One of the neat things about Basic/Expert D&D is the compactness of the rules and how much material they actual contain. This entry in the basic book is a good example of that. What could be a complex and unwieldy series of entries in the monsters section is distilled to one, easily re skinned, listing.
Insect Swarm (from Moldvay)
Move: 30' (10')
Fly 60' (20')
Att: 1 swarm
Dmg: 2 pts
No. App: 1 swarm (1-3 swarms)
Save: Normal Man
So, stat-wise, other than a high morale score it's pretty unimpressive. Of course, that's not the point of the "creature." It's role is to represent one of the more mundane threats in the rules. It gives the DM a handy tool for representing any horde of tiny bitey/sting-y things.
As the book states, "Swarms are not single creatures, but are whole communities of tiny creatures acting together." A swarm might be defending a nest, hungry, or just investigating a curious smell. A normal-sized swarm is 10 x 30', which means more than one PC can be enveloped, and swarms can be larger or appear in greater numbers. Imagine a 30' x 90' area filled with angry bees!
There are a few other things to keep in mind that can make these clouds of critters particularly dangerous.
1) "If a character is within a swarm he or she is automatically hit by the creatures and will take 2 points of damage each round."
Two points doesn't sound like much, but a PC will have a hard time outrunning a flying swarm and a few rounds can seriously mess up a low-level character. Not to mention that unarmored types like magic users take double damage!
2) Running away still means taking a few rounds' worth of damage (albeit halved) as the insects already on the character continue to bite or sting as he swats them away.
3) If you damage the swarm, they will pursue relentlessly. Your best bet is diving in water and submerging yourself.
Now, there are several easy ways to deal with swarms. Fire and smoke are among the easiest. Just swinging a sword won't hurt it, but it will help fend it off somewhat. A sleep spell will knock the whole swarm out, but burning a spell slot to deal with normal bugs seems costly.
Despite not having used these as a challenge in my past games, I hope to spring this one on some hapless adventurers in the not too distant future.