Sources : Tyliacus

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Serpents 8.43): Tyliacus is a worm, as the Liber rerum says, a snake frequently seen in several parts of the world. The tyliacus worm is so called from the linden [tylia] tree, because it is born within the tree. After this is born it eats first the marrow, and finally the wood itself around and within, until the inner tree is consumed, forcing it to fall. Here, therefore, the serpent is said to grow to a great size and to lie in wait for cattle. But it signifies remorse for mortal sin, even when the judgment of the sin is in doubt. No one, then, commits himself to a crisis of doubt, but letting go of doubt, he rests more confidently on those who are certain. For in that he receives the scruple of doubt, he incurs the certainty of danger, just as the prophet coming from Judea to Bethel to rebuke King Jeroboam was received under doubt with the suggestion he was a false prophet, and therefore incurred the certainty of death by the divine judgment by the lion. Such, therefore, sometimes lingering in doubts, gradually through carelessness begin to fear danger less and less, and as they do not pray against evil doubts, certain dangers fall upon them, and the strength of the mind is destroyed by contempt of doubts, which otherwise could stand secure in certain things. - [Badke translation/paraphrase]