The Collaborative International Dictionary of English v.0.48:

Larva \Lar"va\ (l[aum]r"v[.a]), n.; pl. L. Larv[ae]
   (l[aum]r"v[ae]), E. Larvas (l[aum]r"v[.a]z). [L. larva
   ghost, specter, mask.]
   1. (Zool.) Any young insect from the time that it hatches
      from the egg until it becomes a pupa, or chrysalis. During
      this time it usually molts several times, and may change
      its form or color each time. The larv[ae] of many insects
      are much like the adults in form and habits, but have no
      trace of wings, the rudimentary wings appearing only in
      the pupa stage. In other groups of insects the larv[ae]
      are totally unlike the parents in structure and habits,
      and are called caterpillars, grubs, maggots, etc.
      [1913 Webster]

   2. (Zool.) The early, immature form of any animal when more
      or less of a metamorphosis takes place, before the
      assumption of the mature shape.
      [1913 Webster]

WordNet (r) 3.0 (2006):

larva
    n 1: the immature free-living form of most invertebrates and
         amphibians and fish which at hatching from the egg is
         fundamentally unlike its parent and must metamorphose

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "larva":
   Anlage, Masan, apparition, appearance, arachnid, arthropod, astral,
   astral spirit, aurelia, banshee, beetle, blastula, bug,
   caterpillar, centipede, chilopod, chrysalis, cocoon, control,
   daddy longlegs, departed spirit, diplopod, disembodied spirit,
   duppy, dybbuk, eidolon, embryo, fetus, fly, form, germ, germen,
   ghost, grateful dead, grub, guide, hant, harvestman, haunt,
   hexapod, idolum, immateriality, incorporeal, incorporeal being,
   incorporeity, insect, lemures, maggot, manes, materialization,
   millepede, millipede, mite, nymph, nympha, oni, phantasm,
   phantasma, phantom, poltergeist, presence, pupa, revenant,
   rudiment, scorpion, shade, shadow, shape, shrouded spirit, specter,
   spectral ghost, spider, spirit, spook, sprite, tarantula,
   theophany, tick, unsubstantiality, vision, walking dead man,
   wandering soul, wiggler, wraith, wriggler, zombie, zygote