At successive moults the somites increase in number, new somites being added behind those already differentiated, from a formative zone in front of the telsonic region.
Certain races moult or cast their skin three times during their larval existence, but for the most part the silkworm moults four times - about the sixth, tenth, fifteenth and twenty-third days after hatching.
After several moults the rudiments of two pairs of wings appear, and then the insect creeps up to the surface of the earth, and on to the vine.
Both agree in having nothing that can be termed a metamorphosis; they are active from the time of their exit from the egg to their death, gradually increasing in size, and undergoing several moults or changes of skin.
Lubbock traced at least twenty moults in Cloeon; at about the tenth rudiments of the wingcases began to appear.