The six-eyed sand spider (Sicarius hahnii), is a rather large sicariid spider found in southern Africa. It lives in the desert, and hunts by ambush rather than by spinning a web. Unlike most ambush hunters like the trapdoor spider, mouse spider or Sydney funnel-web spider, it does not dig a burrow. Instead, it buries itself just under the surface of the sand. It is covered in small hairs, called setae, which serve to hold particles of sand. This provides effective camouflage even when the spider is not buried.
It does not roam in search of prey, rather it simply lies in wait for an insect or scorpion to happen by. When one does, it seizes the prey with its front legs, kills it with venom, and devours it. They don't need to feed terribly often, either - an adult S. hahnii can live without food or water for a very long time. The venom that it uses is of some note, too. It's highly cytotoxic, containing many of the same toxins as that of the brown recluse. This is no major surprise, as both sand spiders (genus Sicarius) and recluses (genus Loxosceles) are members of the family Sicariidae. Both are considered rather primitive, in an evolutionary sense, and both genera have only six eyes, rather than the eight more typical for araneomorph spiders.
How dangerous would a six-eyed sand spider bite be to a human, then? Well, the answer to that isn't known for sure. There are no confirmed bites in man, and only two suspected ones. However, in one of these cases, the victim lost an arm to massive necrosis, and in the other, the victim died of massive hemorrhaging, similar to the effects of a rattlesnake bite. Its venom has been analyzed and tested on rabbits. The rabbit tests produced extremely severe tissue damage, and eventually death. Thus, the current scientific opinion on the matter is that S. hahnii may well be the most venomous spider in the world. So, why hasn't anyone heard of all this before? Well, the six-eyed sand spider rarely comes into contact with people, and even when it does, it basically never bites. Further, like most spiders, it doesn't always inject venom with every bite, nor, even when it does, does it necessarily inject a large amount.
All that said, this is definitely not a spider that an amateur collector wants, and even semi-professionals and arachnologists should handle it with extreme caution. Consider it to be in the same league as the Australian funnel-web spiders, or the Brazilian wandering spiders, only without antivenin.
Thanks to BookReader for the note of the primitiveness of these spiders.