Antlion

Antlions live in underground colonies, typically situated in sandy areas such as beaches. They usually hide under the sand, waiting for anything unlucky to cross its path and then quickly surface and tear it apart. Antlions sense a victim's presence via vibrations transmitted through loose soil or sand. This makes it possible to safely venture through Antlion territories by walking solely over foreign items placed on the sand, or by walking across rocks. If a swarm of them are disturbed, temporary refuge may be sought in water, since Antlions cannot swim and quickly drown when they come into contact with it. It is also possible to flip the Antlions over with a burst from the Gravity Gun, allowing for a quick escape. Antlions also possess the ability to fly over short distances. Their aggression is not limited to humans or even to the Combine. When undisturbed by human interference, they will also attack other xenofauna such as headcrabs.

The Combine refer to the Antlions as "biotics." When Gordon Freeman assaults the Nova Prospekt maximum-security prison, bringing many Antlions with him, an Overwatch soldier can be heard mentioning a "full biotic overrun" of the facility. In their attempts to fend off the creatures, another soldier over the radio can be heard saying, "Nomad, get those restrictors back online, your sector shows active biotics."

The behavior of Antlions in respect to sound, vibration and aversion to water is analogous to that of Sandworms in the Dune universe. However, while the use of Thumpers in the Dune universe attracts Sandworms, Antlions in Half-Life 2 are repelled by the strong vibrations produced by them, possibly causing either sensory overload to them, or causing them pain due to their sensitivity to vibrations. Other similar fictional creatures include the Graboids of the film Tremors, which react in a similar way to loud noises such as explosions, and the arachnid warriors of the film version of Starship Troopers.

Antlions are the only enemies in Half-Life 2 that gib rather than becoming ragdolls if killed with a stronger weapon (save for zombies sometimes being blown or cut in half, and occasionally the larger synths).

In the second Episode Two trailer, a new variation of Antlions is seen. These Antlions appear to have a glowing green abdomen, similar to a firefly. Antlions are very protective of these young, shown that they become more aggressive with each death. These Antlions in one teaser trailer seemed to explode with an acidic blood which burned the player.