two main body parts,
eight walking legs,
piercing jaws (fangs),
abdominal silk spinning organs,
anterior abdominal genital opening.
Insects have three main body parts, six walking legs, compound eyes, antennae, chewing jaws (mandibles – often secondarily modified), posterior abdominal genital opening.
Relatively speaking – the Arachnida
Spiders and their relatives are called arachnids. Arachnids have the head and thorax combined (cephalothorax) with simple eyes, jaws adapted for tearing or piercing prey, a pair of pedipalps and eight walking legs.
Arachnids include spiders, scorpions, pseudoscorpions, amblypygids( tailless-whipscorpions), schizomids (micro-whipscorpiones), palpigrades, harvestmen, ticks and mites.
Spiders are the only arachnids that have special glands in their abdomen which produce silk
Midgets to monsters
A few spiders are so small and live such hidden lives that most of us never see them. Others are enormous.
Some of the smallest spiders in the world are anapid spiders, sometimes called armoured spiders because of the cuticular plates on their pinhead-sized bodies. Small spiders like anapids are usually found in damp, cool habitats like forest leaf litter and moss because their small bodies can lose water rapidly in dryer conditions. The largest spiders in the world include the South American Goliath Tarantula, some so big their legs can span a dinner plate. Such spiders may take decades to reach such a size. However, spider size is limited, partly because their respiratory physiology becomes less efficient at very large sizes.
Many spiders have unusual body shapes and colours.
Bizarre bodies can be helpful to spiders in various ways – to deceive and ambush prey, to capture particular sorts of prey, to avoid being eaten and to attract mates