SOME REPRESENTATIVE VIRUS DISEASES 39 
These viruses attack many different species of larvae but are ex- 
tremely host-specific and instances of a virus from one species attacking 
the larvae of another are rare. 
Among the insects infected with this type of virus are the following: 
Bombyx mori, the silkworm, Lymantria dispar, the gipsy moth, Lymantria 
monacha, the nun moth, Arctia caja, the tiger moth, Abraxas grossulariata, 
the currant moth, Piychopoda seriata, Neodiprion sertifer, the pine sawfly, 
and T. bisselliella, the clothes moth. A description of the disease as it 
develops in A. grossulariata will suffice, since it is fairly typical. External 
signs of the disease appear in an experimentally induced infection after 
about ten days, and usually take the form of a dark discoloration lying 
dorsally near the head; this is followed soon after by a blackening of 
the whole body and death of the larva. Sometimes the skin ruptures 
before death and liberates vast quantities of polyhedral bodies (Plate 
V1). It is an interesting fact that an infected larva continues to feed and 
live an apparently normal life right up till the last stage of the disease 
when the skin becomes discoloured. Sections of infected larvae show 
that advanced disorganization of the internal tissues has taken place 
some time previous to the first external sign of the disease. The poly- 
hedral bodies are first observed in the nuclei of blood cells, hypodermal 
and fat cells which become greatly enlarged and are packed with 
large numbers of the polyhedra (Plate VII). The nuclei swell up, 
becoming three or four times the size of normal nuclei and com- 
pletely filling the cell. If these ees are placed in a weakly 
alkaline solution and observed under the 7'z oil immersion with dark 
eround illumination, it is possible to see ‘the actual virus particles 
moving rapidly about inside the disintegrating polyhedral body. 
Occasionally, impelled by Brownian movement, they pass right 
outside. 
Shortly after death all the tissues disintegrate and it is very difficult 
to lift up a caterpillar which has died from polyhedral disease without 
the skin rupturing and the contents being dispersed. Indeed, each 
time the caterpillar is touched the skin bursts again, so that in the end 
the insect must be scraped up rather than lifted bodily. 
In the silkworm, infected caterpillars in the later stages of the disease 
become sluggish and cease to eat, whilst lemon-yellow patches 
develop in the skin. It is from this yellow colour that the name silk- 
worm jaundice has been derived. Before death the skin assumes an 
opaque, shiny, and yellowish appearance. 
Not very much intensive study has as yet been carried out on the 
4—(T.502) 
