Publisher Summary From the intensive study of insect viruses by several workers during the past decade, it is becoming increasingly clear that insects, like plants and the higher animals, are susceptible to a multiplicity of viruses, of which the polyhedral viruses are only a part. Before the advent of the electron microscope, there existed much confusion of thought regarding the polyhedral diseases. The actual polyhedra were variously considered to be organisms, to contain the virus, and to be crystalline aggregates of the virus itself. It was shown by Komarek and Breindl with the optical microscope and by Bergold with the electron microscope that the second theory was the correct one and that the virus particles were contained within the polyhedral crystals. Weak alkalis dissolve the polyhedra, leaving the virus particles behind inside the membrane that enclosed the crystal. This procedure is, in effect, a microdissection technique but the pH has to be carefully adjusted, otherwise the virus particles, which are themselves susceptible to the effects of alkalis, would also be dissolved. This fact is shown by the difficulty experienced in dissolving the cytoplasmic polyhedra, which have no protective membrane, without also dissolving the virus particles. The polyhedral bodies from the different types of polyhedroses appear to be genuine crystals. They are not, however, nucleoprotein and the arrangement of the virus particles inside the crystal is not regular but haphazard according to the manner, in which they are drawn into the crystal at the time of its formation. These facts differentiate the polyhedra sharply from the plant virus crystals that are nucleoprotein and are composed only of the virus particles themselves. With the knowledge of the insect virus diseases at present, which is admittedly scanty, it can be put in four arbitrary groups as follows; Group I: The polyhedral virus diseases. These are subdivided into (a) nuclear polyhedroses, and (b) cytoplasmic polyhedroses. Group II: The granuloses or capsular virus diseases. Group III: Viruses without intracellular inclusion. Group IV: A miscellaneous collection of apparent viruses that require further study.