arthropods have the largest number of species . along with ticks , mites and scorpions , spiders fall into the subphylum chelicerata and class of arachnids ( 1 , 2 ) . the arachnid class has very diverse members in a way that more than 80,000 species belonging to that class have been identified to date . spiders are a large group of arachnids belonging to the araneae order ( 1 ) . more than 40,000 species of spider have been identified to date but the real number is estimated at 4 times more than that number ( 3 ) . except for two small groups in the arachnid family , all spiders have poison glands and release their secretions into their venom sacs near their chelicerae . however , the majority of spiders do not bite humans and except for a few cases , they are not harmful to the human being or other mammals ( 2).spider bites are common but the majority of species create little clinical presentations ( 3 ) . there is only one aggressive spider named funnel - web in australia , which attacks the human being without provocation . these large spiders are quite aggressive and their venom is pharmacologically highly toxic . their bite results in severe pain , neurotoxic effects , diaphoresis , severe allergic reaction and priapism ( 1 ) . the severity of reactions to spider venom depends on factors such as its amount , site of biting and its duration and age and health condition . spider venom includes different peptides and substances affecting sodium , calcium and potassium channels in neurons and also glutamate and acetylcholine receptors ( 4).spider bite victims develop symptoms such as pain and swelling in the site of biting , necrosis , pyrexia , pulmonary edema , respiratory distress , hypertension , kidney dysfunction and death . treatment protocol in the case of critically ill patients includes supportive measures and antivenom injection . respiratory support and monitoring the hemodynamic status of these patients highly venomous spiders , funnel - web in australia and armadeiras ( armed spiders ) in south america require antivenom and intensive therapeutic interventions ( 6 ) . three important syndromes are caused by spider bites : latrodectism , loxoscelism , and funnel - web spider syndrome ( 7 ) . here introduction of venomous spiders 
  this section describes different venomous spiders according to their appearance , symptoms after their bites and available treatments . therefore , we collected related articles from pubmed and google scholar ( figure 1 ) . a ) the brown recluse spider 
  the brown recluse spider is recognized by the violin - shaped marking on its back ( figure 2 ) . this spider takes rest during the day and is not aggressive but it will attack in the case of provocation . patients are usually bitten by wearing clothes and shoes with spiders in them ( 2 ) . this spider 's bite is usually painless but it later becomes an inflammatory , hemorrhagic and painful lesion ( 8) . necrosis spreads a few days following the bite and loxoscelism syndrome results in dermatitis necrosis in the site of biting , around which becomes red , white and blue , respectively ( 9 ) . the venom of this spider contains hyaluronidase and sphingomyelinase d enzymes and results in necrosis . local manifestations of the bite of this spider include edema , inflammation , hemorrhage , damage to the vessel wall , thrombosis , and necrosis ( 10 ) but systematic symptoms including acute renal failure , rhabdomyolysis and intravascular hemolysis have also been reported ( 11 ) . in some cases , severe coagulopathy can result in stroke ( 7).considering extensive differential diagnosis for skin necrosis , the standard criteria for the diagnosis of loxoscelism syndrome is capturing the spider during biting or capturing it in the place where biting occurred and its confirmation by a reliable arachnologist(12 ) . dapsone , antihistamines , colchicine , corticosteroids and hyperbaric oxygen have been used for treatment . it has been proven useful during the first 4 hours after the bite but according to an investigation , it was useful even after 12 hours ( 14 ) . the bite - induced necrosis spreads in a few days and completes in a few weeks . treatments include initial debridement and in later stages , after improvement of the inflammation , graft is used in case of severity ( 9 ) . hobo spider is a brown - colored spider with gray marks along its body ( figure 3 ) . symptoms induced by its bite are similar to those of the brown recluse spider but necrosis is rare and permanent scar is probable on the site of biting ( 12 ) . the cause of necrosis is the hemolytic property of the venom or transmission of pathogenic bacteria inside the site of biting ( 15 ) . the systemic symptoms of biting are reportedly cephalalgia and in some , rare cases anaphylaxis and death . moreover , resection should not be conducted before the completion of the necrosis process ( 8) . its major characteristic is a red marking on its abdomen similar to an hourglass ( figure 4 ) . it is not aggressive under normal circumstances but attacks if disturbed , especially while protecting its egg sacs . moreover , it is the most important venomous spider in north america and australia ( 2 , 3 ) . its venom is alpha - latrotoxin ( neurotoxic venom ) which results in the exocytosis of synaptic vesicles from parasympathetic terminals due to the stimulation of calcium - dependent mechanisms , releasing catecholamines and acetylcholine ( 8) . a lesion similar to the target lesion can be observed in the site of biting ( 16).latrodectism starts in a few minutes with the development of pain through the whole body and symptoms such as emesis , respiratory failure , delirium , partial paralysis of limbs , abdominal muscle cramps , hypertension , pyrexia , fasciculation and muscle spasm are developed within a few hours ( 17 ) . mortality following biting is less than 1 % and the risk of death following biting is high in two age spectrums the chelicerae of this spider rarely leave a mark . following the bite , erythema , diaphoresis and piloerection are observed around the site of bite in 25 % of the cases ( 3 ) . while it can be difficult in children , hypertension , distress , diaphoresis and irritability can suggest diagnosis in these cases ( 18 ) . photograph by richard s.vetter.reproduced with permission from : australianmuseum.net.au eratigena - agrestis ( hobo ) spider . reproduced with permission from : www.spiders.us black widow spider ( photograph by eric r. eaton . reproduced with permission from bugeric.blogspot.com phoneutria , commonly known as brazilian wandering spider or armed spider . image source : wikipedia.org funnel web spider ( reproduced with permission from australianmuseum.net.au tarantula spider ( mexican red legged tarantula ) photograph byricbolzan . reproduced with permission fromaustralianmuseum.net.au the treatment of these patients consists of using muscle relaxants , narcotics , analgesics , intravenous calcium and antivenom . antivenom treatment is recommended for children , pregnant women the elderly and also patients with severe local symptoms , severe pains necessitating repeated administration of narcotics and systematic symptoms ( 3 ) . in the case of iv administration however , some centers have recommended i m administration in order to reduce its complications . in the case of i m administration , the effect is delayed and symptoms take longer to improve ( 3 ) ( within 1 - 5 days ) . some may suffer from chronic pains even after proper antivenom treatment ( 18 ) . since they often hide in banana boxes , they are known as banana bunch spiders by locals ( figure 5).the symptom of its bite is severe pain which is observed in 96 % of patients ( 18 ) . the bite of this spider can cause respiratory failure and death at both ends of the age spectrum ( 8) . its venom is neurotoxic thus stimulates the autonomic nervous system ( tachycardia , hypertension , diaphoresis and salivation ) , priapism , dizziness and visual disturbances ( 19 ) . this venom is recommended to be used for controlling permanent pathological pains due to its different compounds and its effect on pain neurons ( 20 ) . there is antivenom for the treatment of these patients but it is used in few cases ( 21 ) . with regard to the probability of respiratory failure in these patients , it is contradictory to use narcotics for pain control . therefore , local nerve block anesthesia is recommended ( 8) . e ) funnel - web spider 
  funnel - web spider , the most dangerous spider in the world , is aggressive in the absence of provocation ( figure 6 ) ( 3 ) . the structure of this spider s web is funnel - shaped , hence the name ( 8).the venom of this spider is neurotoxin and contains a large amount of peptides ( 22 , 23 ) . delta atracotoxin , one of the peptides in the venom , delay the activation of voltage - dependent sodium channels resulting in repeated stimulation and release of massive neurotransmitters in nerve endings ( 24 ) . robustoxin is another toxin in the venom of this spider which is a fatal peptide and can be used for immunization ( 25 ) . systemic symptoms include stimulation of the parasympathetic system ( nausea , emesis , salivation , sialorrhea and tearing ) , cardiovascular system ( hypertension , mostly tachycardia and in some rare cases bradycardia and hypotension ) , nervous system ( fasciculation and perioral paresthesia ) , non - cardiac pulmonary edema ( more prevalent in children ) , agitation , and cephalalgia(3 ) . treatment includes supportive measures , elastic bandage for blocking lymph flow , limb immobilization and rapid transportation to the hospital . antivenom is the definite cure and should be administered up to 15 minutes after opening the bandage . in the case of uncertainty about the bite and exhibition of the first systemic symptoms , 2 vials of antivenom are administered and in the case of severe symptoms 4 vials are administered . other therapeutic measures include hemodynamic support , ventilatory support and the administration of tetanus vaccine . these measures reduce the risk of neurological complications and mortality and improve the performance of patients after being discharged ( 7 ) . tarantula is recognized by its hairy 3-inch brown or black colored body ( figure 7 ) . its venom is not dangerous for the human being and merely creates lesions without any specific systemic reaction except for pyrexia . tarantula s defense mechanism is the hair on its body which stands out and moves when alarmed ( 2 ) . if these hairs enter the eye , they can result in the inflammation of all of the layers of the eye . eye wash and topical corticosteroids are recommended in the case of uveitis ( 8) . recommendations for future 
  the human being has always been frightened of spiders but few of them are venomous and thus real threat to human health . however , since venomous spiders are sometimes fatal , bites should be taken care of . moreover , it is recommended to be adequately familiar with necessary treatments . with regard to the identification of venomous spiders in our country including widow spider , which exists in the majority of provinces , healthcare personnel must be familiar with the symptoms of the bites of venomous spiders and it is essential to prepare antivenoms in the country for the treatment of spider bites . finally , further domestic investigations are necessary on the distribution of venomous spiders and suspected cases of spider bite should be reported to related centers to reduce the damages caused by biting . all authors passed four criteria for authorship contribution based on recommendations of the international committee of medical journal editors .
more than 40,000 species of spiders have been identified in the world . 
 spider bites is a common problem among people , however few of them are harmful but delay in treatment can cause death . since the spider bites are risk full to human , they should be taken seriously , especially in endemic areas . 
 our objective in this review was to study about poisonous spiders and find out treatments of them . 
 therefore , we collected related articles from pubmed database and google scholar . 
 three important syndromes caused by spider bites are loxoscelism , latrodectism and funnel web spider syndrome . 
 many treatments are used but much more studies should have done to decrease the mortality . in this review , we describes different venomous spiders according to their appearance , symptoms after their bites and available treatments .