Isidore of Seville [7th century CE] (Etymologies, Book 12, 3:10): The ant-lion is so called either because it is equally lion and ant, or because it is the lion of ants. It is a small animal that is hostile to ants; it hides in the sand and kills other ants that are carrying grain. In this way it is like a lion to ants though it is like an ant to other animals.

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Worms 9.23): The lion of the ants, which is also called myrmicoleon, is explained by Andelmus from myrmin, which is an ant, and leon, which is a lion, as 'the lion of ants'. This worm is of the genus of ants, but much larger. While it is still small and has little strength, it feigns peace and modesty. But when it has increased its strength, robustness, and greatness of body, it despises its former associations and now accompanies the company of its ancestors. Then, gaining boldness, it hides itself in the secret places, and like a predator it ambushes the ants working for common purposes, and, like a robber, it snatches their burdens or even strangles and eats the ants themselves. But in the winter, the ants have stored food as if in a barn, but the myrmicoleon, which had prepared nothing for itself in the summer for use in the winter, plunders their labors and destroys them. This type of animal symbolizes hypocrisy and deceit in cloisters. - [Badke translation/paraphrase]

Albertus Magnus [ca. 1200-1280 CE] (De animalibus, Book 26, 20): Ant-lion is called the "lion of the ants" and also by another name, "murmycaleon". Despite what some people claim, this creature is not an ant during the first stages of its development. On many occasions I have observed this insect and demonstrated to associates that it resembles a tick and hides by digging a hemispherical hole in the gravel, where the upper pole of the cavity is the mouth of the ant-lion itself. As ants pass over the hole in search of provender, the insect captures and devours them; we watched this drama reenacted time and time again. In the winter the ant-lion is reputed to plunder the food stored by ants because it gathers nothing for itself during the summer. - [Scanlan]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book18.53): Formicaleon, that beast hath ye name of Formica, and of Leo, for as Isidore sayth, libro. 12.sayth, libro. 12. It is a beast with the lykenesse of an Ant, and of a Lyon, and is a little beast, and enimy to Ants: for he cometh théevishly into their place of purveiance, and eateth their corne, and so by wasting of theyr meate he is cause why the simple Antes dye néedes at last: and this Formicaleon is eaten of other beasts, as Antes bée, and may not defend himselfe by his owne strength, & is a maner kinde of Spider. Looke before de Aranea in littera A. - [Batman]