It is generally admitted that the primitive position of the Arthropod mouth and its associated appendages is ventral, this position being evident in the early embryonic stages of all groups and retained in the adult stages of many forms, particularly among the Crustacea. It is by no means to be inferred from this, however, that this ventral position of the mouth is the primitive one in the Insecta as a class. Great variability exists among insects in the postembryonic stages in respect of this feature. The mouth and jaws are usually directed more or less forward and downward but vary from a wholly anterior position to a definitely ventral one, or they may even be directed posteriorly.