In insects, "holometabolism" is the term for insects that pass through a larva stage that is clearly different in form than their adult form. Holometabolist insects hatch into larva, which are often, but not always, worm-like in form, and then go through a pupa phase, before emerging as radically different adults. Some of the most successful insect orders, in terms of speciation, biomass and effect on their environment, are holometabolist. These include beetles, flies, butteries and moths and ants and bees.
Holometabolist insects are more successful, on the whole, than the hemimetabolist insects, despite the fact that they need to invest more energy into growth and development. In the case of many of the hymenoptera (ants, bees and wasps), the young are also extremely altricial, only surviving through group effort. There is no single explanation into why holometabolism is so successful, although most theories seem to revolve around the insects having different, specialized forms for feeding/growing, and mating/dispersal. The origin of holometabolism is also not totally clear, since insects fossilize poorly, and the split between hemimetabolist and holometabolist insects probably occurred sometime in the Permian, when the world had a much different ecology than it does today.