Swarm

This subtype is applied to any collection of Fine, Diminutive, or Tiny creatures that act as a single creature. A swarm has a single pool of Hit Points, a single initiative modifier, and a single EAC and KAC. It attempts saving throws as a single creature. A single swarm usually occupies a square (if it’s made up of nonflying creatures) or a cube (if it’s made up of flying creatures) 10 feet on a side, but its reach is 0 feet. A swarm can move through cracks or holes large enough for its component creatures to fit through.

In order to attack, a swarm moves into an opponent’s space, which provokes attacks of opportunity. Spellcasting or concentrating on spells within the area of a swarm requires a successful caster level check (DC = 20 + spell level). Using skills that involve patience and concentration, such as Computers, within the area of a swarm requires a successful DC 20 Will save.