Spiders ( Araneae )
|Clerck , 1757|
The spiders (Araneae) are the best known order of the arachnids (Arachnida), a class of the arthropods (Arthropoda). They are further divided into limb spiders (Mesothelae), tarantulas (Mygalomorphae) and real spiders (Araneomorphae). After the mites (Acari), the spiders form the most species-rich order of the arachnids. In November 2017, the World Spider Catalog listed 112 families with 4076 genera and 46,986 species for the order of the spiders .
Construction of the weaving spiders
Like all arachnids, spiders have eight legs - in contrast to insects (Insecta), which only have six legs. The head and chest are fused into one piece, the so-called prosoma (cephalothorax). This is followed by the undivided, pedicled and usually much larger abdomen, the opisthosoma .
The front limbs of the head, the large jaw claws ( chelicerae ), are used to grab the prey. They end with a claw that can be folded in like the blade of a pocket knife . The duct of a poison gland ends at its tip . The poison flows into the wound struck by the claw and kills or stuns smaller prey almost instantly. The jaw probes ( pedipalps ) follow as the second head extremity . These are multi-section buttons in the shape of a shortened leg; in the male sometimes with a distended end member serving as a mating organ.
This is followed by four pairs of walking legs on the prosoma. The legs consist of six or seven limbs each and, together with the trichobothria (cup hairs), are an important sense organ. The pairs of legs, which are different in length and shape in the individual genera , end in the net-building Trionycha with two comb-like toothed claws (tarsal claws) and a central claw on the tarsus, through which the thread is usually passed with the rear legs. "Modern" running spiders ( Dionycha ) lack this central claw. In their place, for example, with jumping spiders, tufts of scopula hairs are used which, thanks to adhesive forces, give the legs a secure hold for a catapult-like jump even on vertical glass panes.
The abdomen is connected to the prosoma by a thin stalk (petiolus, formed from the first abdominal segment). He has no legs. The genital opening is on the ventral side of the opisthosoma, and to the side of this are the two stigmas (stomata) of the so-called lung sacs, and often a second pair of stigmas as well. The anus is surrounded by four or six spinnerets at the end of the abdomen , from which the secretions of the spinal glands emerge. The latter are pear-shaped, cylindrical or lobed tubes. Its protein-containing secretion gets to the outside through hundreds of fine tubes; in the air it quickly hardens into a spider thread or many. With the foot claws, if present, the calamistrum, a bristle comb and sometimes with the spinning bobbins, these are woven or glued into a net or web.
The nervous system consists of the brain and a chest ganglia mass.
Behind the forehead there are eight (less often six) point eyes in an arrangement according to the genera and species . The major vitreous (the pair of anterior median eyes , VMA) are located on the top of the prosoma. The retina of the anterior median eyes can be moved laterally by one to six muscles so that the field of vision is expanded. There is no accommodation . The VMA are built relatively uniformly. As with the funnel spiders , for example, they have only a few to many visual cells in jumping spiders and crab spiders . The six-eyed spiders (Dysderidae, Sicariidae, Oonopidae) lack the VMA.
The three pairs of smaller secondary eyes - anterior lateral eyes (VSA), posterior lateral eyes (HSA) and posterior median eyes (HMA) - are, like in mammals, inverse eyes that have a tapetum and therefore reflect the light. The structure of the eyes varies between families. Jumping spiders and lynx spiders have no tapetum. The anatomy of the secondary eyes was therefore also used by Homann for systematic differentiation.
Due to the arrangement of their eyes, spiders have a wide field of vision. Even so, only a few families, such as jumping spiders, are able to see shapes. They can likely see colored, and ultraviolet receptors are present, as in other arthropods.
The lyriform organ for recognizing vibrations belongs to the sense organs . Lyriform organs are found mainly on the spider legs. They consist of several parallel crevices of different lengths in the exocuticle and are usually placed near the joint. Each gap ends in a cylindrical recess, the coupling cylinder, through which a sensory cell is coupled by means of a dendrite . Spiders can perceive web and ground vibrations through their lyriform organs, and they serve as proprioceptors and thus orientation in at least some species .
Although sound is perceived with the lyra-shaped organs , the cup hairs (trichobothria) are more important for acoustic perception . The frequency optimum is 300 to 700 Hz, with air vibrations between 100 and 2500 Hz. The vibrations or sound forces narrow the gaps and thus trigger a stimulus on the tip of the dendrite. Sound waves set the beaker hairs in vibration, which are then transmitted to the dendritic endings. The movements of the hair are perceived by sensory cells that respond to three different directions of deflection. Spiders use vibrations as signals when choosing a partner, to communicate between parents and offspring, to communicate with group members and in defense against enemies.
