Issue 14.2: April - May 2011

The Merry Pranksters

Story by Paul Wood

Photo by David Liittschwager

 

There are 40,000-plus species of spiders on Earth, and nearly every single one of them gives people the creeps. Arachnophobia (fear of spiders) is one of the most common fears among humans, but there is one spider that gets called “cute,” “adorable” and “carefree.” We emblazon its image on T-shirts and caps. It isn’t fair, really. We detest 39,999- plus spiders, but this one, we seem to assume, drew a smiley on its back for the sole purpose of reminding us to have a nice day. What else could we call it but the happy-face spider?

 

But the truth about this spider (which has a far more serious scientific name, Theridion grallator) is more interesting, more puzzling and—yes—more lovable than appears at first glance. The happyface spider is one of the many reminders that the native Hawaiian wilderness is chock full of information about evolution, genetics and biology. An endemic Hawaiian species, it is found nowhere on Earth except in the undisturbed upland forests of O‘ahu, Maui, Moloka‘i and Hawai‘i Island. It is common around Thurston Lava Tube at Hawai‘i Volcanoes National Park.

 

Just five millimeters long—less than one-fifth of an inch—the happy-face spider likes to live on the underside of a single leaf, which it rigs with a slight, almost invisible web. It sleeps by day and gets active at night, darting forth to grab infinitesimal insects, which it consumes in the usual spidery way by liquefying its prey with digestive juices and then slurping the results.

 

The happy-face spider belongs to a large spider family called Theridiidae. These spiders build cobwebs—loose, tangled constructions—and include a variety of house spiders, most notoriously the black widow. But the happy-face, nonaggressive by nature, is utterly harmless to gigantic mammals like us.

 

There is both passion and tenderness in the life of the happy-face. To get the story you must remember that spiders, like insects, live within their own exoskeleton, an armorlike skin that they must occasionally shrug off so that they can grow, suddenly, to their next size. During these periods of skin-splitting, or molting, the spiders are soft and vulnerable. When a male happy-face is ready to make babies, he will search for a female that is ready to molt, and he will stand guard to protect her. When she is quite soft, he will do the deed, placing his charged palps (which look like swollen drumsticks on the front of his head) inside her etigynum (a little pocket). Afterward he loses interest in living while she grows large and aggressive. She’ll lay a pile of eggs, cover them in silk and defend them against anything that moves. When the babies hatch, she’ll pull the silk apart to release them, then feed the spiderlings with her own liquefied diet. This extended parental care is a rare phenomenon among spiders.
 

 

So why the happy face? What’s the evolutionary advantage of having a clownlike grin tattooed across the back of your abdomen?

 

Dr. Rosemary Gillespie has published several scholarly works about that puzzling face and its implications for the study of evolution and genetics. Her interest in the spider dates back to the 1990s when she was a zoology professor at the University of Hawai‘i at Mänoa, and it has continued as she fulfills her many present duties at UC Berkeley: professor in the Department of Organisms & Environment, holder of the Schlinger Chair in Systemic Entomology and director of the university’s seventyyear- old Essig Museum of Entomology. Despite her ability to write papers in the arcane language of real science, she retains a fine ability to explain basic concepts in plain phrases, which she brightens with the crisp tones of her upbringing in Kirkcudbrightshire, Scotland.

 

“In every population it seems that about two-thirds of the spiders are plain yellow, almost translucent,” she says. “The remaining third are all different colors— red in front, red in back, white all over, a whole pile of different colors.” In other words, not all of the happy-face spiders have happy faces. Most are plain yellow, though one-third of them morph into a wide range of colors and patterns, some of which result in smiley faces.

 

Now here’s where the science gets good. The “happy face” or blotch of color on the backs of these basically yellow spiders evolved at least two different times on two different islands and in totally different ways. On Maui when you take a happy-face spider that’s plain yellow and you cross it with a happy-face spider that actually has a face, half of the offspring will turn out yellow, and half will have faces—the results mirror Gregor Mendel’s foundational work in genetics. But if you try the same experiment on Hawai‘i Island— crossing a plain yellow spider with a spider that has a happy face—the outcome is entirely different. The resulting male offspring will have happy faces; the females will all be plain yellow. In other words, the two populations came up with the same results but through entirely different genetic methods: On Maui the happy-face gene is linked to a chromosome that isn’t related to gender; on the Big Island the happy-face gene is linked to a chromosome that is. What is still puzzling the scientists is why the ratio of plain yellow to patterned spiders has remained constant on all of the islands: two-thirds to one-third regardless of where you are.
 

 

Then there’s the even more fundamental question: Why the need for spiders with happy faces on their abdomens at all?

 

Gillespie looked for the “trigger to make this work”—in other words, the evolutionary reason that this was occurring— and focused on the birds that search for and eat the little spiders. As the birds pick their way quickly through the forest vegetation looking for snacks, they react to “search images” by pecking away at all the samelooking ones. So, according to Gillespie, the happy-face pattern gives the spider a “confusion factor. The effect is to stop the birds from developing a search image.”

 

Perhaps that gives spiders a chance to scurry out of range. Perhaps it just forces hungry birds to think, feel frustrated and move on. Either way, this color adaptation, however the spiders create it, works with the birds. And perhaps it will work with humans, too, if Homo sapiens feel more inclined to protect the Theridion grallator habitat because they feel all moist-eyed and cheerful about a little spider that has a goofy grin on its back. Ultimately perhaps the happy-face spider will illustrate a new Darwin-esque principle: the survival of the smiliest.