March 18, 2006
They may be small, but Assassin spiders are among the most dangerous spiders on the planet – if you’re another spider, that is. These tiny arachnids in the Archaeidae family are only about 2 mm (less than 1/8 inch) long, but their bizarre fangs and spider-hunting practices have earned them a reputation as the world’s most grotesque spiders. They hunt by stabbing their prey with venom-filled fangs that are attached to the ends of extremely elongated jaws. These specialized jaws are about ten times longer than the jaws of most other spiders their size. To support these long jaws and prevent them from dragging along the ground, Assassin spiders have also evolved elongated necks. The combination allows them to strike their prey without having to approach too closely.
The fossil record shows that Assassin spiders were once widely distributed across the planet, but today they are known only from Australia, South Africa, and Madagascar. Until recently, only about a dozen species were recognized. However, nine new species are about to be added to the list, almost doubling the known members of the family. For the past few years, Academy entomologist Charles Griswold and San Francisco State University graduate student Hannah Wood have been working to document the spiders of Madagascar as part of the Academy’s ongoing arthropod survey of the country. Since the survey was started in 2000, over 2 million arthropod specimens have been processed. Among them, Griswold and Wood noticed a number of tiny Archaeidae spiders that looked suspiciously like new species.
Wood conducted molecular and morphological studies and found that nine new species of Assassin spiders were included in the specimens from Madagascar. Surprisingly, the DNA data also revealed that the presence of elongated necks among Archaeidae spiders had evolved at least two separate times. A classic example of convergent evolution, her findings suggest that the need to strike out at prey from a distance encouraged the evolution of extended body parts on more than one occasion.