The sarcity of positively associated stages of adults of both sexes and the remarkably little variation shown by the adults of a very few species studied have naturally led to the recognition of the so-called pupal cases, the exuviae of the IV instar nymphs in the matter of generic and specific classification of the aleyrodids—a rather unusual procedure in the taxonomic study of these insects. This stage is the one most often collected and known in the species world over and also possesses a multitude of diagnostic features than in any other stage.