Arachnida (uh-RAC-nuh-duh) is a class in the phylum arthropoda. The study of spiders and their kin is known as arachnology. A scientist who studies spiders is known as an arachnologists. An unreasonable fear of spiders is known as arachnophobia.

There are over 100,000 known species including spiders, scorpions, ticks, mites, sea spiders and horseshoe crabs.

Most live in a terrestrial habitat. Among the exceptions are the sea spiders, the horseshoe crab, and the fishing spider.

Arachnid Characteristics


Spiders are not insects.  They belong to a group of animals called arachnids.  Spiders have no wings or antennae.  They have eight legs, which they use to crawl across their webs.  Spiders' webs are made of silk.  The silk is strong and comes from the spiders body.  Most spiders are helpful because they eat all kinds of insects and rarely harm humans. Some people do have an allergic reaction to a bite.  Most of us never realize that a spider has bitten us, so we call them harmless.  However, the black widow spider does have a poisonous bite.  Spiders are usually small.  But the tarantula could be as large as your hand.

Physical Characteristics:


Spiders have an open, blood circulatory system. This sort of blood system has a heart, arteries and veins but no capillaries. The heart is tubular with a single cavity and with valves to maintain the flow of blood always in the same direction. The spider's blood is pale blue due to the presence of haemocyanin dissolved in the lymph. There are some blood cells but they are for wound-healing and defense against infection - there are no special blood cells like the human red cells which carry oxygen around the body.



Breeding and Young:

Feeding and Digestion

The spider catches its prey with the appendages i.e. the legs. Then it uses its sharp fangs to inject the poison into the prey. This poison also contains enzymes that dissolve the prey. The digestion is partly external in the prey itself. The liquefied contents are sucked into the alimentary canal by the sucking stomach that is located in the cephalothorax. The particles in the food are filtered out by means of many hairs around and in the mouth. The filtering is so efficient that it can separate the particles in Indian ink from the liquid. From the sucking stomach the food is transferred via the intestine to the midgut which is located in the abdomen where secretory cells release enzymes which digest the food further. Resorptive cells take up tiny globules of food and continue the digestion. In this process, the food is broken down to a molecular size. The midgut finally ends in the cloacal chambers where the faeces are stored.



The breathing system of spider is different from ours. There are two separate systems involved, book lungs and tracheae. The book lungs are assumed to be the first respiratory system evolved which is later replaced by the tracheae. A book lung consists of a stack of plates. Blood flows through these plates and the gas is exchanged. The gases pass in and out by a diffusion process and it therefore not very efficient. In more active spiders tracheae replace part of the book lung or the whole book lung. The tracheae are tubes that do not branch but run from the opening on the outside directly into the tissues and organs.

Blood circulation

The spiders blood system is know as an open system. There is no blood vessel system like in man but the blood pours out from the end of the arteries directly onto the tissue where it runs freely. The cavity around the heart is called pericardium. The blood is different from man. The oxygen is bound to hemocyanin, a molecule that contains copper, whereas in humans the oxygen is bound to hemoglobin, a molecule that contains iron. The color of oxidized copper is blue/green. Therefore, spiders have 'blue' blood. The blood contains some cells, though to be mainly concerned with blood clothing, defense against infection and wound healing. The blood pressure of spiders is high and is with 0.18 atm nearly equal to men. During the changing of the skin, this pressure is doubled. It is thought that the blood pressure is used as a hydraulic force for walking.


The reproduction is a dangerous task for the male spider that he often has to pay for with his life. It is a complex process involving a wide range of approaches and tactics.

The male has sperm production organs in it abdomen. Tubes lead to the genital opening that is located in the center of the epigastric furrow that can be found on the ventral side of the abdomen behind the legs. Before the male can mate, he has to transfer the sperms to the reservoirs in its palpal organs. To do this it makes a small web and a drop excluded from its genital opening. He then takes up the semen with its palp. The uptake of semen is probably a combination of capillary and gravitational forces. Then the dangerous part. He has to approach the female with the correct rituals. Failing these conditions is a death penalty. When the females accepts the male, the males connect its palp to the female epigyne and the semen is inserted.


These are the largest spiders. The name tarantula really belongs to members of the family Lycosidae but in the USA the Theraphosidae are commonly called tarantulas. Their hairy bodies may be 10 cm long with a leg span of nearly 15 cm. Some South American types have venom that is dangerous to humans but the bite of a North American tarantula is no more dangerous than a wasp or bee sting. Tarantulas typically hide during the day in burrows or dark holes. They may line their burrow with silk. They do not spin webs. Their jaws - unlike other spiders - move up and down and they feed on large insects and even small vertebrates such as mice or birds. The male tarantula lives only a few years but in captivity a female can live for 35 years.

Wolf Spiders
These spiders live and hunt on the ground amongst dead leaves and debris, on sand dunes, or on the muddy surface of marshes. Most do not spin webs but dig burrows or have no home at all. Female wolf spiders carry their egg sacs around with them until the eggs hatch and then carry the spiderlings on their backs until they disperse. Burrowing members of this family are true tarantulas and belong to the genus Lycos. These large wolf spiders live in burrows with trapdoors and are known to bite humans causing tissue necrosis.

