26 AN INTRODUCTION TO THE STUDY OF VIRUSES 
of poliomyelitis and which presumably ingest the virus from con- 
taminated faeces and thus spread it around on food, etc. 
In those cases where a virus is spread by a large number of miscel- 
laneous insects and other arthropods it is legitimate to suppose that 
the transmission is purely mechanical. The virus of fowl-pox for 
instance seems to be carried by any blood-sucking arthropod which 
happens to feed on an infected bird; it apparently adheres to the 
outside of the mouthparts and is said not to enter the body of the 
mosquito. Since the fowl-pox virus is so infectious, a needle stuck 
first into an infected bird and then into a healthy one might serve the 
same purpose. 
In the case of turnip yellow mosaic virus which is spread by many 
kinds of biting insects we have a possible instance of two kinds of 
mechanical transmission. There is the less usual method where virus, 
contaminating the jaws, is transferred directly to the plant tissues; 
more commonly, however, infection apparently takes place from 
virus which has been ingested and is continually being regurgitated 
from the foregut during the feeding process. There is the third 
possibility of plants being infected from the faeces of insects which 
have fed on infected plants, since the virus passes through the body 
without apparent loss of infectivity. 
It would be legitimate to postulate that all insect-transmitted plant 
viruses are transmitted mechanically except in those cases where it 
may in the future be proved that the virus multiplies in the insect. 
The extreme specificity of insect transmission of some viruses could 
be explained on the presence or absence of certain digestive enzymes, . 
pH suitability, location of feeding and so forth without there being a 
fundamental relationship between insect and virus which involves 
multiplication. On the other hand, even multiplication of a virus 
in its insect vector does not necessitate extreme specificity. Both the 
yellow fever virus and that of equine encephalomyelitis are said to 
multiply in their mosquito vectors and yet the former is spread by 
many kinds of mosquito and the latter apparently by such different 
types of arthropod as mosquitoes, blood-sucking bugs, and possibly 
ticks, 
Retention of Viruses by their Vectors: Persistent and Non- 
persistent Viruses 
In the case of such animal viruses as those of yellow fever and 
equine encephalomyelitis where there seems to be a_ biological 
