Which is the most poisonous spider?
Well, several answers to this question can be correct. Before we can settle the issue, we must decide what we actually mean by the term “poisonous”. A spider can have a really potent venom, but inject such small amounts that a bite will be less poisonous to humans that the bite of many other spider species. The opposite is also true; a spider can produce a relatively weak venom but inject such large amounts that one single bite can be lethal to humans. Spider number two can therefore be considered more dangerous than spider number one, even though spider number one is in fact more poisonous.

The amount and toxicity of the venom is not the only factors to take into account when we discuss spiders, poison and how the human body will react to the poison. In many spider species, the male is considerably smaller than the female. A bite from a male spider will therefore typically not go as deep as a bite from a female. Since the female can deposit the poison further into the skin, her poison will create more damage in the human body. If we would test venom from both the male and the female spider in a laboratory, we might very well find out that both specimens are equally toxic. If we instead want to know for practical reasons which one of them to watch out for, most of us would casually describe the female spiders as “ the most poisonous spiders ”
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Phoneutria – the most poisonous spiders in the world
According to the Guinness Book of Records, the most poisonous spiders in the world are the Brazilian wandering spiders. The name Brazilian wandering spider is used for several different spider species in the genus Phoneutria: Phoneutria bahiensis,
 Phoneutria boliviensis, Phoneutria fera, Phoneutria nigriventer and Phoneutria reidyi.

The Brazilian wandering spiders are believed to be the most poisonous spiders in the world since research performed on their venom showed that it was the most active neurotoxin of any known spider species. No more than 0.006mg (0.00000021oz) of this venom is required to kill a mouse.

The Brazilian wandering spiders are not only the most poisonous spiders; the exhibit a wide range of habits that increase the risk of humans involuntary threatening the spider and ending up being bitten. The Brazilian wandering spiders do not stay in a web or hide in a permanent lair; they will instead wander around in search of prey. This habit increases the risk of human encounters significantly. The Brazilian wandering spiders are also renowned for seeking shelter in dark places during the day, which attracts them to boots, log piles, boxes, garments and similar.   

If you are bitten by any of the poisonous spiders in the world, you should naturally seek immediate medical attention. Try to remain as calm as possible. Fortunately for us, antivenom has been developed that is effective against the most poisonous spiders in the world. Antivenom is today not only available against Brazilian wandering spider venom, but against Australian venomous funnel-web spider venom as well.