Game Mechanics: Entomophobia - the Fear of Bugs

[Vermicelli]

Dwarf guards in Butcherblock won't assist with giant scarabs or large spiders. They will kill most other things you bring to them, including the smaller worker scarabs and rock spiderlings, drakes and snakes, skellies and basilisks and bats, but something about the large bugs got them shaking.

[Sundawg]

Already reported and fixed for next patch.