General process of post-hatching growth in insects
While a young insect hatches out from an egg it is covered by a firm, inflexible, sclerotized cuticle, which because of its rigid structure cannot grow along with the increase in length of the hatched larva. So during development, for the hatched larva to attain its adult size and form, the old rigid cuticle at each stage of growth has to be replaced by a newer, larger one by the process of moulting. As per the development is marked off by a series of moults. The interval between the two moults is termed as stadium and the type that an insect assumes as a result of the moults is called instar. All insects undergo various moults after hatching from the egg and before emerging as adults that are called imago.
The number of moults is 4 or 5 in an insect species and is generally fixed or predetermined, but it is not absolutely constant. Furthermore, the type of the insect changes with each moult in a precise pattern characteristic of the species. Moulting is not just a mechanical process; instead at each moult, the moulted insect goes through changes both in its cuticular covering and internal organization. So you can see that development in insects is generally by the process of metamorphosis. The metamorphic - alterations may be slight and gradual or radical as occurring during the pupal stage. During this stage the lama remains quiescent with no feeding or movement, while several larval characters disintegrate and adult structures are formed. Pupal stage follows the last larval molt and from the pupa the imago emerges.