Together with mites and ticks, the spiders (Araneae) are the most species-diverse and ecologically important order within the class Arachnida, which also contains orders like the harvestmen, scorpions, the tiny pseudoscorpions and a number of less species-rich groups, which for the most part are restricted to warmer climates.
One can easily see that arachnids belong to the arthropods, one of the most diverse groups of animals on the planet.In addition to arachnids with eight legs, the arthropods also include crustaceans (ten legs), insects (six legs) as well as the centi- and millipeds (18-750 legs).
True spiders have a body, which is partitioned into two parts; the front part, the prosoma bears four pairs of legs and another pair of shorter legs – the pedipalps. In front of the pedipalps one finds the downwards-directed chelicerae with a basal part and the poison claw. Most spiders bear eight eyes on the upper side and close to the front end of their prosoma. The the hind part, the opisthosoma, is attached to the prosoma by a short stem, making it quite mobile. This is important to any spider in order to put their spinnerets on the hind end of the opisthosoma into efficient use.
Spiders construct their webs according to a strict behavioral pattern, resulting in species-specific web structures. A given type of web is often characteristic for a certain spider family, recognizable in the family name, e.g. orb-web spiders (family Araneidae), sheet weavers (Linyphiidae). But not all types of prey-catching webs bear sticky threads as the orb webs of the Araneidae do.
In order to make a web, spiders are able to release silk threads “into the void“ and let them drift away in air currents. After the silk thread has become entangled on a physical structure the spider now has a bridge line as a starter for a new web.
Web sizes range from hardly a centimeter up to about 25 meters. The largest webs are made by Darwin‘s bark spider on Madagascar – their webs spanning whole rivers. There are also „social spiders“ with large numbers of spiders in large webs covering branches, brushes and even whole tree crowns. About half of the spider species do not make webs at all. They hunt their prey lying in ambush, running or leaping onto them.
The largest living spiders reach a body length of about 12 cm (the Goliath bird eater: Theraphosa blondi, Theraphosidae) and a leg span of up to 30 cm (the giant huntsman Heteropoda maxima, Sparassidae). The smallest (adult) spiders however, have a body length under a half millimeter.
Most species, even in the tropics, range between 0.5 and 5 cm. Their sometimes considerably long legs can sometimes make a spider appear much larger than it actually is.
You can find more record breaking achievements in a recent article published in PeerJ.
The World Spider Catalog lists the known spider species on a weekly to monthly updated basis. The version 18.5 listed 47,172 species in January 2018. Based on a number of methods, about twice that number are estimated to exist (Agnarsson et al. 2013 in Spider Research in the 21st century). With this number, spiders belong to the six or seven most species-rich animal orders.
According to the online determination website araneae - spiders of Europe, 1589 spider species occur in France, 1016 in Germany, 1023 in Austri, 1002 in Switzerland, 679 in the United Kingdom (as of 1/2018). The Red List of Spiders in Germany lists 992 established species. And even here, in a well-studied region compared to the tropics, more species are discovered almost every year.
Only few species have reduced poison glands, all others utilize poison in order to immobilize or kill their prey. This means that to their prey spiders are indeed deadly poisonous. The poison glands in their front part (prosoma) produce a “cocktail“ of numerous substances (mostly amino acids). As we know, neurotoxins are the most commen type of poison in the species studied so far, meaning poisons, which immobilize the prey very quickly. Other components dissolve tissue, cells or blood.
Naturally, humans do not belong to the prey spectrum of any spider species. Therefore, only the poison of about 40 species worldwide is life threatening to humans. None of these species lives in Central Europe.
Actually, even bites by other spider species are extremely rare. Most species – even if motivated – couldn‘t even puncture our skin with their tiny fangs in the first place. And other species do not inject any poison (or only very little) during a defensive bite, it would only be a waste of precious ressources to them.
In the very few European species able to bite humans, bites usually result in short reactions near the wound: burning sensation, redness, swelling – similar to insect stings. In some cases the patient experiences temporary numbness near the bite. For only two species (yellow sac spiders) in Central Europe more serious symptoms like more heavy pain or bouts of fever have been recorded.
A few species in warmer climates produce substances in their poison, which affect the human organism more severely and can in rare cases even be lethal: the black widows (genus Latrodectus), banana spiders (genus Phoneutria) and the Sydney funnel web spider (Atrax robustus).
