Synchronized Swarming

It's a common sight gag in humorous animation that fish, insects and other swarms have an amazing ability to coordinate movement into very exact shapes. (Especially impressive with fireflies in the night.)

This one is interesting because you can see a little bit of how it works. The idea of fish schooling-up to form words is pretty ridiculous, but because we are used to seeing schools of fish performing coordinated movements, we can suppress disbelief easily to enjoy it. Ditto, the flocking of birds, insects, etc.

This is kind of Truth in Television as real life swarms/schools/flocks can move with amazing coordination, and maintain a shape (usually a sphere or some other simple geometrical pattern) even when disturbed.

Common gags include:

Asking a swarm if they've seen something, and the swarm helpfully forms an arrow pointing to the direction it went.
The target of a swarm (usually bees) is hiding, and when they can't find it they form a question mark for no apparent reason except for the benefit of the audience.

When the swarm takes a human shape and is made of invertebrates, it overlaps with The Worm That Walks. When it's bees that are swarming, it overlaps with Gosh Hornet. Expect this to happen if someone disturbs or walks into the Hornet Hole.

Examples of Synchronized Swarming include:

Anime and Manga

Issue 7 of Blame has the main character briefly interacting with a swarm of microorganisms, which assumes various simple forms -- :) for "hello", O for "yes", X for "no", etc—to answer his queries. Justified, in that the swarm is actually controlled by an AI, in a highly futuristic world where this kind of trick is pretty common.
In SD Gundam Force, the Bagu-Bagu swarms do this.
Panty & Stocking with Garterbelt has a swarm of flies forming into a human hand... who gives Chuck the finger, complete with Written Sound Effect "FUCK!"

Film -- Animated

In Finding Nemo, a school of moonfish (voiced by John Ratzenberger) does a series of impressions for Dory and becomes an arrow to give her and Marlin directions to Sydney. Lampshaded when they mention that they've practiced their impressions.
Disney's Alice in Wonderland. While Alice is traveling through the Tulgey Wood she meets a group of mome raths, who form themselves into the shape of an arrow to lead her to a path. See it here on YouTube, starting around 4:50.
In The Princess and the Frog, the fireflies do this.

Film -- Live Action

In the third movie of The Matrix, the sentinel robots make a hand-like shape. Later, robots form a face and it talks to Neo. Probably Justified because there is just that much computing power guiding them.
In Jaws 2, Brody is alarmed by a shape in the water. It turns out to be a school of fish, and the shark is elsewhere. It is not clear to the audience exactly how similar the shape was to a shark.
In The Nutty Professor with Eddie Murphy, the professor proposes to his girlfriend by attracting fireflies into the words "Marry me?" using a synthesized firefly pheromone.

Literature

There was the book Lorenzo by Bernard Waber, where a school of fish looked like a big fish and scared the sharks away. One of them was an odd-colored fish, so it had trouble staying away from predators until the other fish agreed to make it be the eye.
- Swimmy by Leo Lionni is similar.
- Some fish species in real life employ synchronized schooling like this.
The swarm of "snow gnats" in A Series of Unfortunate Events can take on forms like hoops and arrows when attacking people, whom they enjoy stinging for no good reason whatsoever.
Michael Crichton's Prey has ridiculous magic nanobot swarms. They do this, and it's mentioned that their AI was based on insect swarms. By the end of the book, they do it convincingly enough that a swarm can be mistaken for a human being.
The end of A Hat Full of Sky from Discworld features this.
Ygramul from The Neverending Story, which forms a devilish face, a giant scorpion, a fist...

Live-Action TV

On an episode of Pushing Daisies, following a case where a woman says she was attacked by a "terrifying bee man", Ned speculates about being chased by a human-shaped swarm of bees. Turns out it was actually a person covered in bees.

Newspaper Comics

One Krazy Kat strip, where fireflies write "Illekk Krezy Ket" (it's for an election).
One FoxTrot strip showed fireflies writing "HA HA HA HA" to insult Paige.

