Several types of spiders make their homes within carnivorous pitcher plants found in Southeast Asia, stealing prey that wonder inside. Surprisingly, new research shows, this thievery benefits both partners.

“We found that pitchers containing crab spiders caught significantly greater amounts of certain prey types, especially large flying ones,” says Weng Ngai Lam, a post-doctoral ecology researcher at the National University of Singapore and one of the coauthors of a pair of recent studies.

Aside from the normal diet of water and sunlight, these carnivorous plants supplement their diet by attracting insects to the nectar on the outer lips of the pitcher using chemical and visual cues. These surfaces can become slippery, particularly when it’s raining, sending bugs tumbling into pitcher below— where digestive liquids break them down.

View Images An adult female Thomisus nepenthiphilus crab spider waits for visiting insects at the mouth of a Nepenthes gracilis pitcher, its forelimbs wide open in typical ambush posture. Photograph by Weng Ngai Lam, National University of Singapore

Ambush!

Yellow (Thomisus nepenthiphilus) and red (Misumenops nepenthicola) crab spiders have tuned into this trap and have devised a strategy to take a bite out of the nutritional earnings of the plants by hanging out inside the pitchers in wait of other insects. When bugs are lured near the trap, the hidden crab spiders ambush the prey – the smaller red crab spiders sometimes tackle larger prey directly into the liquid. Both species of spider may extricate themselves from the liquid with the help of silk, he adds.

Lam and his coauthors wanted to see what this relationship meant for the pitcher plants. In the first study, published recently in Oecologia, they conducted laboratory experiments using the slender pitcher plant and the two different types of crab spiders. They put flesh flies, a fearsome insect which plants its maggots on dead flesh or rotting wounds, into cages with the spider-equipped pitcher plants.

Flesh flies are relatively hard to catch for the pitcher but not with the crab spiders waiting in ambush. After the spiders ate their fill, they would drop the leftover carcasses into the liquid for the pitchers to finish off. The researchers found that the plants with spiders scored more bug carcasses than those without.

In the more recent study, published in the Journal of Animal Ecology, Lam and his coauthors took the experiment into the wild. They found pitcher plants with spiders and those without, and observed what type of prey and in what quantity the plants could catch. They confirmed that while the spiders may actually steal some of the nutrients that the plants get by eating parts of the prey, they increase the overall haul by ambushing insects the plant wouldn’t normally attract.

“This kind of thievery can actually benefit the plants – it’s kind of a trade-off,” Lam says.

View Images A Misumenops nepenthicola, the red crab spider, in a Nepenthes gracilis pitcher waiting to catch some prey. Photograph by Weng Ngai Lam, National University of Singapore

A match made in heaven

Ellen Welti, a post-doctoral biology researcher at the University of Oklahoma who was not involved in Lam’s research, says the work is exciting.

“It shows the complexity of how species relationships can change from more antagonistic to mutualistic,” she says, adding that crab spiders “have a very tight evolutionary history with flowers.”

Welti says other research shows some related crab spider species can help attract insects to flowers possibly by mimicking the colors visible to pollinators they prey on, making them more attractive than other flowers without spiders. And some of these (non-carnivorous) likely benefit from having the arachnids around.

Her research shows that at least some crab spiders can actually improve the pollination of these flowers. “We had this very exciting result that when the crab spiders were sitting on the flowers, more of everything was coming to the flowers,” she says.

Another recent study shows that certain plants may even send out help signals when attacked by herbivorous insects like caterpillars or beetles. Crab spiders may pick up on these signals and swoop in to eat the offenders, thereby benefitting the flowers.

To catch a thief

Besides getting help catching meals, pitcher plants may also make for good places to live.

“The pitcher is a very safe environment for the spider because it’s protected from predators. They are out of sight of many potential parasitoid wasps that usually hunt spiders,” he says.