For the bugs, their favorite type of prey was the one with high Psionic potential because the higher the Psionic Potential their target had, the more useful it would be for the swarm. Many talented Psionics or Psykers were kidnapped by the Arachnids because they were the prime biological material for some of the most unholy abominations.
Or they used the brains of talented Psionics and Psykers to advance their bioengineering. In this way, the Arachnids were constantly on the path of evolution to become an increasingly powerful race.
They were devouring everything in their path and assimialting it into their own Gene-Banks from which, then they would be creating more and more fearsome bugs.
