BY ALPER ÖZKAN (MSN/PhD)
d_ozkan@ug.bilkent.edu.tr
I find it unusual that spiders are nearly always presented as dangerous and terrifying predators—they’re that and more in their element, true, and cursorial spiders are capable hunters even without the assistance of silk, but most are in fact little more threatening than a beached shark when out of their webs. A far cry from their image as the apex predators of the invertebrate world, they are also eaten by just about any other predator in their size category, and the conspicuousness of their webs makes them chitinous smorgasbords for their innumerable parasites and parasitoids—which include at least one species of caterpillar. Indeed, your average garden arthropod would have much more to fear from ants than spiders: while a spider can be avoided or overpowered (webs are not inescapable—small spiders will even let larger insects destroy their webs and fly off rather than taking the risk of subduing them), ants are an endless swarm of biting, stinging horrors, and their effect on the ecosystem is nearly comparable to humanity’s impact on vertebrate macrofauna. (Another common misconception is that individual ants don’t pose a threat—don’t forget that ants are small, ground-dwelling wasps, and come with an attitude to match.)
Life in such a hostile world requires either some top-of-the-line defenses, which most spiders don’t have (though spiny orbweavers may beg to differ), or enough panache to bluff—which they most certainly do. Some spiders, for example, exploit the legendary reputation of ants as insects you don’t want to tangle with, living near ant nests and walking with their front legs off the ground to fool predators that are looking for a quick snack but don’t particularly fancy a faceful of formic acid. Others have more elaborate disguises. The spider Pranburia has two “brushes” on its legs, which can be brought together to produce a mask-like imitation of an ant head, while the males of Myrmarachne disguise their massive chelicerae (which are used in mating displays) as a kernel held by an ant. A select few, like the crab spider Amyciaea, even have a “backward” imitation, where the abdomen of the spider imitates the head of the ant; in the jumping spider Orsima this disguise is further enhanced by elongated spinnerets that resemble antennae (this form of defense also has an offensive application in Amyciaea, which feeds on the very ants it imitates).
Ants are not the only objects of spider mimicry. Some smaller species will imitate pseudoscorpions, while the aptly named Arachnura pretends to be an actual scorpion: the abdomen of this spider tapers down into a tail, and the threatened spider lashes its harmless “stinger” to scare off potential predators. (Speaking of scorpions, it’s useful to note that their tail is also just an elongated abdomen—the most direct consequence of this is that a tailless scorpion loses most of its intestines and will invariably die, though those of the genus Ananteris that can willingly drop their tails survive a while longer, collecting their waste into the remaining body segments and dropping them one by one until they run out of these “extra lives.”) Another bizarre modification of the abdomen is found in certain trapdoor spiders, which are often counter-trapped and parasitized in their burrows by velvet ants and spider wasps—but their chitinous, shield-like abdomen, impervious to the stings of their hymenopteran nemeses, can be used to block the door until the danger passes. The most ingenious method for avoiding predation, however, involves the use of actual decoys: spiders of the genus Cyclosa will collect the remains of past prey into roughly spider-shaped lumps and leave them hanging in their webs, causing birds and other visually oriented predators to depart with a mouthful of rotting corpses.
In addition to deploying these surprisingly desperate defense tactics, spiders must sometimes take on prey larger than themselves, and instead of casually drinking their innards as popular culture would have you believe, they’re cowardly on that front too. Ant-eating spiders, for example, will almost never attack their prey without some measure of protection: some will use a dead ant as a shield, while others approach from behind and bite just once in a lightning-fast lunge, retreating and waiting afterward until the ant is too envenomed to fight back. Spider-hunting spiders likewise must contend with highly dangerous prey, and in their tactics can be observed some of the most advanced predatory behavior in invertebrates—the jumping spider genus Portia in particular is famous for adapting to environmental circumstances and learning from past experiences, stalking each target with a method it will be least suited to handle. Web-builders, for example, are lured out by the plucking of their webs in the same frequency as a frantically struggling prey animal, while active hunters with good eyesight are struck from a blind spot after Portia matches its movements with the wind and gets close by appearing to be rolling debris.
Indeed, the very purpose of the spider web is to prevent prey from striking back at the spider, and a great diversity of webs have evolved to tackle different types of both walking and flying insects. If you’re interested in unusual web types, “Prey Specialization in the Araneidae” is an excellent start.