Arachnids

Arachnids are a group of eight-legged invertebrates that includes spiders, scorpions, mites and ticks. There are over 100,000 known species of arachnid, found in almost every habitat on Earth except the open ocean. Arachnids are sometimes confused with insects, but they are a completely separate group: they have eight legs instead of six, no wings, no antennae, and two main body parts instead of three. Almost all arachnids are predators or parasites.

• Known speciesapprox. 100,000+Plus an estimated 200,000+ undiscovered
• Eight legsAlwaysThe defining feature
• Two body partsCephalothorax + abdomenCompared to insects' three
• No wings or antennaeUnlike insects
• Largest spiderGoliath birdeaterUp to 30 cm leg span (Amazon)
• Most dangerousBrazilian wandering spiderPlus a few others

What makes an arachnid?

• Eight legs (four pairs).
• Two body parts: the cephalothorax (head and chest combined) and the abdomen.
• No wings at any stage of life.
• No antennae.
• Multiple pairs of eyes: most spiders have 8 eyes; scorpions have up to 12.
• Chelicerae: special biting or pinching mouthparts in front of the mouth (the "fangs" of a spider, or the "claws" of a scorpion).
• Mostly carnivorous: arachnids eat other animals, often including each other.

The main arachnid groups

• Spiders (Araneae): over 50,000 known species. Almost all produce silk; most spin webs to catch food.
• Scorpions (Scorpiones): around 2,500 species. Famous for their stinger tail and pincer-like claws.
• Mites and ticks (Acari): over 50,000 known species, mostly tiny. Many are parasites of larger animals or plants.
• Harvestmen ("daddy long-legs", Opiliones): around 6,500 species. They look like spiders but are a separate group, with no silk and no venom.
• Sun spiders (Solifugae): not true spiders. Fast, aggressive desert hunters with huge jaws.
• Whip spiders (Amblypygi): strange-looking arachnids with very long thin front legs they use as feelers.
• Pseudoscorpions: tiny scorpion-shaped arachnids without the stinging tail. Live in soil and leaf litter.

Spider silk: the strongest natural fibre

Almost all spiders produce silk: a protein fibre extruded from special organs called spinnerets at the rear of the spider. Spider silk is one of the strongest materials known to science. By weight it is five times stronger than steel and three times tougher than the strongest synthetic fibre humans have made. A pencil-thick strand of spider silk could (in theory) stop a Boeing 747 in flight.

Different spider species make different kinds of silk for different jobs.

• Sticky silk for trapping prey in a web.
• Non-sticky silk for the structural threads of the web (so the spider does not stick to its own web).
• Wrapping silk for bundling up captured prey.
• Egg sac silk for protecting eggs.
• Drag-line silk for safety lines (a spider always trails a drag-line, so it can fall and climb back up).

How spiders catch prey

Spiders are some of the most effective predators on Earth. They have several different hunting styles:

• Web spinners: orb weavers, cobweb spiders. Build silk traps and wait.
• Jumping spiders: pounce on prey with extraordinary accuracy. They have excellent eyesight (better than any other invertebrate) and use a hydraulic jump system.
• Wolf spiders: actively chase down prey.
• Trapdoor spiders: build a hinged silk lid over a burrow and wait for prey to walk past.
• Bolas spiders: swing a single sticky droplet of silk like a lasso to catch moths.
• Spitting spiders: shoot a sticky silk-and-venom mixture from their fangs to glue prey to the ground.
• Diving bell spiders: build an air-filled silk dome underwater and live their entire life submerged.

How dangerous are arachnids?

Almost every spider can technically bite, and most have venom (used to subdue prey). In practice, very few are dangerous to humans. Only about 30 spider species have ever killed a person. The most dangerous include:

• Brazilian wandering spider: powerful venom, frequently encountered in homes.
• Sydney funnel-web spider: aggressive and venomous, but an antivenom exists and there have been no fatalities since 1981.
• Black widow: venomous but rarely fatal with modern treatment.
• Brown recluse: bite causes serious tissue damage, very rarely fatal.

Most other spiders are harmless to humans. The famous daddy long-legs rumour (that it has the most toxic venom but tiny fangs that cannot pierce skin) is a myth: their venom is mild and harmless.

Fact The world's biggest spider is the Goliath birdeater from the Amazon. It has a leg span of up to 30 cm, weighs up to 175 g, and despite its name mostly eats worms, insects and the occasional small frog or mouse (not birds, in fact, despite the name). Its bite is painful but not dangerous to humans. The species is named after an old engraving that showed one eating a hummingbird, which apparently does happen occasionally.

Scorpions: ancient survivors

Scorpions are among the most ancient animals on Earth. They are essentially unchanged from fossils 430 million years old. Modern scorpions still have the same basic body plan: large pincers (called pedipalps), eight walking legs, and a curved tail tipped with a venomous stinger. Scorpions are mostly nocturnal hunters and prefer hot dry climates.

Most scorpion stings are painful but not dangerous. About 30 species have venom strong enough to kill an adult human. The most dangerous is the deathstalker (Leiurus quinquestriatus) of the Middle East and North Africa.

Did you know? Scorpions glow in the dark under ultraviolet light. Their exoskeletons contain chemicals (including a compound called beta-carboline) that fluoresce bright blue-green under UV. This makes them easy to find at night with a UV torch: researchers use this trick to count scorpion populations. Nobody is quite sure why scorpions glow; theories include UV protection, predator detection or attracting prey.

Deeper dive: the truth about house spiders

If you are scared of spiders, you may not enjoy this fact: a typical UK home contains around 30 different species of spider sharing the building with you. Some scientists estimate that there are usually 3 to 6 individual spiders for every adult human in a typical home.

Most are tiny and easy to miss. The biggest and most visible UK house spiders are the giant house spider (Eratigena atrica), with a leg span of up to 10 cm, and the missing-sector orb weaver, often spotted around windows. Both look intimidating but are completely harmless. Their fangs are too small to bite through human skin.

House spiders are doing you a favour. They eat thousands of insects every year, including flies, mosquitoes, moths and ants. A house without spiders quickly fills up with insects. Many entomologists consider spiders to be one of the best natural pest controls in your home, and recommend leaving them alone.

If you really must remove one, the kindest method is to gently catch it in a cup with a piece of paper over the top and release it outside. (Although in winter, that may simply send the spider to its death; many UK spiders are actually indoor-only adapted species that cannot survive cold weather. So sometimes the kindest thing is just to leave it where it is.)

For other invertebrate groups, see insects, crustaceans and molluscs.