ARTHROPOD RELATIONSHIPS WITH VIRUSES 21 
to transmit only one rickettsial disease, scrub typhus or tsutsugamushi 
disease. Transmission of endemic typhus by mites has been demon- 
strated experimentally. 
The vectors cf epidemic or European typhus and of endemic or 
murine typhus are lice and fleas respectively (Kohls, 1947). 
Specificity of Vectors for Viruses and Selective Transmission 
The view was largely held at one time that many insect-transmitted 
viruses could be spread only by one species of insect. Now it is realized 
that this species-specificity is rather rare and that a group-specificity 
is more usual. For example, there are several plant viruses which are 
spread by more than one species of aphis and several which are spread 
by more than one species of leaf-hopper but, so far as we know, no 
aphis-borne virus can be transmitted by a leaf-hopper, nor can a virus 
which is transmitted by a leaf-hopper be carried by an aphis. There 
is one case, however, in which a virus transmitted by a beetle can 
also be carried by an aphis. The virus in question is that of squash 
mosaic (Freitag, 1941). 
This group-specificity of arthropod vectors also exists with animal 
viruses but does seem to break down in some cases. The virus of 
dengue appears to be carried only by mosquitoes but that of equine 
encephalomyelitis, another mosquito-borne virus, has been found in 
a tick and in a blood-sucking bug (Hemiptera). 
Again, the virus of yellow fever can be transmitted by fourteen 
different species of mosquito and also, according to the literature, 
by quite a different type of blood-sucking fly belonging to the 
Muscidae. In addition there are records of the yellow fever virus 
being carried by blood-sucking bugs (Hemiptera) and by ticks 
(Acarina). These last cases are probably examples of mechanical 
transmission and may be purely incidental. 
Among the arthropod vectors of animal viruses, the two most 
important groups are the Culicine mosquitoes and the ticks. Findlay 
(1936) points out the interesting fact that out of the great mosquito 
family Culicidae, the tribe Culicini alone transmits viruses, the equally 
numerous tribe Anophelini being apparently unable to do so. On the 
other hand, the Anophelini comprise all the carriers of malarial protozoa 
with the exception of certain species of the genus Culex which transmit 
the organism of bird malaria. 
Examples of the species-specificity referred to above, where only 
one species of insect can transmit a virus, are found in the insect 
