n medium animal 

Perception +8; darkvision, tremorsense (imprecise) 30 feet

Skills Acrobatics +7, Athletics +6

Str +3 Dex +4 Con +3 Int -5 Wis +3 Cha -5

AC  17; Fort +6, Ref +9, Will +6

HP 20

Avoid the Swat [R] Trigger The giant fly is targeted with a melee or ranged attack by an attacker it can see; Effect The giant fly gains a +2 circumstance bonus against the triggering attack. If the attack misses, the giant insect can Fly up to its fly Speed.

Speed 20 feet, climb 20 feet, fly 40 feet

Melee [>] mandibles +8, Damage 1d6+3 piercing plus fly pox

Fly Pox (disease, virulent) A giant fly could carry any disease, but most transmit a virulent but not fatal infection called Fly Pox with their bite Saving Throw DC 16 Fortitude; Onset 1 day; Stage 1 enfeebled 1 (1 day); Stage 2 as stage 1 (1 day); Stage 3 enfeebled 2 (1 day); Stage 4 as stage 3; Stage 5 enfeebled 2 and fatigued (1 day)

Bestiary 2, page 120. Expanded in LOIL.