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Jumping Spiders Display Elaborate Courtship Dances By JOE EATONSpecial to the Planet

Tuesday June 14, 2005

What caught my eye was the color contrast: something bright red crawling along the green garden hose. It was a creature I had never seen before, a thumbnail-sized spider with a black cephalothorax and a red abdomen, a huge pair of forward-facing eyes, an d a glint of green about its mouthparts. Distracted from watering, I followed it around the lawn as it maneuvered through the grass blades, following one to near its tip where it suddenly pounced on something small, brown, and shiny. 

My find was a jumpin g spider, most likely Phidippus johnsoni, sometimes called the red-backed spider (no relation to the notorious Australian species, an antipodean relative of our black widow). From its coloration, it must have been a male; females have black markings on th e red abdomen. And I learned that it and its relatives in the family Salticidae are no ordinary spiders, if there is such a thing. Phidippus and other jumping spiders have remarkable visual skills and perform elaborate courtship dances; and some of them d o things that you would never expect from a mere arachnid. 

P. johnsoni seems to be common throughout the West; most of the research on it was done here in Berkeley in the ‘70s by arachnologist Robert R. Jackson. Jackson, now in New Zealand, must be a tru ly dedicated scientist. He speculated in one article that johnsoni’s bright color might signal its unpalatability to would-be predators, like the orange of the monarch butterfly. His conclusion: “There is no evidence that their coloration is aposematic, a lthough information concerning this is limited. They do not taste bitter or noxious to humans (personal observation).” 

Since jumping spiders have color vision, the red abdomen probably plays a role in courtship. Male salticids dance for their mates. P. johnsoni’s choreography consists of the linear dance (walking toward the female, then backing away), the zigzag dance (walking side to side while facing her), and gesturing (moving his forelegs forward and up, then down and sideways). In the 19th century, a Wisconsin couple named Peckham studied a whole range of jumping spiders and described fancier footwork, although a later author’s comparison to the hula, the tango, and the highland fling may have been a stretch. For dancing spider videos, check out htt p://tolweb.org/accessory/Jumping_Spider_Image_Gallery?acc_id=59. Males of some species also produce a buzzing or purring sound by twitching their abdomens while dancing. 

Female jumping spiders are tough audiences. The Peckhams introduced two successive m ales to a female Phidippus morsitans, who killed and ate them after “they had only offered her the merest civilities.” But males don’t depend solely on their dancing. When courting a female in her nest, they perform a tactile routine, tweaking her web wit h forelegs and mouthparts. If a male gets a positive response, he enters the nest for mating. When the female is not yet sexually mature (I’m not sure how he can tell, but apparently he can), he builds an annex to her nest and moves in until the time is r ipe. 

Both sexes stalk their prey like eight-legged cats. Unlike some spiders, whose vision is rudimentary, jumpers are sight-hunters. Salticids, typically for spiders, have eight eyes. The six along the side of the carapace, the secondary eyes, are simpl e light-and-motion detectors. But the huge, forward-facing principal eyes are something else again. They’re not compound eyes like those of insects: they’re like a built-in pair of binoculars. Each eye is a long tube with a large corneal lens up front and a second lens in the rear. The front lens has a long focal length and the rear lens magnifies the image from the front lens. Four layers of receptors in the retina allow for color discrimination. In the rearmost layer, a small region called the fovea res olves fine visual details. 

All this appears to be standard jumping spider equipment, good enough for spiders like Phidippus to recognize prey and mates from a considerable distance, for a spider. After his early work on P. johnsoni, Jackson moved on to a new subject, a genus of Old World jumping spiders called Portia. Jackson says Portia’s visual spatial acuity is much better than any insect’s, comparable to that of some mammals. Portia is a spider-hunting spider, and its vision is acute enough to differentiate between a spider and an insect, and between different species of spider; it can also tell whether another spider is holding an egg case.  

And act accordingly. The truly outstanding thing about Portia is the versatility of its hunting tactics. If a potential victim is encumbered with an egg case, it makes a frontal assault; otherwise, it sneaks up from behind. It will approach its prey by circuitous routes, using detours which remove the victim from its line of sight. And it employs what can only be called trial-and-error tactics to hunt spiders ensconced in their own webs. Like a courting male Phidippus, Portia tweaks and tugs the strands of its intended victim’s web. It may in fact be mimicking a male’s signals, with specific messages for differ ent species of prey. Depending on the response, it varies the pattern of tweaks until the victim, expecting to find a suitor at the door, ventures out within pouncing distance. And Portia even uses background noise, like leaves rustling in the wind, to ma sk its movements on the victim’s web. 

All this from a spider? Salticids have comparatively large brains, for spiders. But they’re still operating on a handful of neurons, and their eyes have only 10,000 to 100,000 receptors, compared with over 100 millio n in the human eye. Small doesn’t equate to simple, though. Robert Jackson and his collaborator Stim Wilcox aren’t afraid to use loaded words like “problem solving” and “cognition.” Thinking spiders? There’s clearly a lot more going on down among the grass blades than we would ever suspect.