Video Games

The arcade game 720° has a swarm of bees that chase the player if he hangs around the hub world for too long. The swarm takes on various threatening shapes including a hammer, a skull, an arrow, and a pair of scissors.
The swarms of bees in the Jungle Hut stage of Yoshi's Story form a large open palm to prevent you from passing by them.
The Smorgs in Paper Mario combine like this to form a giant...monster...thingy...
This appears in a level of Logical Journey of the Zoombinis , once a branch with a hive has been angered enough to chase off the nasty Fleens in the shape of scizzors.
- They can also swarm in the shape of an arrow or a storm cloud. They also swarm to heavy metal.
In Evil Genius, bees from the bee hive trap will form a question mark if no targets are in range. When they spot one, they form an exclamation point before attacking.
Realistic schools of fish are found all over Subnautica. Curiously, one game option allows you to turn off their Synchronized Swarming.

Web Comics

Safely Endangered, with Tim and Birds.
In Schlock Mercenary that's how Toughs discovered that the Ot-Skadak are sapient. The team was leaving a camp behind due to giant sea monster killed in self-defence and starting to attract scavengers (the first to arrived were bugs with metallized exoskeletons and assumed to be the same species Toughs have seen killing a creature the size of a gunboat by flying through it like a cloud of flechettes, thus on top of their "carnivores to avoid" list), so Schlock said goodbye to the local landmarks, ending with "goodbye, swarm of bugs" and waved a pseudopod. That's when the swarm of bugs assumed a recognizable shape of Schlock and waved back, leading to Beat Panel.

Western Animation

The episode "Risky Beesness" from Chip 'n Dale Rescue Rangers has a swarm of bees forming not only flying hearts on behalf of the queen, but also stealing musical instruments while hypnotized by a thief playing a mind-controlling tune.
Used all over the place in the Schoolhouse Rock short "Busy Prepositions."
If a character in an old MGM or Warner Bros. cartoon encounters a beehive, it will almost inevitably cause the swarm of bees to change into some familiar shape; arrows, anvils, boxing gloves, even a flyswatter for an ironic twist. Seen it happen to Tom and Sylvester the cats, Daffy Duck, Yosemite Sam, Elmer Fudd, Foghorn Leghorn etc.
One of the Domo shorts on Nicktoons TV has the title character going ice-skating and following a school of fish swimming just beneath the ice.
In the Donald Duck cartoon "Inferior Decorator", Donald gets into a feud with a bee. Toward the end of the cartoon, it calls on all its friends, which swarm out of the hive and form a question mark, as if to say "Yeah, what do you want?"
Lampshaded in an episode of SpongeBob SquarePants. Plankton rallies his extended family to aid him in finally stealing the Krabby Patty secret formula.

Mr. Krabs: Curse you Plankton, and your ability to join together to form a working human ear!

- Also done by jellyfish in the episode "Jellyfish Jam".
Very common sight in The New Adventures of Winnie the Pooh.
One of The Pink Panther cartoons has the title character annoy a swarm of bees. While taking cover inside a house blocks them, the bees take a form of a drill, and create a hole in the door that they fly through.
In Tom and Jerry, the ants that invade Spike's picnic are quite organized, which helps them walk off with the entire food supply...and Spike's son.
One episode of Life with Louie had a scene in which Andy teaches a swarm of bees to spell his name. More conforming to the trope when you consider that he had to correct their spelling after the first attempt ("Andie").
Done in My Little Pony Friendship Is Magic. In the episode "Fall Weather Friends", Rainbow Dash gets chased by a swarm of bees, and they form a "?!" when she manages to hide from them.
- Also done by the Parasprite swarm in "Swarm Of The Century". They attack Rainbow Dash and form things like a beard and a bikini on her body before she shakes them off.

Real Life

starlings, sardines^{[dead link]}, mackerel, bees, siafu, robots.
- Starlings, Flipping the Bird.
Swarming in military strategy is the act of striking at an enemy force with mobile elements which hit and run, perpetually disrupting the enemy and destroying command and control by preventing the enemy from being able to react before the situation has changed. This is not a simple Zerg Rush of throwing troops at an objective in rapid waves, but an extremely complex, difficult to coordinate, and devastatingly effective plan to cause confusion and disorientation.