There are cultures, in which fear of spiders is unknown, and others, where spiders are considered holy creatures. Some specialists are convinced, arachnophobia is a „primal fear“, triggered by certain events, e.g. when observing another human being scared by a spider. Being afraid of spiders as a possible danger is of no protective value in most parts of the earth.
Only few spider species utilize our homes as habitats, mostly species unable to live outdoors for extended periods in our seasonal climates. And during summer or fall, the occasional vagrant might erroneously find its way inside. For most species, our homes with their (for spiders) dry climates and scarcity of prey are unsuitable and spiders quickly die in heated rooms with their dry air. Cellars and sheds, however, present quite hospitable conditions for cave-dwelling species. On the other hand, sunny stone walls of houses or seminatural gardens are quite good habitats for a large number of species.
Everybody has heard of the male-killing “black widows“, but in most species partners part peacefully after mating. In some species, where the male is significantly smaller than the female, the male can, however, find itself being a good fodder after mating. It is true that male spiders in search of females do not hunt prey anymore and therefore have only little resources left and hence short life expectancy. Most often they die after mating anyways. Being eaten by the female after mating might therefore be a better contribution to their offspring because the female can utilize the energy from his body tissues to feed their mutual offspring.
After hatching from the egg, the yound spiderling already looks very much like a spider, only it is very tiny and transparent with its thin and not yet hardened skin. Having a rigid exosceleton made of chitin, spiders, like other arthropods in order to grow, must shed their skin (molting) several times during their life – small species about four times, larger species more often. Tarantulas are the only spiders, which can molt even after becoming adult; their females molt up to 20 times.
Most species complete their life cycle within a single year; some smaller species can even have up to three generations within that period. Larger species, on the other hand, sometimes need two year until becoming adult and only few species live longer than one year. Record holders are a tarantula, having lived 27 years in captivity and a certain trap door spider has been observed to have lived for that period in nature.
In higher latitudes like in Europe, spiders hibernate according to their life cycle, either well protected as eggs in their egg sacs or as juveniles or adults hidden in protected places.
Spiders are known to have existed for over 350 million years and can nowadays be found all over the world in many species. This evolutionary success is surely due in large part to their and right from beginning to their being able to produce silk.
And also from an ecological perspective, spiders are a very successful group of predators in all terrestrial ecosystems. Their preference for insects is of considerable benefit to mankind, taking into account their consumption of enormous amounts of prey; among which we find many insect species considered pests on agricultural land. It has been calculated that some biotopes harbor about one million spiders per hectare, consuming one billion insects (50 tons!) of insects each year.
In total this amounts to 400 to 800 million tons of prey each year, a huge number in comparison to the 400 million tons of annually consumed meat by humans. At least that is the number published by the two scientists Martin Nyffeler (University Basel) and Klaus Birkhofer (University Gießen) in the journal Science of Nature in early 2017.
Not all spiders can be found everywhere or are equally abundant in a certain habitat. Some species are widely distributed across a biogeographic reagion, others have such specific requirements that we find them only in certain environments (biotopes). Many types of biotopes have become rare in Central Europe, their area and quality reduced, e.g. mires, inland dune landscapes. Accordingly, species restricted to certain ecological niches and micro climates can be found only in a small number of locations. As a result the species composition in such locations, the spider community, is changes after changes in landscape and environment. Therefore spiders are suited for assessing the condition of biotopes. An intensively used piece of land, say for agriculture, is home to only 20–40 spider species, at a forest edge or in pristine forests it can be up to 200!
An essential book for any work on spiders is the book “Biology of Spiders“ by Rainer F. Foelix (2011, Oxford University Press).
Excellent review articles of the latest research on spiders in areas such as biodiversity, systematics, ecology and biogeography, genetics, behavior, spider silk and paleontology can be found in the recently (2013) published book “Spider Research in the 21st Century – trends & perspectives“ (Siri Scientific Press, Manchester, UK).
A number of recommended study books dealing with certain aspects and topics:
Spiders in ecological webs von David H. Wise (1993), Cambridge University Press.
Ecophysiology of Spiders edited by Wolfgang Nentwig (2013), Springer Verlag, Berlin.
Spiders of Britain & Northern Europe by Roberts, M. J. 1996. Collins Field Guide. Harper Collins Publishers, London.
Interested readers find scientifically sound and peer-reviewed articles in our journal Arachnologische Mitteilungen - Arachnology Letters (in English and German).