdomen. Irritatingly, nobody is quite sure what exactly a 'black widow' is; some people use that term to refer to that whole family of spiders - including redbacks - and others use it to refer solely to one certain species. Anyway, though far less dangerous than the wandering spider, many more people come into contact with the black widow/red back spider and hence, these species have claimed more victims. It is believed that the black widow caused the plague for which the wolf spider was blamed.

Scorpions

Unlike spiders, scorpions have proper brains. They are almost all nocturnal carnivores, and tend to have five eyes: two large, powerful ones for hunting, and three little ones for sensing changes in light. They have pincers at the end of their pedipalps and smaller ones at the chelicerae, but by far the most distinctive feature of the scorpion is its tail. A scorpion's unique tail, known as a teslon, is one of the most recognisable symbols of danger in the natural world - so much so that a type of fly, called the scorpion fly, has evolved a fake teslon to scare off enemies.

The teslon is divided into several jointed bulges, which allow it to move in any direction, and dart forward extremely quickly. In some scorpions, it is so fast that they can accurately flick their venom for great distances, sometimes up to 45m. The terminal bulge contains a nasty sting, which is full of venom. Scorpions eat insects and spiders, though some of the largest can also eat frogs, rodents, and small reptiles. Though their carapace is very tough, there are stretchable sections along the sides of the abdomen to allow extra food to be eaten. Oddly, scorpions glow bright green when UV light is shone on them.

Mating

Scorpions have one of the most unique mating habits on earth. While other arachnids mate in a fashion similar to that of vertebrates, the male scorpion begins by placing a drop of sperm on the ground. He then locks pincers with a female and tries to position her over this sperm. To an observer, it looks as if they are performing a long, complicated dance. This can go on for a few minutes or for up to two days. During the 'dance', the partners hold their tails low in order that they do not sting each other. If they did, however, it would not be a serious problem, as all scorpions are immune to venom of their own species.

Scorpions give birth to live young in large batches. After they are born, the female carries her soft-skinned young around on her back for protection until they moult for the first time. After that, they are on their own, but by now they have hard carapaces and are quite capable of hunting and looking after themselves.

Dangerous Scorpions

At only a few centimetres long, the golden scorpion is the smallest scorpion of all, yet its venom can easily kill a human. By contrast, the imperial scorpion grows to about 18cm long, making it the largest scorpion of all; however, it also has the weakest venom. The reason for this is that the imperial scorpion has huge pincers which are quite capable of ripping prey (or rivals) to shreds, and it has little need of venom. In general, an imperial scorpion will only sting if its prey struggles. The golden scorpion, meanwhile, has only weak pincers, and so must rely on powerful venom to bring down its prey. This relationship between size and venom potency is pretty much the same for most scorpions; the smaller the scorpion, the worse its venom. The exception here is the fat-tailed scorpion. At 12cm, it is quite large for an arachnid, and it is also the most venomous scorpion on Earth; a sting from this scorpion can kill a human in as little as six hours.

Holy Scorpions

The scorpion is one of the few invertebrates to have been worshipped as a god. In ancient Egypt, scorpions were associated with the goddess Isis. This is likely to be because scorpions carry their young around on their backs for so long, and Isis was the archetype of a devoted mother.

Interestingly, Isis was also associated with the star Sirius. Guess which constellation Sirius is in? That's right - Scorpio!

Harvestmans

The harvestman is also called a daddy-long-legs because of its especially long legs, which appear even longer when compared to its tiny body. The body is not usually much more than half a centimetre long, though the legs can reach about 3cm. On a harvestman, it is very difficult to see the division between the cephalothorax and the abdomen. Though they resemble spiders, they do not produce silk. Like spiders, their pedipalps are used as feelers, though these are very short. They dehydrate very quickly, and so must drink often; they have even been known to drink ink. Harvestmans feed primarily on small bugs, though will also often eat bits of plants. If cornered by something that wants to eat them, they will either spray a smelly chemical or play dead. They are also quite willing to lose a leg in order to escape. They have no venom, and possess ganglia instead of brains.

Ticks

Ticks are among the smallest multicellular organisms in existence. Few can be seen without a microscope. Their chelicerae have evolved into a hard, sharp, stabbing needle, which they use to suck blood. A hungry tick is almost flat, but as it feeds, it swells up to dozens of times its former size. A tick's abdomen is very elastic, allowing it to gorge itself on blood quickly and then survive without feeding for quite some time. Its internal organs are squashed into a fairly small area to allow more room for blood. Millions of ticks are found on the skin of mammals, but they conceal their presence with a chemical in their saliva which numbs the area where they feed. Their chelicerae are barbed, meaning that scratching or rubbing will not dislodge a tick. Attempting to pluck a feeding tick out with the fingers will only remove the abdomen, leaving the cephalothorax still sucking. The tick itself, however, can remove itself from its victim at any time. Since ticks often pick up germs from their victim's blood, they are among the most potent disease vectors around; the most known being Lyme desease and meningitis.

There are two main types of tick: the hard tick and the soft tick. The main difference is that the hard tick possesses a carapace on top of its abdomen, but the soft tick does not. While a soft tick generally has one host for life, a hard tick can have quite a few. After a hard tick has finished sucking blood, it dislodges and, when it is hungry again, climbs onto the edge of a leaf or blade of grass to await a new host. Both types can suck relatively vast amounts of blood; females who are about to lay eggs can suck up to 100 times their own body mass. However, since a tick is very tiny, this is not a serious loss for the host. Drinking this much blood can take several days. After mating, the female sucks herself full, then spends a few days laying a batch of several hundred eggs. After this, she shrivels and dies.

