There are approximately 40 species of funnel web spiders, in two taxa. The Sydney Funnel web is a large hunting spider and the male is known to be the cause of human deaths. The Hadronyche species are widely distributed along eastern Australia, from Queensland, Tasmania and even South Australia, while the Atrax species range from New South Wales to Victoria. The Sydney Funnel web, however, is limited to a radius of 160km from Sydney.
Funnel web spiders are nocturnal, insectivorous, ambush hunters who weave a funnel shaped web surrounded by sticky trip lines. They tend to prefer moist environments, although the Sydney funnel web has adapted well to urbanization and is often found in suburbia.
Venom
Venom of the Funnel web spider is delivered upon biting prey, through paraxial chelicerae, which can be up to 5mm in length. There is some voluntary control over the delivery of venom, and droplets can be drawn back into the fang. The spider often has to be prised from the victim as Funnel web spiders bite with such tenacity.
The lethal part of the Sydney Funnel Web (Atrax robustis) venom is the protein Robustoxin (atraxotoxin) that contains 42 amino acids. It affects primates, newborn mice and other mammals at high doses. Other vertebrates are unaffected due to naturally occurring venom-inhibiting immunoglobins present in serum, or differing sensitivities of neurons.
Robustoxin is a presynaptic neurotoxin. It induces spontaneous, repetitive firing of action potentials in autonomic and motor neurons. Venom from the male has a higher concentration of robustoxin and is 4 -19 times more potent then that of the female. It is also believed that variations in size, health and feeding habits of spiders can affect the lethality of the venom.
Envenomation
Upon receiving a bite from a Sydney Funnel web spider, the victim would likely feel pain and discomfort. There will be puncture marks and or bleeding evident and swelling and redness around the bite.
Envenomation is defined by autonomic effects, such as diaphoresis (artificially induced perspiration), salivation, lacrimation (tear secretion), piloerection (hair standing on end), miosis (constriction of pupils) and mydriasis (dilation of pupils). Cardiovascular effects, such as hypertension, tachycardia and brachycardia.
Neurological effects like oral paraesthesia (abnormal skin sensation), muscle fasciculations (twitching of muscle fibers) and muscles spasms, as well as pulmonary oedema, headache, vomiting and abdominal pain is also likely.
Studies have shown that the Sydney Funnel web has a severe envenomation rate of 17%, although the difference of rates between various types of funnel webs can be attributed to spider size, behavior, ability to bite and fang size, rather the toxicity of the venom.
Interestingly, severe envenomation by females has not been reported, which supports studies that have shown female Funnel webs have a less potent venom then their male counterparts and perhaps can be attributed to the fact that the males tend to wander around at maturity to look for a mate, while the females stay at home.
Treatment
It is important in management of spider bites to immobolise the patient, not just the affected limb and to apply a pressure bandage. Next step is venom neutralization and dealing with any other complications of envenomation.
Severe envenomation can be quite rapid, deaths can occur within an hour. Since the introduction of antivenom in 1981, no confirmed deaths have been reported from Sydney Funnel web spider bites.
Antivenom reverses the effects of envenomation and is successful in 97% of cases, where the bite has been expertly identified as a Sydney funnel web spider bite. Use of antivenom also reduces the hospital stay for most patients.
Funnel web spiders can be milked twice in succession and the same amount of venom is produced each time. This shows that there is some voluntary control upon release of venom and that they could bite two times before running out of venom. It is likely that these spiders can deliver more venom during a bite then they do during milking.
Further information can be obtained regarding envenomation and clinical cases.
The fear elicited by the site or thought of a spider is often attributed unfairly due to the lack of good clinical trials, as many spiders aren’t dangerous to humans, or they lack large enough fangs to pierce human skin.
More studies on all species of spider bites would then provide a basis for diagnosing spider bites without the need for collecting or identifying the guilty spider and that more knowledge would minimize irrational fear of spiders.
Although not all bites produced result in envenomation, all should be treated as potentially fatal. However, much of the populations fear is based on the morphology of spiders; their beady eyes, or far too many hairy legs, for example; then the possibility of envenomation, and that may be difficult to change.
Roberta Goli - Roberta has a Bachelor of Science Degree (Zoology), has worked with animals for over 10 years and enjoys writing short stories and flash ...
