Objection 1. It would seem that the worm by which the damned are tormented is corporeal, because flesh cannot be tormented by a spiritual worm. Now the flesh of the damned will be tormented by a worm: "He will give fire and worms into their flesh" (Judith 16:21), and: "The vengeance on the flesh of the ungodly is fire and worms" (Sirach 7:19). Therefore that worm will be corporeal.
Objection 2. Further, Augustine says (De Civ. Dei xxi, 9):... "Both, namely fire and worm, will be the punishment of the body." Therefore, etc.
On the contrary, Augustine says (De Civ. Dei xx, 22): "The unquenchable fire and the restless worm in the punishment of the damned are explained in various ways by different persons. Some refer both to the body, some, both to the soul: others refer the fire, in the literal sense, to the body, the worm to the soul metaphorically: and this seems the more probable."
I answer that, After the day of judgment, no animal or mixed body will remain in the renewed world except only the body of man, because the former are not directed to incorruption [Cf. 91, 5], nor after that time will there be generation or corruption. Consequently the worm ascribed to the damned must be understood to be not of a corporeal but of a spiritual nature: and this is the remorse of conscience, which is called a worm because it originates from the corruption of sin, and torments the soul, as a corporeal worm born of corruption torments by gnawing.
Reply to Objection 1. The very souls of the damned are called their flesh for as much as they were subject to the flesh. Or we may reply that the flesh will be tormented by the spiritual worm, according as the afflictions of the soul overflow into the body, both here and hereafter.
Reply to Objection 2. Augustine speaks by way of comparison. For he does not wish to assert absolutely that this worm is material, but that it is better to say that both are to be understood materially, than that both should be understood only in a spiritual sense: for then the damned would suffer no bodily pain. This is clear to anyone that examines the context of his words in this passage.
Source: Summa Theologica (New Advent)