This giant mosquito(ish)/bird-bat thing is a classic dating all the way back to the Greyhawk supplement. The stirge is a great monster for low-level parties (and an easy early introduction to grapple checks and Con damage).
As for making them more interesting for your game—well, it would be damn hard to top Brandon Hodge’s Feast of Ravenmoor, featuring a village full of pet stirges, stirge sausages, and stirge-masked cultists. And this Creighton Broadhurst article from back in the day, “Weird and ‘Wonderful’ Stirges,” is a great example of using 3.5’s templates to great effect—let’s see anyone sneer at the ghost brute stirge or the two-headed half-green dragon stirge. But let’s see if we can’t think of some stirge adventure ideas for ourselves…
The local lizardfolk are experimenting with the same electric-blue mold of which derros are so fond. The first hint is that the area stirges have all turned turquoise and are acting erratically.
Sent to retrieve the skin of a sasquatch, a party of adventurers finds out that the creature is far more clever than usually reported. Not only is it almost impossible to track, but it lures them into several deadfalls and other traps, including the mud-packed breeding ground of a storm of stirges. The thick muck has a quicksand-like effect on anyone wearing metal armor, and the stirges are sure to take advantage of any such immobilized meals.
The stirge’s half-insect, half-mammal physiology is a mystery to most natural philosophers. Alchemist Abraham the Vivisectionist and the druid Green-Hand Jonar seek several live specimens for experiments. Unfortunately, their research has caught the attention of cleric Pedrus Michel, an Antiresurrectionist who believes that exploring the origins of species is just as blasphemous as returning them from the grave.
—Pathfinder Bestiary 260