From Citizendium, the Citizens' Compendium
Arthropod metamorphosis is the process by which arthropods change form before they reach the adulthood.
Also called incomplete metamorphosis, the young resemble adults but are generally smaller and of different proportions. Often they differ in subtle ways, such as lacking wings (cockroaches), or having fewer legs (Myriapoda, spiders).
Complete metamorphosis occurs only in insects. This type of metamorphosis is characterized by a larval stage that is used primarily for eating and growing, during which they are called a grub, maggot, caterpillar, or simply larva. This stage looks entirely different from the adult stage. After the larval stage obtains the necessary amount of energy to metamorphose into the adult form, it pupates into a form called a pupa or chrysalis. After a period of time, the adult emerges.