Larvae are the form that evil souls take upon their arrival in the lower planes, assuming that no higher power has (or bothers to) claim them. These semi-sentient souls made manifest are the common petitioners of the evil planes, and the basis of its entire economy.
Additional Limbs: larvae have six small legs, all towards their head. These tiny, segmented joints allow them to crawl along as opposed to merely undulating (though they can do that as well). These appendages cannot be used to fight or manipulate objects at all.
Disease: it's possible that, in addition to inflicting direct damage, the bite of a larvae can expose its victim to a painful wasting disease. Those who fail a Fortitude ACT against this rank value 10 power will lose 1 Health point per day until they die or are treated.
Fangs: though they lack useful appendages, larvae can nonetheless bite anything foolish enough to provoke the nearly mindless creatures. They can wield their wretched mouths to inflict their Brawn value in Slashing damage, causing wounds which bleed until bound.
Stampede: larvae are stupid, but anything riling them up (which is just about anything) can cause them to stampede, which they'll do en masse. This is treated as a charging attack, with each doubling of larvae so stampeding raising the damage +1 RS (maximum of 30).
Limitations / Enhancements:
Unattractive 3: there's no two ways about it, larvae are disturbing to look upon. Even if the creatures could somehow communicate with non-larvae, they would be at a distinct disadvantage while doing so, and NPCs should react to even exceptional larvae at a -6 RS.
Larvae are near-mindless. They seem capable of communicating amongst themselves with strange, instinctual movements, but no one seems able (or cares enough) to translate the meaning of these. They usually lay dormant and quivering until herded, eaten, or otherwise manipulated.
Larvae are horrific. They are five foot long slimy worms, of a sickly yellow pallor. The creatures have six small, segmented legs that allow them to move along slightly faster than a worm's normal undulation. They also possess a humanoid face resembling what they had in their past life.
Larvae are the form that evil petitioners (the souls of deceased mortals) take in the lower planes, assuming that none of the powers claim them for their own realms or uses, instead. Larvae are also the lynchpin of the lower planes' economies.
They are herded up by night hags for sale to whoever wants them, whether the buyers wish to consume them, slay them, or transform them. The former are usually liches and other undead, who can feed upon the larvae with ease (and without hassle from silly heroes).
The latter, however, are usually baatezu and tanar'ri. These races of fiends will collect larvae to evolve them into more of their own kind, though the mortality rate involved is rather staggering. Naturally, this doesn't seem to bother either group very much.
Mind you, larvae can eventually evolve if left to their own devices. Those on Baator can ascend into nupperibos (much to the baatezu's consternation), those in the Abyss can evolve into manes, and those on the Gray Wastes, they can eventually transform into hordelings.