Many ages ago, when the grasses and plants were different than the ones we know now, the First Scorpion was called Rai. He was an immense creature of darkness, with seven poisonous tails. Now, during that age of the world, there were also creatures called the Nyms: They were the builders who made the mountains, one stone at a time. At first, Rai the Scorpion was free to roam the land, but death and evil followed wherever he went, so the Nyms banished him to a cave in the mountains, away from everything else. But Rai was crafty, and he found the means to cover the world with his presence, in a way that could never be contained. He created an army of children.
Rai cut off one of his seven tails, and from it he made a Spider. Yes, that was the birthplace of the Spider, in all of its revulsion. Then Rai cut off another of his own tails, and it became a different creature: the Harvestman (or, daddy longlegs). Rai continued in this way, severing one tail after another, birthing new entities with his body and with his own suffering.
After creating the Mite, the Tick, the Camel Spider, and the False Scorpion, Rai was left with only one tail to call his own.
Arachnids spread across the world like a shadow. No one could escape them, in any land. They hid under the rocks, climbed the trees, and even walked on the waters. The Nyms were powerful enough to eradicate them, but not without killing all other life in the process. And so, the Nyms made an army of their own, to fight the arachnids.
A Nym called Terrenbowur took a blade of grass and tore it into tiny pieces, and he flung the pieces into the wind, to be carried across the world. Those bits of grass became living creatures, and that was the birth of insects.
Since then, there has been a blood grudge between our kind and theirs, and insects live by the rule of killing arachnids on sight, without context or question.