Digestion, breathing and circulation
The intestinal canal runs relatively straight from the mouth to the anus . It is divided into the esophagus , the stomach with five pairs of blind tubes and the intestine. The liver ducts and two ramified urinary canals open into the intestine. The liver juice acts similarly to that of the pancreas of the higher vertebrates .
The respiratory organs are usually peculiar so-called fan trachea or book lungs , also called lung sacs. But there are also common tracheas , into which the air enters through air holes (spiracles).
The heart is a pulsating back vessel ( artery ) located in the abdomen . It has lateral stomata (ostia) that allow the blood to enter. When the heart contracts, the blood is forced into the artery while the lateral stomata close like valves. The blood flows through arteries to the limbs and head, flows back around the sacs of the lungs and passes through the lateral stomata into the dorsal artery. So it is an open blood circuit .
Reproduction and development
The males are often recognizable by external features and are usually smaller than the females. They have a smaller abdomen than the females and have paired testicular tubes , but usually no penile structure , so that sometimes distant limbs such as the jaw palpation take over the transfer of sperm to the female as secondary reproductive organs . The thickened end link of the jaw probe is hollowed out in the shape of a spoon and contains a spirally bent thread along with protruding attachments. During copulation , the male fills this member with sperm and introduces it into the female genital opening, where there is a special container for storing the sperm (the seed pouch). The females have unpaired or paired ovaries , the fallopian tubes of which usually open out together at the beginning of the abdomen.
In the case of spiders, the behavior of the male animal is important for successful fertilization: If the male does not adhere to the species-specific ritual, the female cannot recognize it as a sexual partner and often becomes a victim of the same ( cannibalism ). All spiders lay eggs . The development in the egg is interesting insofar as the embryo has an abdomen clearly consisting of ten to twelve segments for a while, on which the appendages of limbs can also be seen, which, however, disappear again along with the structure. The females often carry the young around with them in an egg cocoon until they hatch. The hatching young do not experience any metamorphosis ; so they usually already have the shape of the adult animals, but remain in the web of the egg cases until after the first molt.
Young spiders of different species produce long threads in autumn ( see Indian summer ), by means of which they rise high into the air to be carried to other places.
Sexually mature animals also molt in certain periods of time depending on the food available. The females of some species are able to reproduce for several years. Some can exist for months without food. Your ability to regenerate is enormous. Lost limbs can be replaced at early moults (no longer at maturity moults).
Growth and molt
As with all arthropods , body growth through the solid exoskeleton (outer skeleton) is only possible to a very limited extent. In spiders, the soft-skinned abdomen can expand. The extremities and prosoma, however, are surrounded by a hard exocuticle so that they can only grow by moulting . The number of moults required to reach maturity depends on the size of the spiders; smaller species only need five moults, larger ones up to ten. Some tarantulas shed their moult even after sexual maturity. In the early nymph stages, the time between moults is very short. The frequency of moulting decreases with age and depends on the food supply (speed of growth). Tegenaria species, for example, double their weight with each molt.
The molt is announced by the spider's retreat into its hiding place and refusal to eat. The legs become darker and the receding abdomen reveals the petiolus . The new cuticle is already folded under the old one. Hair and bristles can be completely or partially renewed. Most web-building spiders let their hind legs hang down by the moulting thread to moult . Tarantulas lie on their backs.
In the first phase, the back shield folds down like a lid. The heartbeat accelerates and hemolymph is pumped into the front body by the heart, but also by contractions of the dorso-ventral abdominal muscles, until its weight has doubled and the pressure has risen to 200 mbar (150 mmHg). Although the cuticle withstands five times this pressure, the exoskeleton has already been weakened by the breakdown from within. The abdomen shrinks considerably with the old shell and looks wrinkled. Finally, the old shell of the front body bursts at the side edges.
Then the abdomen is freed. The cracks in the flaked back armor also reach the abdomen. The abdomen contracts in a wave-like manner and is pulled out. Shortly beforehand, the spinnerets attach another thread to the old shell so that the spider can abseil out of the old skin.
At the same time as the abseil out of the old skin, the extremities are pulled out. This is the hardest part of the process, and this is where the most serious complications can come. If the spider cannot pull its legs out of the exuvia , it dies. It is useful that the new cuticle has not yet hardened and thus slides more easily through narrow areas such as the tibia (especially important for male bulbs , the palpentarsi). Rising hemolymph pressure can also cause swelling. Downward-facing bristles prevent the legs from sliding back. Legs that have already been skinned form an abutment to pull the remaining legs out of the old shell with great effort. The order can be species-specific.
