Invertebrates
Spider
Araneae · also called spiders
Every spider is a predator, and almost every spider carries venom — yet out of some 52,000 species only a couple of dozen can do a person real harm. What defines the group is not danger but two other things: silk, spun from the abdomen in up to seven varieties, and a stomach that works outside the body.
Overview
Spiders are the order Araneae, around 52,000 named species, on every continent except Antarctica. They are arachnids, not insects — eight legs, two body sections, no antennae or wings, usually several pairs of simple eyes. All have fangs, most have venom glands, and all have spinnerets. About half build capture webs; the rest are active hunters.
Silk and external digestion
Dragline silk is around five times stronger than steel by weight and tougher than Kevlar, drawn from a liquid protein that hardens as it is pulled. A spider’s gut can only handle liquid, so it pumps or injects digestive enzymes into its prey and sucks back the broth, leaving a drained husk or a wad of fibre.
Hunters without webs
Many spiders never spin a snare. Jumping spiders stalk and pounce with sharp vision, wolf spiders run prey down, crab spiders ambush inside flowers, and trapdoor spiders explode from a lidded burrow. Young spiders of many species disperse by ballooning — releasing silk that catches the wind and the air’s electric charge, sometimes carrying them hundreds of kilometres.
An enormous appetite
Spiders have hunted for over 300 million years, and the world population runs into the trillions. Together they are estimated to eat 400 to 800 million tonnes of insects and other prey a year — a mass comparable to all the meat and fish eaten by humanity, and one of the largest natural controls on insect numbers.
Status
Spiders as a group are Not Evaluated. Most are common and adaptable, but cave species, single-island endemics, and specialists of bogs and old forest are declining, and a few cave and dune spiders are legally protected.
Spider: common questions
Are spiders insects?
No. Spiders are arachnids, in the class Arachnida along with scorpions, ticks, mites, and harvestmen, and the arachnids are only distantly related to insects within the arthropods. The clearest differences are that a spider has eight legs to an insect's six, two main body sections rather than three, no antennae, and no wings, and that most spiders have several pairs of simple eyes instead of the compound eyes of insects. Spiders also have fangs and, uniquely, spinnerets that produce silk from the rear of the abdomen. All spiders are predators, whereas insects fill every dietary role.
How dangerous are spiders to humans?
Far less than their reputation suggests. Of around 52,000 species, only a few dozen have venom that matters medically to people, and deaths are very rare, especially now that antivenoms exist for the worst offenders. The widow spiders can cause painful, systemic illness, recluse bites can occasionally cause a slow-healing skin lesion, and the Australian funnel-webs and Brazilian wandering spiders are genuinely dangerous but bite few people. Most spiders are too small to pierce human skin, are not aggressive, and bite only when squeezed against the body. Many bites blamed on spiders, including most so-called recluse bites outside that spider's range, are actually infections or other skin conditions.
How do spiders make webs?
Silk is a liquid protein stored in glands in the abdomen and drawn out through nozzles called spinnerets, where the pulling force itself makes the protein chains line up and harden into a fibre. A web-building spider has a repertoire of silks, using a strong dragline for the frame and spokes and a stretchy, glue-coated thread for the spiral that catches prey. An orb weaver works to an inherited plan, laying down a bridge line, a frame, radial spokes, a temporary spiral, and finally the sticky capture spiral, eating the old silk as it goes. The whole structure can take under an hour and is often rebuilt daily.
What do spiders eat, and are they good for the garden?
Spiders are almost all predators of living animals, overwhelmingly insects and other spiders, and collectively they are one of the most important checks on insect numbers anywhere. Studies estimate the world's spiders eat hundreds of millions of tonnes of prey a year. In a garden or on a farm this means fewer aphids, caterpillars, flies, and mosquitoes, with no cost to the plants, since no spider damages vegetation apart from the single mostly vegetarian species in Central America. Encouraging spiders by leaving some rough cover and cutting pesticide use is a recognised part of natural pest management.