The terms instar and stadium are not synonymous. An instar forms at the moment of apolysis or as soon as the 1st layer of a new cuticle is laid down underneath the old one. This period in the insect's life is always initially concealed. A stadium begins at the moment of ecdysis, when the instar emerges from either an egg shell, an old larval cuticle, or a pupal case. It is the period of an arthropod' life between ecdyses. If one knows when apolysis occurred, one can use the term instar specifically to indicate this fact. When the moment of apolysis is unknown, it is better to use the word stage. This simple distinction allows one to avoid an elaborate and clumsy terminology.