n large animal swarm 

Perception +11; darkvision, scent (imprecise) 30 feet

Skills Athletics +7

Str -2 Dex +4 Con +4 Int -5 Wis +2 Cha -4

AC  21; Fort +13, Ref +11, Will +9

HP 55; Immunities precision, swarm mind; Weaknesses area; Resistances bludgeoning 2, piercing 5, slashing 5

Cling [R] Trigger A creature leaves the swarm's space; Effect The swarm takes 1d6 damage as ants cling to the creature and continue biting, dealing 3d6 persistent piercing damage. High winds or immersion in water reduces the DC of the flat check to end this persistent damage to 5. Any area damage dealt to the creature destroys these clinging ants.

Speed 30 feet, climb 30 feet

Swarming Bites [>] Each enemy in the swarm's space takes 3d6 piercing damage (DC 21 basic Fortitude save). A creature that fails its save against Swarming Bites becomes clumsy 1 for 1 round. If the creature attempts a concentrate or manipulate action while affected, it must succeed at a DC 5 flat check or the action is lost; roll the check after spending the action, but before any effects are applied.

Bestiary 2, page 20.