Mites

Mites are somewhat larger than ticks, but still very small. The largest are rarely much more than a centimetre across, and the smallest are positively microscopic. Mites eat practically anything, though many are fond of sap-sucking insects. They will also eat plants, carrion, and human remains such as hair and skin flakes. The microscopic dust mite eats only skin flakes, and causes asthma in people who are allergic to them. Their droppings constitute a large proportion of household dust. Mites do not usually carry serious disease and this, coupled with their extremely small size, means that they are the least harmful arachnids of all. Their chelicerae are almost identical to those of spiders, but the venom of a mite is far weaker. Anatomically, they are quite similar to spiders, but cannot produce silk.

Young mites are almost always parasites. When they mature, some species retain their parasitic ways, but others become omnivores, eating whatever they can lay their pedipalps on. Since many enjoy the eggs of pests such as locusts, mites are seen by many farmers as helpful bugs.

Vinegaroons

The vinegaroon gets its name from the fact that, when threatened, it can spray a liquid not unlike vinegar at its enemy. It also uses this liquid to subdue its own prey, primarily insects. It is also known as the whipscorpion due to the presence of a long, whip-like tail through which it sprays its liquid. Though the vinegaroon grows to only about 6.5cm long, its spray is accurate over a distance of 500cm. Its pedipalps take the form of vicious, jointed claws, which are not quite as effective as proper pincers, but are nonetheless very useful for catching prey. The vinegaroon tends to go around with its abdomen curved and its tail held high, and so is sometimes mistaken for a scorpion. It has similar mouthparts to a spider, but without the venom. Like scorpions, vinegaroons have true brains, and they eat the same type of food as scorpions.

Raising Larvae

Vinegaroons reproduce the same way as most other arachnids - ie, by a simple insertion of sperm, not the complicated 'dance' of scorpions. When the female vinegaroon becomes pregnant, she makes a little home in the undergrowth which no predators can find. She then waits patiently for the eggs to hatch, and when they do, she carries them around on her abdomen and gives them food. When they are old enough to fend for themselves, they leave. When all her larvae have left, the female vinegaroon dies.

Tarantula?!

Though tarantulas do not truly exist, one species of tailless vinegaroon has the scientific name Challus tarantula. It is the only animal on Earth with this spelling. By contrast, the European wolf spider has the scientific name Lycosa tarentula, and the bird-eating spider has the name Euthlus smithi.

Windscorpions

The windscorpion is proportionately the fastest land animal on earth. It can hit over 15kph - not bad for something whose body is only 7cm long. It can also run over 330 times its own body length in 10 seconds; a human can run about 50 times their body length in the same amount of time. This great speed is what gives it its name; it is also sometimes referred to as a sunspider due to the fact that it lives in hot, desert areas. Compared to body size, the windscorpion's jaws are proportionately the largest and most powerful of any animal. Most windscorpions are nocturnal, but a few are diurnal. They eat anything that moves - worms, spiders, insects, scorpions, other windscorpions, small lizards, salamanders, birds, and mice, provided it is not too much bigger than them. A windscorpion eats until it is so fat it can hardly move.

Windscorpions have true brains. They only have two eyes, but they are by far the best of any arachnid. As they run along, they only use six legs for movement; their two front legs as well as their pedipalps are held outstretched, feeling for prey. Windscorpions are covered in many tiny, sensitive hairs, which allow them to easily pick up vibrations in the ground made by other organisms.

Reproduction

Mating is a dangerous proposition for male windscorpions. The females are especially vicious, and will not hesitate to eat them. For this reason, when a male wishes to pass on his seed, he slowly sidles up to a female and starts stroking her. This relaxes her, and it can seem almost as if the female is in a trance. However, this does not last long, and so the male quickly does his business and gets out of there. When the female snaps out of it, she eats as much as possible to give her energy for the job ahead. After a while, she digs a burrow with her jaws and lays between 100 and 250 eggs. She then guards the burrow, driving away any predators that get too close. When the eggs hatch, the mother goes out hunting again to bring back food for her brood. She stays with them until they can look after themselves, then gives them the boot.

Pseudoscorpions

Pseudoscorpions are very small, and are generally only about a centimetre long. In appearance, they resemble scorpions without tails. Their name means 'false scorpion', and it has this name due to the fact that it is not what it looks like. The pseudoscorpion walks around with its pincers held high, but if it finds itself in danger, it plays dead rather than fights. Its pincers contain venom, but not nearly strong enough to harm a human. They are also covered with tiny, sensitive hairs, which pick up vibrations of other animals walking around. They are in fact so sensitive that a pseudoscorpion can feel the vibrations in the ground given off by a single ant. A pseudoscorpion can also produce silk from its chelicerae. If two pseudoscorpions of the same species meet, they lock pincers and appear to be shaking hands. It is believed that the purpose of this ritual is for males to warn each other out of their respective territories. The pseudoscorpion eats mainly small insects and spiders.

Going Places

At only 1cm long, a pseudoscorpion cannot travel very fast on its own. However, it is quite good at grabbing onto other animals and getting a lift on them. Its hairs tell it when another animal is near, so it grabs on with its pincers and does not let go. Since the pseudoscorpion is so small and light, the carrier often does not notice.

Mating

Pseudoscorpions have many different mating habits. Often, the male simply leaves a drop of sperm on the ground and leaves it up to the female to find it. Some species attempt to guide the female using the 'dance' of scorpions. This is often preceded by a display of waving pincers, tapping legs, and shaking abdomens in order to get the female's attention. Still others string a series of silk threads up to the sperm.

When the female becomes pregnant, she collects tiny scraps of wood and debris and arranges them in a nest around her, stuck together with silk. Her eggs hatch in a pouch in the underside of her abdomen, and the young grow inside her, where they are fed on fluid from the mother's body. The babies do not emerge until they are ready to face the world.