20 AN INTRODUCTION TO THE STUDY OF VIRUSES 
viruses fall into the latter category, although there are one or two 
authentic instances of plant virus transmission by biting insects. 
The mechanics of inoculation by a sap-sucking insect are briefly as 
follows: there are two pairs of very fine stylets, mandibles, and maxillae, 
interlocking on their inner faces, which fit closely together. Two 
pairs of grooves in the faces of the stylets thus come together and form 
two channels up one of which flows the sap of the plant, while the 
saliva ows down the other. These stylets are contained in an outer 
beak or rostrum which acts as a supporting structure for the stylets 
and does not enter the plant at all. When the insect presses on the leaf 
surface with its beak (labium), the latter bends and becomes fore- 
shortened, whilst the stylets enter the plant. The fact that the labium 
has this dorsal opening allows the stylets to become free over part of 
their length during the process of feeding (Plate I, upper photograph). 
In the case of one plant virus, that of tomato spotted wilt, the 
vector, a thrips, has slightly different mouth parts. They consist of 
a hollow cone, in which work the maxillae and one mandible, the 
other being vestigial and non-functional. The feeding motion 
consists of a kind of pickaxe movement with the mandible which 
rasps off the epidermis, the sap being sucked up as it is liberated. In 
this insect, therefore, there is not the deep penetration of the plant 
tissue during feeding which is characteristic of the feeding of such 
insects as aphides and leaf-hoppers. 
Finally we have the biting insects such as the mustard beetle (Phaedon 
cochleariae) and its larvae and the flea beetles, Phyllotreta spp., which 
transmit the virus of turnip yellow mosaic. Here the mouth parts. 
are entirely different and consist essentially of a pair of strong, chiti- 
nized jaws working horizontally in a scissor-like action. The mechanics 
of infection with this type of insect are thought to be of two kinds, 
first a direct transmission of virus which contaminates the mandibles; 
this is a rather rare method of infection. The second kind is a some- 
what delayed form of transmission and is thought to be brought 
about through regurgitation of part of the contents of the foregut 
during feeding, whereby some of the virus already ingested comes 
into contact with the leaves of healthy plants. 
The rickettsial diseases of man are transmitted by four groups of 
parasites, ticks, mites, lice, and fleas. 
Diseases of the spotted fever and boutonneuse fever groups are 
transmitted by ticks and it is thought highly possible that ticks are also 
concerned in the transmission of “Q” fever. Mites have been proved 
