ARTHROPOD RELATIONSHIPS WITH VIRUSES 27 
relationship between virus and insect, in that the virus can multiply 
inside the vector, it is perhaps to be expected that the insect should 
retain the virus for long periods, frequently for the rest of its life 
without recourse to a fresh source of virus. In the case of a virus like 
that of fowl-pox which may be carried mechanically on the insect’s 
mouth-parts, it is reasonable to suppose that infectivity is rapidly lost 
by the insect. 
So far as the plant viruses are concerned, we can divide those which 
are insect-transmitted into two classes, although there is no hard and 
fast dividing line between them. In a series of progressive 24-hour 
transfers of infected insects from plant to plant, there is one class of 
viruses which is not carried beyond the first plant and is thus rapidly 
lost by its insect vector. This type of virus is mostly aphis-transmitted 
and has been named non-persistent (Watson and Roberts, 1939). In a 
similar series of 24-hour transfers, the second type of virus, called 
persistent, is not as a rule transmitted to the first plant of the series, but 
is transmitted to the succeeding plants and is usually retained by the 
insect for long periods if not for the rest of its life. Intermediate 
between these two types is the virus of turnip yellow mosaic previously 
mentioned which seems to be partly non-persistent and partly per- 
sistent. Occasionally the virus is transmitted in what seems to be a 
purely mechanical method, by adhering to the mouthparts of the 
beetle vector; this might be termed non-persistent. On the other 
hand, the vector can retain the virus for several days and continue to 
infect plants in progressive series; this is the persistent phase and can 
be accounted for by regurgitation at intervals of infected material 
from the foregut of the insect during mastication of the leaf. 
When the virus passes into the body of the insect vector and is 
rapidly lost it is reasonable to suppose that digestive enzymes in the 
insect’s body inactivate the virus. This supposition is supported by 
the fact that continuous feeding of the aphis vector, whether on a 
healthy or infected plant, greatly reduces the efficiency of the vector, 
whilst a period of starving before feeding on a source of virus followed 
by an extremely short—about two minutes—feed on the healthy 
plant are the optimum conditions for infection. Starving the insect 
after feeding on the source of virus also prolongs the period during 
which the aphis is infective. All these facts support the suggestion 
that non-persistent viruses are rapidly digested in the insect’s body; 
starving both before and after feeding would presumably reduce 
the flow of digestive enzymes whereas long periods of feeding, either 
