ARTHROPOD RELATIONSHIPS WITH VIRUSES 29 
only the larvae are parasitic on animals. So far as is known, the eggs 
are deposited in the soil; the almost microscopic larvae feed on 
vertebrate hosts of different kinds, become fully engorged and drop 
to the ground. The nymphs and adults live in the soil, but little is 
known of their feeding habits. Since the larval mites do not move 
from host to host, it is clear that the disease agent must pass from 
parent to offspring for any transmission to occur. This has been 
proved experimentally when laboratory-reared larvae from female 
mites collected in the field produced scrub typhus infection in a white 
rat on which they were fed. Another example of virus inheritance 
by mites is shown by the virus of St. Louis encephalitis which is 
transmitted through the eggs of the chicken mite Dermanyssus gallinae. 
Syverton and Berry (1941) have demonstrated that the wood tick, 
Dermacentor andersoni Stiles, can acquire the virus of the western type 
of equine encephalomyelitis, pass it to later stages in its life-cycle and 
to its progeny and transmit it. Ticks of the genus Dermacentor have 
essentially the same seasonal occurrence and geographical distribution 
as the equine encephalomyelitis virus and their habits make them 
almost ideal vectors or reservoirs of the virus. 
The tick-transmission of viruses may be rather complicated; it 
has been shown by Daubney and Hudson (1931) that infection with 
Nairobi sheep disease virus of any instar of Rhipicephalus appendiculatus 
results in transmission of the virus by the succeeding stage which then 
loses infection at the next moult. A female tick, infected as an adult, 
thus only passes infection to the eggs. 
Nature of Relationship between Viruses and Vectors 
It has already been suggested in this chapter that, so far as the insect- 
transmitted plant viruses are concerned, it may be legitimate to regard 
them all as examples of mechanical transmission in the sense that 
there is no direct biological relationship between virus and vector, 
such as a cycle of development inside the insect. What happens, then, 
to the virus which is swallowed by the insect? In the case of persistent 
viruses with sap-sucking insect vectors presumably something of this 
sort takes place. After the virus has been swallowed by the aphis, it can- 
not be regurgitated, since there is an oesophageal valve which prevents 
this, but must travel on down the alimentary canal (Plate I). Now in 
the case of a sap-sucking insect, the only way a virus can reach the 
plant again is via the saliva and in order to reach the salivary glands it 
must presumably pass through the wall of the alimentary canal into 
