Arachnids fact sheet

What are arachnids?

Arachnids are a group of animals from the animal class Arachnida which includes spiders, scorpions, harvestmen, mites and ticks. Their body is made up of two parts called the cephalothorax and abdomen and they have eight legs.

Arachnids also have a pair of chelicerae which are mouthparts that are used for catching and holding prey and a set of pedipalps which look like another pair of legs, often smaller, that act as feelers for their surroundings and locating prey.

The habitat of arachnids is mostly terrestrial environments however some species are located in freshwater and marine environments.

The majority of arachnids are predators, feeding on mainly on invertebrates and other small animals, with some species being parasites such as mites and ticks.

Every species of spider has its own unique web. The golden orb weaving spider has a large round shaped web that is gold in colour.

Scorpions are a type of arachnid. Female scorpions give birth to 25 live young. The babies ride on their mothers back until they are independent.

All arachnids belong to a larger group of animals called arthropods. This means they have a segmented body covered in an exoskeleton and paired jointed limbs.

Spiders are the most familiar group of arachnids. The local Darug people of the Sydney basin refer to spiders as  marigung, marrayagong  or  danganuwa.

Case study – huntsman spider

Appearance

Huntsman spiders are large, hairy, flat-bodied spiders. Their legs fan out sideways enabling them to walk forwards and sideways. They are found in a variety of colours and patterns but are mostly brown, black or grey. Some species are very large reaching over 16 centimetres in leg span.

Huntsman spiders are one of Australia’s largest native spiders.

Habitat

Huntsman spiders occur Australia-wide and are usually found on tree trunks, under bark, beneath stones and even on the walls inside houses. Their flat body allows them to squeeze into narrow places.

Huntsman spiders live on tree trunks and under bark.

Diet

Huntsman spiders eat insects and other invertebrates as well as small lizards and frogs. Their fangs are large and can be used to hold the prey until it is paralysed by their venom.

Role in the ecosystem

Huntsman spiders control invertebrate populations and pest species such as the household cockroach. They are also food for other animals such as birds, geckos and spider wasps.

Huntsman spiders hunt prey such as cockroaches at night. This dead huntsman spider was found at Field of Mars Reserve.

Spider anatomy

Cephalothorax

The cephalothorax is the head and thorax fused together. Huntsmen have 8 eyes to spot approaching prey and predators.

Located at the front of the spider are the mouth parts called chelicerae that includes their fangs which they use to inject venom into their prey. Beside the chelicerae are the pedipalps which the huntsman uses to feel and touch things around them and also helping to catch and hold onto prey. Males of some species use their pedipalps when mating.

All the spiders' jointed legs are attached to the cephalothorax. The legs have sensitive hairs to pick up sounds, vibrations and air currents.

Huntsman spiders have 8 eyes. The mouth parts are called chelicerae. The leg hairs sense vibrations.

Abdomen

The abdomen has no appendages except for small spinnerets at the rear base which produce silk.

Huntsmen spiders build silk egg sacks the size of a 20 cent piece to protect their eggs. These can often be found under bark.

The abdomen – back body part – of a huntsman spider has no appendages except tiny spinnerets.

Related fact sheets

More invertebrate fact sheets

• Invertebrate fact sheets – overview
• Insect fact sheet – six-legged invertebrates that are often prey for spiders.

Spider species fact sheets

• Huntsman spider fact sheet – detailed species page linked to directly from the invertebrate index.
• Golden orb weaving spider fact sheet
• St Andrew’s Cross spider fact sheet – another web-building spider example.
• Trapdoor spider fact sheet – contrasts web-building spiders with ambush hunters.

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