THE INSECT VIRUSES 381 



B. mori and Abraxas grossulariata. In sections of B. mori infected with its 

 nuclear polyhedrosis, extremely long virus rods are sometimes observed of a 

 size never seen among rods obtained from polyhedra. Similar long rods, which 

 appeared to be breaking up into two more normal-sized rods in the nuclear 

 ring zone, have also been observed. 



Bergold (1950) and Bird (1952) have described V-shaped forms which, in 

 the writer's opinion, consist of either two half-length rods within one capsule 

 or two bundles of half-length rods, each bundle within its own inner capsule 

 and the two bundles lying alongside each other at an angle (Smith and Xeros, 

 1954c). 



As regards the outer membranes and capsules of the nuclear polyhedral 

 viruses, there is, first, the intimate membrane, which actually holds the 

 constituent protein and nucleic acid components of the virus. Then comes the 

 inner capsule, which is acquired by the virus rod after its formation and after 

 it has been liberated into the nuclear ring zone, (Smith and Xeros 1954a). 

 Finally, there is the polyhedral crystal with its own encircling membrane. 



In the rather atypical nuclear polyhedrosis of Tipula paludosa, the inner 

 capsule behaves in a manner quite different from other nuclear polyhedroses. 

 During the formation of the polyhedral crystal, the virus rod without its 

 inner capsule can be seen in the center of a large vesicle (Fig. 17). This vesicle 

 may be the greatly enlarged capsule, as suggested by Xeros (1957), connected 

 with the peculiar elasticity of the polyhedral crystals. On the other hand, 

 sections of the virus particles in the insect's tissues (Fig. 18) sometimes 

 reveal a somewhat similar dilation of the capsule in the absence of polyhedral 

 bodies. 



B. Cytoplasmic Polyhedral Viruses 



Although there is some variation in the length of virus rods in the nuclear 

 polyhedroses, this does not approach the degree of variation in morphology 

 which occurs in the cytoplasmic polyhedral viruses. The virus particles from 

 three separate cytoplasmic diseases are briefly described to illustrate this 

 diversity of form. The difficulty of isolating some of the cytoplasmic viruses 

 from their enclosing polyhedra has been previously commented upon (Smith 

 and Xeros, 1954c), so it is less easy to make a comparative survey of the virus 

 morphology of this group than with the nuclear polyhedral viruses. 



The first example is a cytoplasmic polyhedrosis of the larva of Sphinx 

 ligustri, the privet hawk moth. At first, owing to the difficulty of isolating this 

 virus, thin sections were cut of the polyhedra and observed on the electron 

 microscope. As can be seen from Fig. 10, it is difficult to observe virus 

 particles in the sections of the polyhedra. However, after some experimenting 

 with the polyhedra both in bulk and on the electron microscope grid, using 

 very weak alkali for very short periods, the virus was eventually isolated. It 



