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He creeps and he crawls,
Spiders are arthropods which means they are related to insects and crustaceans (including shrimp, crab, and lobster), but unlike insects, spiders have 8 legs (insects have 6), no wings or antennae, and most possess fangs that inject venom.
While insects have three body parts (tagmata) spiders and other arachnids (including ticks and mites) only have two.
Spiders are predators that hunt and kill their prey. Spider's abdomens have spinnerets which provide silk to build the webs they use for shelter, and to 'trap' insects to eat.
Typically, spiders only eat insects or other spiders, but some large spiders can eat birds and lizards!
Most spiders are not dangerous to humans at all, but there are some species that have venomous bites that can be harmful. The black widow spiders, which are common in the United States are known to have a venomous bite:
Black Widow and Egg Sac
(if you see a spider with this red hour-glass figure on its abdomen, it would be best to avoid it).
Some people believe that "daddy long-legs" are venomous spiders, but they are actually not venomous at all.