ARTHROPOD RELATIONSHIPS WITH VIRUSES 23 
together in the same plant, and if this plant is colonized by the common 
cabbage aphis (Brevicoryne brassicae), or the potato aphis (Myzus 
persicae) both viruses are transmitted. If, however, the plant is colonized 
by Myzus ornatus, only one virus is picked up since this particular 
aphis does not transmit the second virus of the complex (Kvicala, 1945). 
In the third instance, selective transmission is brought about by 
varying the length of time the aphis is allowed to feed on the plant, 
in this case a strawberry plant, containing the virus complex. Some 
viruses are best transmitted if the aphis feeds for five minutes only, 
whilst others require a minimum time of feeding of some hours for 
transmission to take place. Such viruses are known as non-persistent 
and persistent viruses respectively and this phenomenon is more fully 
explained in a later paragraph. 
A fourth anomaly arising from the aphis transmission of virus 
complexes may appropriately be considered here, although it is not a 
question of selective transmission by the insect itself. It sometimes, 
though rarely, happens with a virus complex that one of the com- 
ponent viruses is only aphis-transmitted if the other virus is present 
with it in the plant. The best-known example of this phenomenon 
is tobacco rosette, the constituent viruses of which are the mottle and 
vein-distorting viruses respectively, and it is the mottle virus which 
is not aphis-borne unless the vein-distorting virus is present with it in 
the plant (Smith, 1946). 
Multiplication of Viruses in their Vectors 
The evidence which suggests that viruses may multiply inside the 
bodies of insect vectors is stronger in the case of the animal viruses 
than in that of the plant viruses. It has been shown by Merrill and 
Ten Broeck (1934) that the virus of yellow fever multiplies inside 
the body of the mosquito vector and Trager (1938) has actually culti- 
vated the virus of equine encephalomyelitis in the tissues of the 
mosquito. 
It would perhaps be more interesting if it could be proved that 
plant viruses multiply in their insect vectors because that would relate 
them more closely to the animal viruses. There is a certain amount 
of rather circumstantial evidence which supports the multiplication 
theory. For example Black (1941) carried out the following experi- 
ment with the aster yellows virus and its leaf-hopper vector (Cicadula 
sexnotata). He selected a number of insects uniform in size, age, and 
sex, and fed them for a given period on a yellowed aster plant. He 
3—(T.502) 
