Their epidermal layer secreted a viralsuppressant that could be used as a salve for treating cuts and bruises. They also had a more obscure property: if they were injected with DNA from another organism, the larvae rapidly transformed themselves into clones of that organism that grew, matured, and died in fifteen days. These clones were called "mimetic simbiots" and were practically identical to the original organism, including, in the case of Humans, sharing the original individual's memories.
Although "larvae" is usually the plural form of "larva", the Lyssarian Desert Larvae was consistently called that in the final draft script of "Similitude". The organism's full name was scripted to be said twice. However, the second of those instances ultimately ended up in a deleted scene. Though the only remaining on-screen reference to the lifeform's full name was pronounced as per the script, the pronunciation of the name in the deleted scene calls it a "Lyssarian Desert Larva" instead, differentiating from how the name was formatted in the scripted version of that scene as well as the rest of the episode. (The deleted scene can be found in the ENT Season 3 DVD and Blu-ray.) Similarly, though the name "Lyssarian larvae" was also scripted to be said once, the latter word is pronounced "larva" on screen in that instance too.
In the final draft script of "Similitude", the Lyssarian Desert Larvae was described as "an anemone-like creature the size of a man's fist. Its body is nearly transparent (like some species of tropical fish), the internal organs visible to the naked eye. Light hitting the creature splits into rainbow patterns, like a prism... quite beautiful."
The aforementioned deleted scene from "Similitude" briefly showed a schematic picture of the larvae on a screen.