Finally, a fresh spider hangs limply on the old skin. She then begins gymnastics to keep the joints flexible during the curing process. The moulting process can take from ten minutes for small spiders to several hours for tarantulas. After the gymnastics there is often a body cleansing in which the legs and palps are pulled through the chelicerae and the mouth opening and rubbed together.
Way of life of the web spiders
Spiders usually live predatory and feed mainly on captured arthropods , especially insects that they suck. For this purpose, the prey animals are first dissolved with an enzyme-containing digestive juice, which the spider brings into its victim, which has been killed with its jaw claws ( see extraintestinal digestion ). Many species build nets to catch their prey.
In addition to active hunting, there are exceptions in diet. Almost all spiders eat freshly killed animals and are therefore also scavengers . There is also evidence that a not inconsiderable part of the diet of spiders consists of pollen, which is ingested with old web material. A special case is the jumping spider Bagheera kiplingi , which feeds almost exclusively on the Belt body of some acacias ; These are special food corpuscles directly on the leaflets, which these acacias form for the ants in particular as attractants and food. The webs of the weaving spiders differ in appearance depending on the genus and species. The animals often stay in tube- or funnel-like hiding places near the nets. There are also many species that stray and overwhelm their prey while running or jumping.
Almost all spiders are land animals. They are distributed all over the world, but most and largest species are found in the hotter zones. The only species of spider that lives in water is the water spider ( Argyroneta aquatica ). However, there are a number of species that hunt on the surface of the water.
Birds in particular play an important role among vertebrates as spiders' predators . Amphibians, reptiles (geckos, iguanas, salamanders, etc.) and bats prey on spiders less often. Other important predators are other spiders, e.g. B. the Mimetidae , which feed exclusively on spiders. Some insects, such as some tropical dragonflies and various ants, eat spiders. All wasps (Pompilidae) and some grave wasps catch spiders for their offspring. They capture a spider, which they stun with their venomous sting, and then bring it into their nest, where the wasp larva eats the spider. Some parasitic wasps lay their eggs in living, often previously anesthetized, spider bodies, and the developing larvae then live as parasitoids in these hosts. The ball flies also develop into spiders in this way. There are also various parasites such as roundworms and mites .
Systematics and evolution of the weaving spiders
See main article: Systematics of weaving spiders
The approximately 47,000 species worldwide are divided into around 4,000 genera in 112 families (as of November 2017). In Central Europe there are 43 families of real spiders and one family of tarantulas , namely three types of wallpaper spiders (Atypidae).
|Central Europe as a whole||43||1313|
The systematic classification is based, among other things, on the shape and size of the spinneret glands, the arrangement of the eyes, the structure of the chelicerae and pedipalps and the presence of a cribellum ; but more and more recently based on genetic analyzes. In this way there are three groups classified as subordinates:
The limb spiders are the only recent weaving spiders to show clear segmentation , a primeval (plesiomorphic) feature that is also common in other chelicerates. The external segmentation of the link spiders is also reflected in the internal organization. The abdominal muscles connect the segments between the sternites and tergites and the dorso-ventral muscles run from tergite to sternite. The heart is also segmented. The articulated spider Liphistius has four pairs of active spinnerets, which are formed from segments ten and eleven (two pairs per segment) on the ventral side (ventral side) from extremities; in all other spiders one or two pairs of spinnerets have been reduced secondarily. The fourth, inner pair on the tenth segment has no function in Liphistius either .
The segmentation can only be recognized to some extent in the tarantula-like and in the real spiders, for example in the sternites, relief and patterning of the abdomen. In these suborders the segments supporting the six extremities fused to form the prosoma (front body); the seventh became the Petiolus. The sac-shaped opisthosoma (abdomen) of the spider formed from the remaining segments, as can be seen from their embryonic development. The second segment carries the epigastric groove (genital opening). The breathing openings to the book lung are located between the second and third sternite .
Due to a strong stretching of the third sternite and a shortening of the tergites 13 to 17, the spinnerets migrated backwards, where they are now located directly under the cloaca in the mygalomorphs and araneomorphs. Between the third and fourth sternite, directly in front of the spinnerets, in addition to the outwardly migrated respiratory opening of the book lung between the second and third abdominal sternite, there is another respiratory opening to the tubular tracheal system . The genital opening also moved back to a more favorable place on the abdomen. Both suborders have an endoskeleton to which the suction stomach muscles attach.
