30 AN INTRODUCTION TO THE STUDY OF VIRUSES 
the blood. Some evidence for this sequence of happenings is given 
by the work of Storey (1932). Working with the streak disease of 
maize which has a specific leaf-hopper vector, he found that these 
insects could be divided into two races, one which could transmit the 
virus, termed active, and one which could not, termed inactive. Both 
races appeared identical and were undoubtedly only one species. 
Now both the active and inactive races of this leaf-hopper ingest the 
maize streak virus and pass it out with the faeces where its presence 
can be ascertained, but the virus is present in the blood only of the 
active transmitting race. If, however, a puncture is made with a 
sterile needle in the inactive insect’s abdomen in such a way as to 
rupture the abdomen, the insect will then behave like the active type 
and can transmit the virus. The physiological basis of inactivity thus 
lies in some property of the intestine that prevents the outward 
movement of virus through its wall. Storey (1939) goes farther and 
suggests that in this insect the protoplasm of the cells forming the 
intestinal wall inherits a susceptibility or resistance to invasion by the 
virus. 
A very interesting parallel to this phenomenon is found in an animal 
virus, that of equine encephalomyelitis and its insect vector, the 
mosquito Aédes aegypti. It will be remembered that there seems to 
be a definite relationship between these two since the virus is known 
to multiply inside the mosquito (Merrill and Ten Broeck, 1934). 
Two strains of this virus exist, the “‘eastern’” and the “western,” of 
which only the western strain is normally transmissible by the mosquito. 
Merrill and Ten Broeck showed (1935) that the mosquito could be: 
induced to transmit the eastern strain if it received virus by inoculation 
into the abdomen, or if it were punctured in the abdomen after a virus 
feed. In this case it appears as if it was the virus itself which determined 
activity or inactivity and suggests a closer biological relationship 
between the virus and the insect’s protoplasm than seems to exist in 
the case of the maize streak virus and its insect vector. 
For the transmission of a plant virus by a sap-sucking insect, it is 
therefore a fundamental necessity that the virus must pass through the 
wall of the alimentary canal and enter the blood if it is to reach the 
exterior by way of the saliva. But for the transmission of the turnip 
yellow mosaic virus by beetles and some other biting insects, this 
condition does not apply. We know that the virus passes down the 
alimentary canal and can be recovered, still highly infective, in the 
faeces but we do not yet know if it reaches the blood. However, this 
