The world’s most venomous spiders are the Brazilian wandering spiders of the genus Phoneutria , and particularly the Brazilian huntsman Phoneutria fera , which has the most active neurotoxic venom of any living spider.  Its venom is so potent that only 0.006 mg (0.00000021 oz) is sufficient to kill a mouse.   Phoneutria spiders are usually dark in colour and have a body-and-leg spread of approximately 17 cm (6.75 in).  

These large and highly aggressive creatures often enter human dwellings and hide in clothing or shoes. When disturbed they bite furiously several times, and hundreds of accidents involving these species are reported annually. Fortunately, however, an effective antivenom is available, and when deaths do occur they are usually of children under the age of seven.