It's interesting to me how games handle swarms of things: bugs, piranhas, spiders, birds, and so on. They're a classic threat, but the rules to support them vary widely between systems. Here's what's generally the same:

* They fill an area like a bigger creature. Even though individual swarm members are really small, the swarm itself is the size of a large creature (sometimes, its squares can bend around a lot, so long as they're contiguous).

* They can fit through small spaces, and other creatures can fit in their space along with them (though they might get bitten/stung to do so!).

* There are too many of them to count. This generally makes them immune to single-target spells like disintegrate, and sometimes makes them more susceptible to area-effect spells like fireball. 

Different game systems have different rules for swarms.

D&D 5E lists the "size" as something like "Medium swarm of Tiny beasts" to let you know how small the swarm's critters are and how big the swarm itself is. Swarms also get a "swarm" trait that usually reads the same: "The swarm can occupy another creature's space and vice versa, and the swarm can move through any openings large enough for a Tiny creature. The swarm can't regain hit points or gain temporary hit points." They're resistant to physical damage (bludgeoning, piercing, and slashing attacks) and immune to lots of conditions that would usually affect a single target, like being charmed or paralyzed or knocked prone. A swarm attacks like a normal creature, but does less damage if reduced to half its hit points or fewer (to reflect the fewer number of viable creatures in the swarm, presumably). All the rules are right there in the monster stat block.

Pathfinder First Edition has...problems. "Swarm" is a subtype, and although the stat blocks of most swarms aren't large, the description of the subtype is almost a full page of text. That's a ludicrous amount of detail, and it usually means going back to the book when running a swarm. Swarms are resistant to weapon attacks, but some swarms are wholly immune to weapon damage. Some of these very low-CR monsters, which has spelled death for many characters and led to a paranoid fixation on grabbing splash weapons (like alchemist's fire) as soon as possible in an adventurer's career. That doesn't seem good for the game.

Starfinder has some of the same architecture as Pathfinder 1st Edition, but it breaks up the pieces into "Swarm Attack," "Swarm Defenses," and "Swarm Immunities," which seems a little easier in play--you don't have to reference a full page, just the relevant section (attack when it's the swarm's turn, defenses and immunities when the heroes are trying to do something to the swarm). It's still a full column of text in total, though. There are also more rules for a swarm subtype graft, which loops in lots of the complications from Pathfinder First Edition.

Pathfinder Second Edition makes things a little easier.  Swarm is a trait, but it's only three sentences long: "A swarm is a mass or cloud of creatures that functions as one monster. Its size entry gives the size of the entire mass, though for most swarms the individual creatures that make up that mass are Tiny. A swarm can occupy the same space as other creatures, and must do so in order to use its damaging action. A swarm typically has weakness to effects that deal damage over an area (like area spells and splash weapons)." Swarms usually have a "swarm mind" ability, making them immune to mental effects that target a specific number of creatures. They're usually immune to precision damage, have a weakness to area damage and splash damage (as the trait recommends), and have resistance to physical damage (though resistance to bludgeoning damage is usually lower, presumably because it's easier to smash up swarm critters than stab them). I wish "swarm mind" was right in the stat block--it's not that hard to say "immune to mental effects that target a specific number of creatures"--because this would then almost be as easy to use in play as D&D 5E.

Torg Eternity has "swarm" as one of its few creature special abilities: "Swarms are composed of hordes of smaller, individual creatures. Eliminating one, or even dozens has no real effect on the swarm as a whole. Unarmed and crushing weapons deal base damage normally. Area attacks deal damage normally. Blades, bullets, or weapons that can't hit dozens of creatures at once deal no damage at all. Each round, any character in contact with the swarm on its turn suffers two Shock. Large swarms have +5 Shock and +2 Wounds." Actual swarm monsters give this in shorthand, although they don't always follow these rules, making it puzzling why it would be set out with such specificity as a special ability if it was deployed differently in practice.

That's what swarms are all about; I'll talk next time about applying these rules to make lots of other swarm creatures--swarms of swarms, you could say.