NEW SPECIES OF SOCIAL SPITTING SPIDER DISCOVERED IN MADAGASCAR
Academy Scientist Jeremy Miller surprised by spiders' communal behavior
SAN FRANCISCO (July 28, 2006) – In January of 2006, Academy entomologist Jeremy Miller traveled to Eastern Madagascar to study the region's spiders. He and his Academy colleague Hannah Wood were the first arachnologists to survey the dry, deciduous forests in the area, so he wasn't necessarily surprised to find a new species living there. He was, however, amazed by the behavioral traits he witnessed when he encountered a new species in the spider family Scytodidae, the members of which have earned the nickname "spitting spiders" by literally spitting for their supper. These talented arachnids can trap prey from as far away as 60 millimeters (about ten times their own body length) by spewing out a mixture of venom and glue. As impressive as this spitting tactic is, however, it was not the behavior that made Miller stop in his tracks. The big surprise was that the spiders were living together in colonies.
Communal living may seem mundane compared to projectile poison, but it is actually incredibly rare among arachnids.
Most spiders are solitary creatures - once they reach adulthood, they seek each other out for one of only two reasons: to mate or to eat one another. However, a handful of species have developed a more tolerant attitude toward their kin. Of the approximately 40,000 known species of spiders on the planet, about 20 live in cooperative family groups that stay together for generations. This social behavior has evolved independently several times and probably develops gradually, as females begin extending maternal care to older and older juveniles. By living together, social spiders can share the responsibility of building webs and capturing prey.
The new spitting spider that Miller discovered in Madagascar (Scytodes sp.) lives in groups of up to 16 individuals, including juveniles and adults of both sexes. The species builds webs by weaving together debris, leaves, and branches with strands of silk. Besides providing shelter for the spiders, these roughly baseball-sized webs also help to ensnare passing prey. Once a potential meal lands on the web, the spiders team up to help tackle the tasty morsel. Approaching from multiple angles, they use their hind legs to comb silk over their prey and - if the prey is particularly feisty - they spray the entangled intruder with toxic, sticky spit. Mature males, mature females, and juveniles all participate in the prey capture, and they share the fruits of their labor quite freely - even spiders that did not assist in the tackle are permitted to partake in the meal.
These social spitting spiders are able to subdue much larger prey by working as a team than they could by attacking alone. On one occasion, Miller watched a single female attempt to capture a roach that was about twice her size. She managed to tenuously entangle the roach in her silk, but the bigger bug eventually escaped. Roaches of the same size were successfully captured, however, when two or more spiders worked together. Teams of spiders were also able to hunt other large prey, including moths and flies.
Most species of spitting spiders do not actually build webs to capture prey. The new species that Miller encountered, however, follows the trend of almost all other social spiders by building communal webs. The webs, which vibrate when anything hits them, seem to assist in prey capture by simultaneously alerting multiple colony members to the presence and location of potential prey.
Although they toe the party line when it comes to web building, Miller's spiders differ from most other social spiders in another regard: sex ratio. As a general rule, most social spiders have little or no contact with spiders outside their own colony, so mating occurs between family members. As a result, colony members carry virtually identical genes. Under these circumstances, the competitive pressure for individuals to pass down their own genetic information disappears, and the needs of the colony as a whole take precedence. Colonies can grow faster if most of the offspring are female, so most social spiders live in colonies that are about 90% female. Only a few males are needed in each colony for egg fertilization. Miller was therefore surprised to find that in the 61 colonies of social spitting spiders he documented, the sex ratio was almost equally split between males and females. This suggests that there is likely a substantial amount of interaction and gene flow between colonies of Scytodes spiders. It also implies that - despite their table manners - these spitting spiders may be the top socialites in the spider world.
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The California Academy of Sciences is home to Steinhart Aquarium, Morrison Planetarium and the Kimball Natural History Museum. The Academy is in the midst of an extensive rebuilding project in Golden Gate Park. Pritzker prize-winning architect Renzo Piano is designing the new Academy, which is scheduled to open on September 27, 2008. www.calacademy.org (415) 379-8000.