THE INSECT VIRUSES 383 



for electron microscopy, which also reveals the crystal lattice of the granule, 

 see Fig. 23. 



D. Viruses without Intracellular Inclusions 



According to Wasser (1952), the size of the small virus isolated from 

 Cirphis unipuncta (Haworth), the cosmopolitan army worm, is approximately 

 25 rn.fi in diameter. It is described as a more or less regular, spherical to slightly 

 ovoid body. 



The other virus in this category is the tipula iridescent virus (TIV) and its 

 size and shape have been very carefully studied. In air-dried and frozen- 

 dried preparations the virus appears as a five-or six-sided particle and in 

 sections of the particle itself some degree of nonuniform structure can be seen. 

 There appears to be an outer envelope, inside of which there is a relatively 

 transparent region; the central area of the particle is rilled with opaque 

 material, tentatively identified with its nucleic acid portion. An unexpected 

 observation is the fairly frequent appearance of six-sided contours when the 

 virus particles are seen in section. 



Purified preparations of TIV appear quite monodisperse in the electron 

 microscope, with each virus particle having a diameter of about 130 rn.fi. 

 Even in specimens dried out of a water suspension in the usual way, and hence 

 suffering the distortions brought about by surface tension, the characteristic 

 contour of the particles is six-sided, rather than circular (Fig. 24). This 

 appearance is quite unique among the known viruses of comparable size; 

 vaccinia virus for example, is brick-shaped and nonuniform in size, while 

 influenza virus appears quite circular and heterodisperse. Since the particles 

 of TIV appear to have six sides, it is reasonable to suppose they are polyhedral 

 in external shape. Other cases of polyhedral-shaped virus particles are known 

 (Williams, 1953) among the bacterial and plant viruses. But up to now it has 

 not been possible to arrive at a convincing notion of the exact form of the 

 polyhedron. An indirect approach is to infer the three-dimensional shape of 

 the polyhedra from the shapes of the shadows found by application of the 

 shadow-casting technique (Williams and Wyckoff, 1954). The large size and 

 regular shape of the TIV particles are suitable for the determination of their 

 full polyhedral shape by analysis of shadow shapes (Figs. 25 and 26). This 

 has been done by Williams, who has shown that the virus particles, when 

 frozen-dried and shadowed with azimuth angles 60° apart, cast two shadows, 

 one five-sided and blunt on its end, the other four-sided and pointed. The 

 only polyhedron which will do this is a twenty-sided figure, so that we can 

 conclude that the three-dimensional morphology of one virus is now known 

 (Fig. 25) and that its shape is that of an icosahedron (Smith and Williams, 

 1958). 



