Hive Crone
The Hive Crone is the Tyranid's answer to a fighter jet. No, seriously. Created to allow the Hive Mind to establish aerial dominance over a world it has targeted, they fight with tooth, nail and claw for the Hive. That, and a variety of nasty living weapons; living missiles known as Tentaclid,; a Drool Cannon (lol) which fires acidic spit, and bladed spurs on their wings. Despite how they look (and considering they are flying by flapping their wings) they are freaking fast and have to be to catch enemy fighters.
These creatures also serve as the Hive Fleet's jet escorts in space as well, protecting their bio-ships and ripping open the hulls of enemy vessels. This is a bit retarded though when you consider the vastness and vacuum-ness of space - and, as mentioned, these things are literally flapping to get around, which we are expected to believe makes them a match for fast moving space attack craft. Regardless these pests are a danger and anti-Nid spray should be applied at the same sighting.
They are cousins to the Harpies, another Tyranid aerial strain. This means that Tyranids are content with building a single base creature that works, and slap a variety of bio-weapons onto it (although not to the extent of the Imperial Guard- seriously, how many different guns can you put on a Leman Russ chassis?).