Trap-Door Spiders
These quite large spiders have spines on their jaws that they use to dig burrows in the ground. They seal the opening with a hinged lid or trapdoor. Some types have a delicate silken trapdoor while others have a thick, tightly fitting lid made of earth and debris. The spiders wait in their burrows until they sense that a suitable prey is close by. Then they rush out, seize the victim, inject it with venom and drag it into the burrow.

Jumping Spiders
This is the world's largest family of spiders. These small to medium sized spiders (usually well under 10mm long) are called jumping spiders because of the spectacular leaps they make as they pounce on their prey. Their leaps can be up to forty times their body length. They have quite large eyes and better than usual spider eyesight and are excellent hunters. As a jumping spider leaps onto its victim silk streams from its spinnerets and serves as a life-line. Some species of jumping spiders are beautifully colored and patterned, sometimes with irridescent hairs.

Nursery Web Spiders
These spiders spin webs not to catch prey but to care for their young. The web is constructed in low plants or shrubs and the female guards the eggs until they hatch and the tiny, perfectly formed spiderlings emerge and disperse. Some members of this family are called fishing spiders. These spiders can run over the surface of quiet water and can creep below the surface for as long as 30 minutes to catch a small fish or tadpole.

Spitting Spiders
A spitting spider has special glands in its cephalothorax which produce a sticky substance that is squirted onto its prey. As the spider spits out the gum it turns it head from side to side so that the gum falls in a zigzag pattern and holds down the prey so that the spider can give it a paralyzing bite. Once the prey is dead the spider frees it from its sticky bonds, pours digestive juices over it and starts feeding.

Crab Spiders
The Crab spiders are a large family distributed worldwide. These spiders have a sideways scuttling movement similar to that of crabs. A few members of this family habitually rest on flower heads waiting for unwary insects. These spiders tend to be colored to match the flowers that they visit. Most members of the family though are drab colored and rest on vegetation or on the ground.

Orb Web Spiders
The members of this large family vary greatly in size, shape and color. Orb weavers spin webs with non-sticky radiating strands and spiraling sticky threads. The spider often rests near the center of the web waiting for an insect to become caught . But others like the barn spider wait above the web during the day connected to it by a silk signal line and is thus alerted when an unsuspecting insect disturbs the web. The Garden spider eats its old web and spins a new one each day. The Bola spider does not spin a web. Instead it dangles a silk thread with a sticky globule on the end and swings it to capture a passing moth.

Funnel Web Spiders
These small to medium sized spiders - under 20mm long - spin sheet webs of non-sticky silk with a funnel extending from the center to one edge and a barrier web over the top to catch insects. The spider sits in the funnel. When a flying insect hits the barrier and falls onto the sheet web the spider rushes out of the funnel, bites the victim to paralyze it, and drags it into the funnel to feed on it.
An unusual member of this family is the water spider (Argyroneta). This spider feeds on small water organisms and spends its life below the surface of lakes and ponds living in a bubble of air held in the water by a silken diving bell or thimble-like structure. A good description of how the spider constructs this astonishing structure is given in Spiders of the World by Ron & Ken Preston-Mafham.

Daddy Long Legs
Sometimes thought to be spiders, Daddy Long-legs are not spiders. They are not included in the Order Araneae. They have a very superficial similarity to spiders in that they have 4 pairs of legs but their legs are stilt-like and are kept bent with the body close to the ground. The daddy long-legs eats insects and other invertebrates as well as the tender gills of fungi and soft decaying matter.


The Arachnids, evolved in the sea, but now they are almost entirely terrestrial and have developed several important features to help them survive on the land. These include: a waterproof (waxy) exocuticle, internal fertilisation, malphagian tubules as a metabolic excretory system and internal organs for breathing and gaseous exchange

The class Arachnida is extremely diverse in form and in lifestyles and little more can be said that includes them all. This diversity is reflected in the classification scheme shown below.

The Class Arachnida is divided into 13 subclasses, (or orders depending on which classification scheme you are following) of which the Araneae (spiders) and the Scorpiones (scorpions) are the best known.

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Spider Facts

The smallest spiders are tiny specks: As adults, they're less than a millimeter long – legs and all. The biggest spiders have a four-inch-long (10 cm) body and a leg span that stretches more than ten inches (25 cm).

In certain South American cultures, people roast and eat tarantulas. The meat is said to have a nutty flavor.

A spider under attack can cut its losses by deliberately breaking off a leg. A shut-off valve at the joint seals the wound. Many spiders can grow a new leg to take the place of the old one.

Tarantulas can send predators packing with poisonous, barbed hairs. The spiders use their hind legs to aim a cloud of these tiny, painful "spears" at their attackers.

The male European house spider can run 330 times its own body length in ten seconds. To do the same thing, a person would have to run farther than the length of six football fields in the same amount of time.

Certain spiders have a unique way of hiding: They look like bird droppings! Their appearance keeps them safe from predators. And their prey doesn't realize that the "droppings" are dangerous.

Bolas spiders are cowboy arachnids: They use a single silken line – dabbed at its end with a droplet of spider glue – as a kind of lasso. When an insect comes close, they swing the line around a few times and then throw it at their intended prey.

Net-casting spiders catch their meals by building a tiny web between their four front legs and throwing the web over passing prey.