The Mygalomorphae usually have three pairs of spinnerets. In members of the wallpaper spider (Atypidae), three pairs of spinnerets are only active in the juvenile stage; in adult animals, the third pair remains passive. In other tarantula species, only the two pairs of the eleventh segment are left. The real spiders usually have three pairs of spinnerets. The fourth pair partially transforms itself into further organs. In the cribellate spiders , this pair formed into a cribellum , on which the spinning bobbins sit folded when at rest. Contrary to the traditional view, it is generally assumed today that the Cribellum is a plesiomorphic stem characteristic and was often lost independently of one another. The homologous organ in some Ecribellates is the colulus (a hill with an unclear function, probably without function); in other Ecribellata this fourth pair is completely absent.
Furthermore, the tarantulas differ in the position of the jaw claws; Because of this striking distinction, this used to give the subordination its name. With the real spiders, they work against each other and can also be used as a multifunctional tool. In contrast, the more powerful chelicerae of the tarantula fold like a pocket knife onto the base of the jaw.
Araneae's oldest fossils come from the Carboniferous . Like the recent mesothelae, they have clearly recognizable opisthosomal segmentation. Devonian , spider-like fossils, described as Attercopus fimbriunguis , are no longer regarded as belonging to the actual weaving spiders, they are now placed in their own order Uraraneida , which could be a root or sister group to the actual weaving spiders. The Uraraneida already had spinning power, but no spinnerets. Just like today's mesothelae, they did not have a poison gland on their chelicera claws. Fossils that can be safely assigned to the more modern suborders Mygalomorphae and Araneomorphae date from the Mesozoic era. The radiation of the spiders seems to follow that of the insects, their most important prey group, with a certain delay. The spider fossils of the Triassic , e.g. B. from South Africa and North America (Virginia) are predominantly assigned to modern families. At the end of the Cretaceous almost all modern families have been found to be fossilized, recent forms show hardly any changes in comparison to them. Most of the most significant fossil spiders exist as inclusions in amber . In addition to the well-known Baltic amber, older deposits are significant. For example, the Linyphiidae family , which has so far only been documented in fossil form, can be dated back to the Cretaceous through finds in Lebanese amber. According to molecular biological analyzes , the origins of the most species-rich family, the jumping spider , can also be dated in the Cretaceous, even if there are no fossils here.
Spiders and humans
Spiders as disgusting animals, delicacies or deities
In the societies of the western industrialized countries there is an irrational aversion to this group of animals, which can go as far as pathological arachnophobia , although there, with the exception of Australia, hardly any human pathogenic spiders occur. Fewer than a dozen of the 40,000 or so species are dangerous to humans, and this “dangerousness” only rarely has serious consequences. On the other hand, in other societies in whose environment dangerous spiders are more common, they are tolerated, eaten as a delicacy or even worshiped as a god.
In Cambodia, tarantulas are collected in some areas and fried after removing their jaws. Prepared this way, they are considered a delicacy and snack on the roadside or in markets. In many cultures in Asia, spiders are tolerated near humans as they are useful insect killers. In West Africa the spider Anansi is worshiped as a high deity . Anansi is considered the originator of knowledge and wisdom, inventor of agriculture, god of rain and weather.
In Japan there is the legend of the giant spider Tsuchigumo , in Nauru that of the giant spider Areop-Enap .
A distinction must be made between neural toxins ( neurotoxins ) and necrotic, i.e. cell-destroying toxins. Most of the poisons of the spiders are not used for killing, but for stunning in order to preserve the prey alive as a reserve. Only consumption or a killing bite kills the prey.
The poison of the Loxosceles species native to America and Africa also has a hemolytic effect . The brown recluse spider ( Loxosceles reclusa ) native to the southern United States is known for the characteristic death of the skin around the bite site, which in rare cases can lead to life-threatening sepsis (but only after a few days). The Chilean angle spider ( Loxosceles laeta ), a very common relative in South America, especially Chile , is even more dangerous and occurs mainly in human living rooms and bedrooms. Their bites cause similar symptoms and result in death time and again.
Only a few spiders native to Central Europe are even able to penetrate human skin due to their size. The dose of poison injected through the opening of the chelicerae is minimal, although the poisons are highly potent. Overall, the probability of being bitten by a spider is negligibly small, because spiders first either flee or pretend to be dead ( paralysis ).
The only spider species native to German-speaking countries, although only found regionally here, whose bite has a certain medical relevance, is the wet nurse's thorn finger ( Cheiracanthium punctorium ). Its bite is quite painful in the first place. In rare cases, nausea, headache and vomiting have been reported, and more rarely fever and chills. The symptoms subside after about three days. The bite site can be swollen or reddened for a long time. The field-angle spider ( Tegenaria agrestis ), which is feared in America and introduced there, has slight local symptoms such as numbness, which disappear after a short time - only in the case of an allergy to the poison can a bite be accompanied by further complications. The field-angle spider is also widespread in Central Europe, where its habitat, however, is predominantly limited to the free and less frequented nature; It is rarely found inside houses, not least because there it would be in direct competition with the other species of the genus Tegenaria, which are again missing in North America, but frequent and relatively larger in Central Europe, but all harmless to humans ( see above about T. atrica , the giant house spider ). Bite accidents associated with these spiders are accordingly unknown in Europe.
