Spiders are the most numerous order (O. Araneae) of Arachnida class, Artropoda phyllum with more than 39 000 species that have been described until today. They inhabit almost all ecosystems – they have been recorded on the Arctic and Antarctic, in deserts, steppes, caves, lakes and even at the altitude of 7 000 m. They are primarily ground dwelling animals although there are also those that have moved into water during the secondary process.

We distinguish 2 regions/tagmata on a spider's body – head-thorax (cephalothorax or prosoma) and stomach (abdomen, opisthosoma) regions that are connected with a thin waist called the pedicle. Body surface is covered with chitin layer which represents and egzo skeleton.

The eyes, mouthparts and 6 pairs of extremities are located on prosoma. Spiders usually have 8, less frequently 6 eyes, while cave species do not have them at all. The first pair of extremities are chelicerae. They are robust and can be found in front of the mouth. Spiders use them to catch and kill the prey.

The second parts of extremities are padipalps, which are placed laterally from the mouth. In females they are very similar to extremities used for walking, while in males their structure is complex with the role in copulation and in taxonomical identification.

Four pairs of legs are placed laterally from the head-thorax region. Spiders have different groups of specialised hairs and spines on them and the longest among them are the sensory hairs (trichobothrium) providing the spiders the capability to feel vibrations of the web or the ground. In addition to tactile sense, spiders also have the scent sense organs, which resemble to crack openings on the legs or some other parts of their body. Males are sensitive in particular to a female scent. The taste sensory organ is placed in the pharynx and it helps them to select food, which means that if there is something on the menu that they do not like they shall not eat it. Some spiders have also got the sound producing organs and even a man can hear sounding of the big bird spider.

Spiders' abdomen is oval in most cases, without traces of segmentation. It is mainly soft, diverse in shape, size and colour and sometimes with very specific spines. The structure called epigastric fold is placed on the ventral side of their abdomen. In the centre of that fold there is the genital opening while respiratory openings (branchial operculum) of leaf like lungs or tracheas are placed laterally from it.

Spiders' are of divided sexes, very often with highly emphasised sexual dimorphism. All species show typical and specific behaviour during mating. Smaller males usually wait for a female to fall into a kind of cataleptic state so that they can fulfil their function and run away on time to avoid being eaten by a female.

Families of Lycosidae and Salticidae are known for their wedding dances. Males wave with their palps and legs in a certain rhythm, moving constantly to the side, or in the forward-backward direction. The males of Pisaura mirabilis species bring a »wedding gift» to their chosen one, a fly wrapped in the web, while the males of Xisticus gulosus species spin their web around the head and front legs of their brides.

Spiders lay eggs – some species a lot of them while others lay only some. Eggs are large, rich with yellow substance and they are in cocoons, silk bundles, the shape and consistence of which are specific for certain species. Some females take their egg sacs with them until the youngs come out, others keep them in the vicinity of the hunting webs. There are also those that take no care of them at all.

The youngs grow up without metamorphosis and they moult several times. They stay in their cocoons until the first moulting while the total number of moults depends on the species and the length of their lives. Spiders living for a season usually moult 5 to 10 times. The secondary sexual characteristics appear only after the last moulting and this is when sexes can be distinguished.

Spiders are best known after their web spinning capability. The spines with openings of spinnerets or web silk producing glands can be found on the ventral side of abdomen, closer to the posterior part of the body. Out of them, using their rear legs, the spiders extrude the web silk and make the most diverse structures. They make the hunting webs, dwelling webs, different shelters and cocoons for their eggs. There is no universal type of a web that would be typical for all species – the simplest are the webs of a non-uniform shape with threads leading to all sides while the most perfect are those round and vertical ones, such as those made by cross spider or wasp spider.

Spiders are predators and they feed on other organisms. Their hunting techniques differ – the wanderers attack their victims in a run or a jump while dwellers spin the hunting webs and signalling threads around their houses. Most spiders eat whatever they can catch. However, there are those that are specialised for certain types of food. Some representatives of Salticidae family hunt exclusively ants and they even remind us after them with their appearance. The largest spider on Earth, the Giant Tarantula (Theraphlosa blondi) is strong enough to feed on frogs, lizards, mice and even smaller rattle snakes while its relative, Tarantula Avicularia avicularia builds its nest between the banana tree leaves stalking in an ambush for smaller birds. Argiroweba aquatica, which lives in water, hunts smaller fish and tadpoles.

Spiders kill their victims with chelicerae and venom from the venom producing glands that are placed in the anterior part of prosoma. The venom acts either on the nerves or on both the nerves and muscles. The venom made spiders dangerous and unpopular animals. Its quantity is very small but it can still be dangerous even for humans with some species.

Poisoning with spiders' venom is called araneism. There is a larger number of poisonous species in Europe. More attention is paid to studying of the Black Widow's, i.e. Latrodectus mactans venom because the epidemics of its bite occur in the Mediterranean from time to time during the summer months. Lactrodectism is the name of poisoning with venom of this spider. Very often one can find the quotations in bibliography saying that its venom «drop by drop is 14 times stronger than the rattle snake's venom».

Contemporary trends in spiders' studies are not directed only towards the faunistic and toxicological researches. The materials are being improved that could be similar to the spiders' web because it is firm and elastic at the same time and still resistant to water, bacteria and germs. The genetic researches are also conducted that should provide answers to the questions related to the reason because of which some spiders make webs while others do not or why some live for just one season while tarantulas can live up to 25 years. There is also an interesting possibility of using spiders as bio-pesticides in greenhouse production of organic food.