The Robber Fly is an interesting critter. It's got a great "Gygaxian Naturalist" feel to it. It's got a camouflage adaptation. It's an effective predator. All good stuff.
Basically, it looks like a Giant Killer Bee. Which it hunts for food! While it's not super-tough individually, it has a few tidbits that can make it a fun low level encounter. But first, the stats:
Robber Fly (From Moldvay)
No. App: 1-6 (2-12)
Move: 90' (30')
-Fly 180' (60')
Att: 1 bite
So it's a 3' long carnivorous fly imitating a 3' long bee. Like the Cave Locust, it's another nice "Mysterious Island" vibe monster. Up to a dozen appearing is pretty respectable, though at 2 HD each they are vulnerable to Sleep spells. It's fast, and can jump 30' for a nice surprise attack. Speaking of which, they are patient, stealthy and surprise on a 1-4! They are also immune to the killer bees' stings, but that's unlikely to impact the characters situation. The fact that they can be easily confused with the GKBs (Giant Killer Bees) might make for a fun time, too.