Here's one answer: Our ancestors - and the spiders' ancestors - with different numbers of legs didn't live and reproduce. 8-legged spiders and 2-legged people survived and reproduced. ... Spiders have 8 legs, because their ancestors had 8 legs. Spiders and horseshoe crabs evolved from the same ancestors!

Do only spiders have 8 legs?

Myth: You can always tell a spider because it has eight legs. Fact: Not exactly. Scorpions, harvestmen, ticks, and in fact all arachnids—not just spiders—have four pairs of legs (see illustrations).

What do spiders have 8 legs?

How many legs does a spider have? They have 10 legs! This is no joke; spiders have 8 legs that they walk with, however, they also have a pair that they use sort of like hands. These front pair of legs are referred to pedipalps or just palps for short.