Antlions are social insectoid creatures that appear as enemies in Half-Life 2 and its episodic sequels. They are large creatures that dwell underground and operate under a hierarchy similar to Earth ants.
It is not known where Antlions originate from, though it is known that they do not come from Xen. It is strongly suspected that they may come from the same planet/dimension as the Vortigaunt. Some Vortigaunt mention to Gordon Freeman that they once farmed Antlions for their larval extract and that Antlion husbandry was once an ancestral practice of theirs.
Antlions were among the various alien fauna brought to Earth during the Portal Storms that occurred after the Black Mesa Incident. They appeared in large numbers and multiplied exponentially afterwards, likely causing the extinction of many of Earth's native fauna.
Antlion Soldiers are the most visible element of Antlion society, most of which is kept hidden in fiercely defended underground nests. It is not clear how Antlions breed; one resistance member is heard referring to a "spawning season" at the beginning of the Highway 17 chapter, but whether Antlions mate separately, have a queen/drone system or practice something completely alien to Earth sensibilities is not known. Their young begin life as larvae in blue honeycombs similar to those of bees.
The extract of these larvae is highly prized by the Vortigaunt for its ability to "divide the false veils of the vortessence" when imbibed. This seems to augment their natural powers. It should be noted that when they imbibe the extract, they glow purple and look very similar to how they did in the beginning of Episode One. It may be speculated that they were in this state when rescuing Gordon.
The larvae grow into large grubs, which are attended to and protected by Antlion Workers and Antlion Guardians. Within this system, common Antlions presumably serve as hunter-gatherers. Inside their nests, decomposed bodies of other creatures can often be discovered, trapped in the viscous silk the grubs produce as a food source, and some of their deepest chambers are pooled with blood.
The grubs also produce a substance that possesses healing properties.
Barnacles appear to be one of the few effective predators for Antlions, as they are commonly found in Antlion cave systems. The Antlions, being blind, are often unable to detect their 'tongues,' and as a result become easy meals. They also serve, however, to deal with any invaders hiding in the water, as Barnacles often hang over pools of water.