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I have always been told that a spider is not an insect, since the former has 8 legs and the latter 6 legs.

Is it just a matter of definition and spiders could have been defined as "8-legged insects"? Or are there more fundamental differences between the two kinds of animals and the distinction makes sense from the viewpoint of biology and evolution?

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Spiders and Insects have been separate independent linages of animals for 500 million years. Spiders have two body parts (cephalothorax and abdomen), while insects have 3 (head, thorax and abdomen). Besides the differences in number of legs, spiders and insects have different forms of respiration, different mouthparts, and different exoskeletons. Insects have antennae, while spiders don't. In short, spiders and insects are more different from one another than mice are from sharks.

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