With a few levels under their belts, your players will probably scoff at giant insects. But these particularly nasty bugs (CR 6 and 3, respectively) should get under their skins.
Faerie’s Queen of Summer sends a heat wave to punish mortal loggers for despoiling her forests. Aside from rising temperatures, the first sign of this is the rampant hordes of mosquito swarms moving up the coast, spreading disease as they go.
A traveling zoo and circus brings a megatherium to town. The giant mosquitoes follow in its wake, driving the animals mad and killing many onlookers.
A strange, cloaked entity called the Mosquito Lord menaces the land and leaves blood-drained corpses in his wake. Thought to be some kind of twisted druid, daemon, day-walking vampire, or even an intelligent gryph, he is constantly surrounded by mosquito swarms.
—Pathfinder Bestiary 2 193
My former roommate is a malaria researcher. Now there’s a man who knows his Culicidae from his Plasmodium.
Backlog alert: I finally got around to writing up the annis hag.