CHAPTER III 
ARTHROPOD RELATIONSHIPS WITH VIRUSES 
Tue relationship of viruses with the insects and other arthropods which 
transmit them is an interesting and important subject, and one which 
is far from being clearly understood. We do, however, know a great 
many facts about this relationship and they are discussed, under 
various headings, in this chapter. 
Whereas the number of plant viruses which depend upon insects 
for their dissemination in the field is much greater than is the case 
with animal viruses, it seems that the relationship between virus and 
insect is closer with animal than with plant viruses. This is shown by 
the apparent multiplication of certain animal viruses in their insect 
vectors, evidence for which is more easily obtainable and more 
convincing than is the case with the insect vectors of plant viruses. 
Different Types of Arthropod Vectors 
As would be expected, the great majority of insect vectors transmit 
the virus which they carry to the susceptible host during the process 
of feeding upon that host. There are, however, one or two exceptions 
to this general rule, such as the apparently mechanical transmission of 
poliomyelitis virus by non-blood-sucking flies and the possible 
transmission of trench fever in the excrement of lice rather than by 
their blood-sucking habits. Occasional spread of the virus of turnip 
yellow mosaic by means of the faeces of the insect vector may also 
occur, since passage of the alimentary canal of the insect seems to 
have little effect on the infectivity of the virus. 
The sucking insect, whether its food be blood or sap, is obviously 
well adapted for transmitting viruses, which are mostly injected into 
the host together with the saliva. In the case of the animal viruses, 
the arthropod vectors are, of course, all blood suckers, and although 
there are differences in the mouth parts, those of the mosquito are 
perhaps the most typical. Insects which feed on plants, however, 
can be divided into two categories: biting insects such as caterpillars 
and beetles, which actually eat the leaves and ingest the tissue, and 
sucking insects such as aphides, leaf-hoppers, whiteflies, etc., which 
feed only on the sap. The vast majority of the insect vectors of plant 
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