Spiders, scorpions, ticks, and mites have two body parts and four pairs of legs. They breathe using lung books (that look like an open book) in the abdomen. The front part of the body, known as the cephalothorax, bears the legs and two pairs of mouthparts: the chelicerae, which are like either pincers or fangs, and the pedipalps, which look like either legs or claws. Most arachnids live on land, but some live in water.


The red-kneed tarantula pounces on prey that comes close to its lair. Like most types of spider, the tarantula paralyses and kills its prey with venom, which it injects using fangs. The venom also breaks down the prey’s flesh, so that the spider can suck it up as a liquid. Spiders are carnivorous, eating mainly insects.


Most arachnids are predators, but some scavenge for food and a few mites are parasitic (live on another animal and feed off that animal). There are 75,500 species of arachnid, in 12 orders, including the following three main ones.

Order: Scorpionida


Features: predators, sting-bearing tails, large, claw-like pedipalps, bear live young

Order: Acarina

(mites, ticks)

Features: body not distinctively segmented, many are pests and parasites

Order: Araneae


Features: mostly eight-eyed, able to produce silk


A tick’s soft, flexible abdomen can expand to 10 times its normal size as the tick sucks in blood with specialized piercing and sucking mouthparts. The tick fastens itself to a sheep while it drinks in blood, then drops to the ground. When it needs more food, it attaches itself to another animal that is passing by.


The imperial scorpion is one of many arachnids that care for their young. A female scorpion carries about 30 young on its back until they have moulted (grown a new, larger skin) for the second time. The scorpion has a hard, black carapace (shell), large claws, and a poisonous sting.


A spider’s spinnerets produce liquid silk that hardens in the air. Many spiders spin a web with this silk, to catch prey. When an animal gets caught in the web, the spider wraps it in silk and kills it with venom. Spider silk is the strongest-known material – if a web were made with silk threads the diameter of a pencil, it would be strong enough to stop a plane in flight.


Animal Anatomy

Copyright © 2007 Dorling Kindersley


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