THERE was no splash, just a flash of red as the body plunged into the pool and disappeared. At first, Simon Pollard thought his eyes were playing tricks on him. Spiders don’t commit suicide, yet this one – a small red crab spider about a centimetre across – seemed to have thrown itself into the mouth of a carnivorous plant.
Pollard, a spider expert from Canterbury Museum in Christchurch, New Zealand, had never seen anything like this before his visit to Sarawak, in the Malaysian part of the island of Borneo. The slimy fluid inside the trap of the pitcher plant spells death to any unsuspecting insect. Lured to the rim by sweet drops of nectar, most are unable to keep their footing on the slippery surface and fall to their doom.
But Pollard soon discovered that reckless behaviour was quite normal among the local red crab spiders. Whenever he peered over the edge of a pitcher, he saw spiders hurl themselves into the void and vanish under water. And if he waited, he saw them surface, bedraggled and sometimes covered in bits of drowned insect, but otherwise unharmed. Intrigued, Pollard cut off a pitcher and stood it upright on the forest floor. Then he placed a spider on the rim, watched it jump in and waited to see how long it stayed under. “It was 40 minutes before it came up for air,” he says.
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The next stop on Pollard’s spider-watching trip was Singapore. There, a quick search in the university library revealed more about his pitcher-diving spiders. They went by the name of Misumenops nepenthicola, were discovered more than a century ago, and their apparently death-defying leaps were well known. M. nepenthicola is associated with several species of nepenthes pitcher plants in forests across Asia, and like other crab spiders it ambushes its prey. But while many of its kin lurk around flowers and grab insects attracted by nectar, this beast lies in wait under the pitcher’s rim and intercepts the plant’s prey before it falls in.
Malevolent lifeguards
The spider also seems at home deep inside the pitchers. Occasionally one will climb down to the water’s edge and haul out a struggling victim. “It saves the insect from drowning, but is a malevolent sort of lifeguard,” says Pollard. “Instead of giving the kiss of life, it gives the kiss of death.” Females even attach their egg sacs inside the pitcher close to the waterline. And when danger threatens, the spiders jump right in, newly hatched spiderlings as well as adults. The pitcher makes a perfect refuge, for no prospective predator could follow and survive. “But how do the spiders stay under for so long and emerge unscathed? No one knew that,” says Pollard. His trip was almost over, but he was already planning his return to find out.
Soon he was back. In the forests of Sarawak and Singapore, he searched for specimens of the pitcher plant Nepenthes gracilis with resident spiders, then snipped off the pitchers and hung them by their hook-shaped stems from a twig at eye level. By shining a torch into the pitcher’s opening, he could see what the spiders were doing while submerged. “They seem to be totally comfortable under water,” says Pollard. “They go right to the bottom of the pitcher, burrowing down through the insect bodies to get as far away from danger as possible.” He also found that both adults and spiderlings regularly stay submerged for half an hour or more. Why, he wondered, don’t they die like the plant’s insect prey?
“The first thing to realise is that the pitcher fluid is not some evil brew filled with killer acid or flesh-dissolving enzymes,” says Pollard. “Nepenthes victims die by drowning, and their corpses are broken down mainly by bacteria and enzymes released from their own bodies.” But the plants encourage their victims to drown faster by secreting a detergent-like wetting agent into the pitcher fluid. This surfactant reduces the fluid’s surface tension so that insects slip quickly beneath the surface rather than walking about on top. Then it helps the fluid to move easily into their breathing tubes.
Famously, some mosquitoes lay their eggs in pitcher fluid: the larvae that hatch survive because they have a tube that connects them to the air above. (They don’t survive so long if they share a pitcher with M. nepenthicola, though.) The crab spiders don’t have a mosquito-style snorkel but, as Pollard discovered when he examined them under a microscope in the lab, they have something better – the arachnid equivalent of an aqualung. “When they dive, they take a bubble of air with them,” he says. “The bubble is in the same place every time, in a small pit on the abdomen just over the openings to the book lungs.” Could that keep them alive long enough to evade their enemies?
Pollard took a fresh look at what happened when the spiders dived in, this time with the aid of see-through “pitchers” – test tubes part-filled with nepenthes fluid and fitted with an escape-way made from a strip of real pitcher. In almost every case, after 40 minutes the spiders had exhausted the air supply. “Then they surfaced. If the coast was clear, they’d climb out and dry themselves off. If it wasn’t, they grabbed another bubble of air and went back under.”
Something still puzzled Pollard. Some large aquatic spiders, such as Dolomedes fisher spiders, create a bubble around their whole body, trapping enough air in their thick fuzz to keep them going for several hours. The crab spider may be a tenth the size of a fisher spider but it has a respectable covering of hair, so why doesn’t it do the same? Pollard suspected it might have something to do with the surfactant in the pitcher fluid: low surface tension makes it easy for even the smallest spiderling to slip underwater, but it could spell trouble for bubbles.
To check his hunch, he asked Dolomedes specialist Nancy Kreiter at the College of Notre Dame of Maryland, in Baltimore, to lace her spiders’ water with commercial surfactant. “If I added a few drops, they only managed to take down a partial bubble,” she says. “At first it was just missing from around their feet. If I added more, the bubble got worse and worse. Instead of one big bubble it became a patchy bubble.” For the spiders it was a disaster. “They couldn’t stay under. They just popped back up again.”
“The crab spiders don’t have a mosquito-style snorkel, but they do have something better”
Crab spiderlings, at just 1 to 2 millimetres across, are small enough for a bubble to hold together around most of their abdomen, and that’s enough to supply their tiny bodies for 40 minutes. For adults the solution to the surfactant problem is the abdominal pit: they can’t keep an intact bubble over their whole body, but they can trap a small bubble in a depression. Long, water-repellent hairs lining the pit push the bubble up against the pit wall and hold it in place. “The pit functions like an empty glass when plunged upside down into a sink full of water,” says Pollard.
There is still one nagging question. The inside of a pitcher has evolved ways to foil almost every possible means of escape. Even insects with the surest, stickiest or sharpest-clawed feet are almost invariably defeated by it. How do the spiders get out? “They don’t have some miracle-grip sort of feet, but they do have silk,” says Pollard. Although the silk is too fine to see, the spiders almost certainly have safety lines fixed around the inside of the pitcher. “They can’t take a line under water but when they surface, you see them go to the wall and move around as if they are looking for something. Then suddenly they just walk out.”
