336 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1940 



The insect feeds on a virus-diseased plant and moves over to a healthy 

 plant, at which point one of two thing may happen. The insect may 

 inoculate the plant with the virus at once, or a period of time varying 

 from a few hours to many days may elapse before the insect is capable 

 of transmitting the virus. In either case there is a period of develop- 

 ment in the plant before disease symptoms occur. 



Generally speaking the insects capable of transmitting a virus with- 

 out any delay are the aphis or plant lice and those that transmit viruses 

 only after retaining them in their bodies for a long period are the 

 leafhoppers and thrips. There is a certain compensation, however, 

 for the long delay, since the aphis can usually only inoculate one plant, 

 whereas the leafhoppers and thrips can inoculate a long series of 

 plants, often throughout their remaining life. 



The period of time which the virus must spend in the insect has been 

 called the incubation period because it was assumed that during that 

 period the virus developed, i. e., reproduced to a point where it could 

 be injected by the insect in sufficient quantity to establish the virus 

 in the plant. Recent work has challenged this point of view however, 

 and it is doubtful whether many viruses multiply in the insects' bodies 

 at any time. 



The matter has by no means been settled, however, since much of 

 the evidence for multiplication has not been satisfactorily explained 

 away. There is, for example, a virus disease of rice in Japan which 

 is transmitted by a leafhopper. This virus passes through the eggs 

 of the virus-carrying female, and experiments have shown that leaf- 

 hoppers can thus acquire the virus from their maternal parents. A 

 family of these leafhoppers can therefore be carrying the virus even 

 though their grandparents were the original feeders on the diseased 

 plant. This is considered real evidence for multiplication because 

 otherwise the original quantity of virus would have been diluted to a 

 degree which no known virus can stand and yet remain capable of 

 infecting a plant. 



The path of the virus in the insect's body is, of course, only a matter 

 of surmise although there is experimental evidence that it enters 

 through the mouth and passes to the midintestine and out through 

 the wall of the midintestine to the blood, in which it is carried to the 

 salivary glands, whence it passes out with the salivary secretion 

 injected by the feeding insect. 



It is a matter of great interest to learn just where in the insect's 

 body the virus is stored, for if it does not multiply in the insect, it 

 must be held in some tissue from which it is gradually released. The 

 best evidence thus far is that the blood is the reservoir for the virus. 

 Certainly viruses have been found in insect blood, for by employing 

 ingenious techniques blood has been removed from virus-bearing in- 



