THE INSECT VIRUSES 379 



that they rupture, liberating a mass of granules into the cytoplasm of the 

 ceU (Fig. 15) (Smith and Rivers, 1956). 



C. Viruses without Intracellular Inclusions 



As mentioned earlier, the number of viruses so far discovered, occurring 

 freely without crystalline inclusions in the tissues of insects, is not large. 

 There are two in which the virus itself has been well-characterized on the 

 electron microscope, one attacking a lepidopterous larva, Cirphis unipuncta 

 Haworth, and the other infecting the dipterous larva, Tipula paludosa. 



Larvae of C. unipuncta infected in late third instar appear swollen and 

 somewhat darker than normal insects. The cuticle of the diseased larvae has 

 a waxy appearance, and in some cases the mid-portion is slightly enlarged. 

 The liquefaction and disintegration characteristic of the nuclear polyhidrosis 

 of the same insect are absent. Deaths from the disease may occur in either the 

 larval or pupal stage. As the disease progresses, the Cirphis larvae become 

 sluggish, are soon eating little, and gradually succumb, usually within 6 to 14 

 days following infection (Wasser, 1952). There seems to be no information as 

 to the site of multiplication of this virus in the insect's body or whether it is 

 nuclear or cytoplasmic in origin. By the analogy of shape, however, this virus, 

 which is very small and apparently spherical, should multiply in the cell 

 cytoplasm. 



The second virus of this type, which attacks the larva of the crane fly, 

 T. paludosa, is most unusual in many ways and is of great scientific interest. 

 It was first discovered by the Virus Research Unit at Cambridge in 1954 and 

 was later briefly described (Xeros, 1954; Smith, 1954). 



The change induced in infected larvae is very striking and enables the 

 disease to be diagnosed with ease. The normal color of these larvae, which are 

 known in England as "leather jackets," is a brownish gray, while the diseased 

 specimens exhibit a somewhat opalescent blue-indigo. This color is rendered 

 more striking if the insect is placed in a test tube with moistened sides and 

 viewed through the glass. The site of multiplication of the virus is in the cyto- 

 plasm of the fat body cells and it is here that the blue color originates. When 

 sections of the diseased fat body are viewed in the electron microscope, it is 

 seen that the cells are filled with the darkly staining virus particles. No virus 

 occurs in the cell nuclei and there are no polyhedra such as would be present in 

 a cytoplasmic polyhedrosis. The amount of virus in the fat body is very great 

 and the blue or violet color is due to the optical effect given by the virus, 

 which actually begins to crystallize in the living insect. The quantity of virus 

 produced by each larva is extremely high and measurements suggest that one- 

 quarter of the dry body weight is converted into virus particles. Pellets 

 formed from this virus, which is known as the tipula iridescent virus (TIV), 



