How to stalk a spitting spider: on a mountain in the Philippines, the hunter risks becoming the hunted

Natural History, Nov, 2001 by Robert R. Jackson, Simon D. Pollard

   Eye to eye and head to head,
   This shall end when one is dead;
   Turn for turn and twist for twist--
   Hah! The hooded Death has missed!

--Rudyard Kipling, The Jungle Book

Rudyard Kipling's story of the battle between the mongoose Rikki-tikki-tavi and two cobras has a surprising parallel in the spider world. Portia labiata, a spider with the cunning of a mongoose, hunts and is hunted by Scytodes pallidus, a spider with the defenses of a spitting cobra. While Kipling set his story in an Indian bungalow and garden, our story takes place in a rainforest on the slopes of Mount Makiling, about forty miles southeast of Manila on the Philippine island of Luzon.

Having built its web in the cup of a partly folded leaf, Scytodes sits with its feet splayed on the strands of silk, "listening" for telltale vibrations that will signal the arrival of a possible prey or predator. It is oblivious to being watched by Portia, a clever jumping spider that views Scytodes as a potential meal. Surveying the surrounding vegetation with its keen eyes, Portia initiates a plan of attack that will thwart its target's formidable defenses. First it approaches the edge of the web and, using its palps (small, leglike appendages close to the mouth), softly plucks the web's silk lines. The finely tuned signal has the effect of bringing Scytodes into the open without provoking an attack. Portia then starts to climb toward a leaf directly above the web. Often losing sight of its prey, Portia spends twenty minutes detouring through the foliage to reach the optimal vantage point. Peering down on its intended victim, Portia situates itself so that it can land in a prime position to grab its prey from the rear. Then it leaps.

In an instant, the tables are turned. As Portia jumps, one of its legs touches a silk strand, spoiling its aim and alerting Scytodes to the presence of an intruder on the web. Whirling around to face the danger, Scytodes fires a stream of sticky glue at Portia from its mouthparts. The jumping spider is hopelessly trapped, pinned down by the glue. The web's resident spider delivers the coup de grace by burying its fangs in Portia's head and injecting venom. This time, the would-be diner has become dinner.

Both these species have a predilection for arachnid flesh but are otherwise quite different. Instead of constructing a web, Portia uses its eyes and brain to stalk and pounce. Unlike most spiders, Portia has eyesight that rivals our own; it is capable of seeing size, color, and shape. Web-building spiders such as Scytodes have much weaker eyesight and instead rely heavily on interpreting the signals from movement and variations in tension of the strands of their webs. Portia exploits this sensory system by manipulating the web silk in numerous and often subtle ways as it seeks to gain control over the other spider's behavior. Sometimes, perhaps expecting to catch a struggling prey insect, the duped Scytodes rushes into Portia's waiting arms. At other times, Portia entices its victim out into the open to investigate and then positions itself for the kill.

While Portia's advantage is its acute eyesight, which allows it to monitor every move of its prey, Scytodes's advantage is its headful of ammunition. Most spiders produce venom and all have fangs, but Scytodes has an additional weapon: Connected to the venom glands inside its disproportionately large head are glands that contain a sticky gum. By contracting muscles around these glands, Scytodes can fire a wad of venom-laced glue over distances of an inch or more within half a second, quickly paralyzing both prey and predators.

About 130 Scytodes species exist worldwide, and they are the only spiders that spit. S. pallidus is found in different parts of Asia, but as far as we know, the only ones whose standard fare is jumping spiders are those that live on and around Mount Makiling. They weave sparse, nonsticky webs on the large leaves of trees and shrubs. Jumping spiders are very common in the same habitat and often trip over Scytodes's web lines, eliciting spitting attacks.

There are about 15 species of Portia in the world. All of them specialize in eating spiders--other jumping spiders as well as web builders. In the area of Mount Makiling, P. labiata is the only jumping spider that routinely gains the upper hand and overcomes S. pallidus. Part of its success appears to depend on its ability to stay out of the spitting spider's line of fire.

But Portia also exploits the maternal behavior of Scytodes. Female spitting spiders are unusual in the degree of care they give their offspring. The mother carries a cluster of developing embryos in an egg sac that she holds in her chelicerae (stout mouthparts ending in fangs). And while the juveniles of most web spiders disperse after the first molt, Scytodes spiderlings stay with the mother until they have molted three or more times. During this period of dependency, the mother brings prey to her babies, sharing the meal with them or allowing them to eat all of it.