Large specimens of the garden spider ( Araneus diadematus ) at least lead to an inferior, sometimes painful pinching, which is only used in the greatest need and is very rare. The poison of the water spider ( Argyroneta aquatica ) has a similar effect , but due to its way of life it will hardly come into contact with humans. A European representative of the black widow is the Latrodectus tredecimguttatus found in the Mediterranean countries . The spider, which is also misleadingly and incorrectly called “Malmignatte”, used to build its web under toilet lids of simple latrines due to the high number of insects that arose there; Occasionally poisonous bites occurred when using the latrine.
Some representatives of the genus real widows ( Latrodectus ) such as the black widow ( L. mactans ) native to North America or the Australian red-backed spider ( L. hasselti ) have strong poisons, so that their bites are life-threatening for sick people, for example those with allergies can. At first there is no trace of the actual bite of a black widow, so the bite often goes unnoticed.
Even bites from the Sydney funnel- web spider ( Atrax robustus ), which is native to the region around Sydney in Australia, are usually mild. Since this species is a pronounced cultural follower , encounters with it are frequent. Even so, only 13 fatalities have been counted since 1927.
In the past, when tropical fruits were imported, there were occasional bites by comb spiders ( Phoneutria spp. ). After hardly any import events could be documented in the 1980s and 1990s, these increased again at the beginning of the 21st century. For example, Phoneutria boliviensis, which was brought in with a banana box , was only discovered in Hesse in 2009 .
Contrary to popular belief, the commonly feared tarantulas do not pose any mortal danger. Only a few genera such as B. Poecilotheria spp. are able to trigger poisoning with chills, paralysis and convulsions, which is more severe for healthy people.
Allergy sufferers also have to be careful of more harmless spiders, whose bite effect is often compared to a bee sting, because even the weakest venom can in rare cases trigger an allergy with the risk of anaphylactic shock . The pain caused by a dry bite alone should not be underestimated, however, as the biting tools of large species such as Theraphosa blondi can reach a length of up to 2.5 cm. The species of the New World subfamilies Theraphosinae and Aviculariinae pose a greater danger . These have very fine stinging hairs , which they can usually break off with their last pair of legs in quick jerky movements over their abdomen so that they fly towards a troublemaker with the movement of the air. However, there are six different types of these hairs and most of the species of the genus Avicularia have particularly thick stinging hairs (exception Avicularia versicolor (Bertani et al. 2003)), which do not spread through the air so easily, so that the spider has its abdomen Troublemaker reaches out with the aim of providing him with stinging hairs. Another exception is the genus Ephebopus , which has stinging hairs on the palps instead of on the abdomen. With a length of 0.1 to 1 mm (Marshall & Uetz 1990), these hairs are very fine and have barbs that can dig into soft tissue like a harpoon. They cause purely mechanical irritation, so that the affected areas are more or less reddened, swell and very itchy. Eyes and mucous membranes are particularly at risk. If the stinging hair gets into the windpipe, it can cause breathing problems.
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- ↑ a b Currently valid spider genera and species in the World Spider Catalog, Version 18.5. Natural History Museum Bern. Retrieved November 6, 2017
- ↑ Randall, JA (2014). Vibrational Communication: Spiders to Kangaroo Rats. In: Witzany, G. (ed). Biocommunication of Animals. Springer, Dordrecht, pp. 103-133. ISBN 978-94-007-7413-1 .
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- ↑ An overview in: Paul A. Selden & DavidPenney (2010): Fossil spiders. Biological Revues 85: 171-206. doi: 10.1111 / j.1469-185X.2009.00099.x
- ↑ Paul A. Seldena, William A. Shear, Mark D. Sutton (2008): Fossil evidence for the origin of spider spinnerets, and a proposed arachnid order. Proceedings of the National Academy of Science USA, Vol. 105 No. 52: 20781-20785. doi: 10.1073 / pnas.0809174106
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- ^ David Penney & Paul A. Selden (2002): The oldest linyphiid spider in lower cretaceous lebanese amber. Journal of Arachnology 30: pp. 487-493. doi : 10.1636 / 0161-8202 (2002) 030 [0487: TOLSIL] 2.0.CO; 2
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