How many arachnids different from insects?

Firstly, all arachnids cannot fly. they all have 8 legs and 2 body parts

Insects have 3 body parts, 8 legs, and some have wings (Spiders are often mistaken for insects).

Arachnids get their name from an ancient Greek legend. It was said that Athena found a woman called Arachne boasting that her weaving was better than anyone, even the goddesses weaving. Greek gods and goddesses did not take well to upstart mortals, so Arachne was turned into a spider. She could weave only her webs, and all spiders now carry her name.

Hope this helps :D