370 K. M. SMITH 



Several hundred virus diseases of the polyhedral type have been recorded 

 and about thirty or so granuloses. The number of viruses without accompany- 

 ing intracellular inclusions, so far described, is small but this is probably 

 because of the greater difficulty involved in their diagnosis. There is little 

 doubt that many more insect viruses have yet to be described. 



The distribution of the known viruses in the insect kingdom is very uneven 

 and this is probably more a reflection of our lack of knowledge than a true 

 state of affairs. However this may be, viruses are only known for certain to 

 affect the Lepidoptera, Hymenoptera, and Diptera. None have so far been 

 reported from the Hemiptera which includes the aphids and plant bugs, the 

 Orthoptera containing the grasshoppers and locusts, or the Coleoptera with its 

 forty thousand species of beetles, though a rickettsia-like organism has 

 recently been described in Melolontha sp. (Krieg, 1955). 



It is an interesting fact that in almost every case it is the larval stage only 

 of the insect which is susceptible to infection. Occasionally, caterpillars may 

 pick up the virus in a late instar and complete their development as far as the 

 pupal stage, but the pupae die of the disease. Very rarely the adults may 

 become infected. On one occasion in this laboratory a number of larvae of the 

 privet hawk moth, Sphinx ligustri, were infected with a nuclear polyhedrosis. 

 The majority died but a few survived sufficiently long to pupate; of the 

 resulting pupae one female moth emerged but died shortly after. The tissues 

 of this moth were found to be filled with polyhedra. 



There seems to be only one case where the adult form of an insect is 

 susceptible to a virus but not the larva; this is the so-called bee paralysis where 

 the adult bee is attacked (Burnside, 1933). 



As we shall see later, there is a good deal of variation in the morphology of 

 insect viruses, as is the case with the viruses affecting plants and higher 

 animals, but within each group the morphology is fairly constant. It is 

 interesting that, up-to-date, the viruses which develop in the cell nuclei 

 appear to be all rod-shaped, while those multiplying in the cytoplasm are 

 spherical or near spherical. 



Between them the various types of viruses seem to attack practically all 

 the tissues of the larval insect, and polyhedra have been observed in the skin, 

 tracheae, fat body, blood cells, gut, gonads, muscles, imaginal wing and limb 

 buds, nerve ganglia, Malpighian tubules, and silk glands. 



II. Different Types of Insect Viruses and the Diseases Caused 



A. Polyhedral Viruses 

 1. Nuclear Viruses 



Most of the early work on polyhedral diseases was carried out on the so- 

 called jaundice of Bombyx mori, the silkworm, which is the classic nuclear 



