You might think you would notice an eight-legged animal the size of a dinner plate, but in fact many of the world's biggest spiders are easily overlooked.

In Europe, the Desertas wolf spider is only found in a single valley on the island of Madeira, Portugal. It is one of the world's largest wolf spiders, with a body length of up to 1.6 inches (4cm) and distinctive black legs with white polka dots.

The family name conjures up fierce and predatory associations, and rightly so. Wolf spiders are named for their active hunting style. Instead of capturing prey in a web, these arachnids pounce on their meals.

Like any self-respecting legendary spider, it lives in a cave

However there are ways in which they differ from wolves. The spiders are solitary rather than pack hunters, so they ambush their prey or chase it down over a short distance. Also, they hunt millipedes, not mammals.

The most distinguishing characteristic of wolf spiders is the layout of their eyes. They have a row of four small ones, with two larger eyes above, topped by another two slightly smaller ones that boost their predatory senses. The Desertas wolf spider's excellent eyesight allows it to hunt fast-moving beetles and even small lizards.

But conservationists are now keeping a keen eye on Europe's biggest spider. It has been classified as "critically endangered" because its unique habitat is being overgrown with invasive grass.

A similarly remote location is home to one of Asia's biggest spiders: the giant huntsman spider.

It has a leg span of up to 30 cm (1 foot), which has been championed as the world's biggest. Like any self-respecting legendary spider, it lives in a cave.

In 2001, Peter Jäger discovered the species in a collection at the National Museum of Natural History in Paris, France, before heading out to Laos to visit its secluded habitat. Why it grows to such a significant size remains something of a mystery.

When approached, a king baboon spider rears up to expose its fangs and hisses loudly by rubbing its legs together

"A straightforward explanation is difficult," says Jäger, "But I guess one reason in the case of H. maxima is certainly the cave-dwelling habit… prey is scarcer than outside, [meaning] growth is slower and this may result in bigger size."

Unfortunately the media spotlight on the giant huntsman has had negative consequences. Jäger reports that its numbers are dwindling due to unregulated demand by the pet trade.

There are also large huntsman spiders in Australia. They usually hide under loose tree bark, but their long legs have also been spotted behind wall clocks and sun-visors in cars.

Since they prey on pests such as flies, they should be considered welcome guests. But their crab-like appearance, leg span of up to 6 inches (16cm) and ability to deliver a nasty nip when provoked can be alarming.

The huntsman might have the best pins, but the true heavyweight champions are the tarantulas.

One of Africa's largest living spiders is the king baboon tarantula. It has a leg span of up to 8 inches (20cm) and is rusty red-brown in colour. In the wild it digs into the soil of grasslands, then constructs webs across its burrows to catch prey.

Hysterocrates hercules has a name which is bigger than the actual spider!

Their size makes them popular pets, but their temperament is less appealing. Their common name refers to the fact that baboons often eat them, and as a result they are highly defensive towards primates, including humans. When approached, a king baboon spider rears up to expose its fangs and hisses loudly by rubbing its legs together.

There is another species of African tarantula revered among collectors. There is a thriving unscrupulous online trade in "super-sized" Hercules baboon spiders, even though they have not been seen in the wild since 1900.

"Hysterocrates hercules has a name which is bigger than the actual spider!" says Richard Gallon from the British Arachnological Society. "Although the only known specimen has the largest carapace of any African spider, there are other species of tarantula in Africa which are heavier and with larger leg-spans."

The first part of the Goliath birdeater's name is no exaggeration

That single specimen resides in a jar of preserving alcohol in the Natural History Museum in London, UK.

There are unlikely to be more anytime soon. "Hysterocrates hercules was collected from central Nigeria, an area which is now politically unstable, so not a place for researchers to risk their lives visiting," says Gallon. "Grassland tarantulas are also difficult to find, often needing several hours of searching by an experienced team to locate specimens."

In 2011, staff at the Natural History Museum brought in a Guinness World Records expert to witness a showdown. They measured the volume of their Hercules baboon spider against a similarly-preserved Goliath bird-eating spider. This settled the matter: H. hercules was less than a third of the size of Goliath.

The first part of the Goliath birdeater's name is no exaggeration. Its legs span 11 inches (28cm), just shy of the giant huntsman, but it is a much bulkier animal.

Size is much debated, particularly among the tarantula breeding community. But one of the heaviest captive-bred specimens was identified by Guinness experts as Rosi. She weighed in at 6 oz (175g) and had a body length of almost 4.7 inches (12cm).

In the wild, Goliath birdeaters are rarely encountered. They live in the upland rainforests of South America and are nocturnal. To maintain their impressive physiques, they are voracious predators – of earthworms.

The B-movie moniker comes from early explorers, who reported having witnessed one eating a hummingbird. The naturalist Maria Sibylla Merian created a dramatic illustration of this incident, which promptly did the Victorian equivalent of going viral.

In truth, being a ground-based predator the Goliath birdeater's diet is unlikely to include many birds. But the opportunistic spiders have been recorded snacking on frogs, toads, lizards and small mice.

In the wild, Goliath birdeaters are rarely encountered

This diet obviously helps to boost their size, but it also makes them a bigger target for predatory mammals and birds. Fortunately, they have powerful defences.

"The large South American species possess particularly irritating hair on the abdomen, which they rake off into the air with the spines on their rear legs," says Gallon. "These tiny irritant hairs waft in the air and settle on the mucus membranes – eyes, nose – of would-be attackers and discourage them."

For true giants, it is not enough to be big: they have to be hairy too.