662 
DIPTERA. 
the practical outcome is seen in the growing body of knowledge relating 
to the transmission and spread of disease among men and cattle. There 
are two ways in which insects may carry the “ germs ” of disease from 
one place to another. They may alight upon the excrement of diseased 
persons or animals, or upon sores on the body or on any other infective 
matter, and may then convey the infection elsewhere on their contami¬ 
nated bodies or in their excreta. The transmission in this case is purely 
mechanical and it is immaterial by what kind of insect it is effected, 
though owing to the nature of their habits it is the Diptera which are 
chiefly concerned. It is not however in this connexion that the chief 
importance of blood-sucking insects lies, but rather in the part they play 
in the propagation of diseases which are due to the presence of certain 
microscopic parasites in the blood. It seems that in general these para¬ 
sites can infect a healthy animal only by being directly introduced into 
its blood, and in the absence of blood-sucking insects it is difficult to see 
how this could very often occur : on the other hand, if blood-sucking 
insects are present they afford at once a ready means whereby 
a blood-parasite might be sucked up from one animal and 
introduced into another at a subsequent bite. It is in this way that the 
parasites appear to be usually transmitted, but there is still uncertainty 
as to the details of the process in many cases : the chief difficulty lies in 
deciding whether the parasite is carried by the insect from one animal 
to another in a simply “ mechanical ” way, undergoing no change en 
route , or whether, as in the case of the malarial mosquitos, the parasite 
on entering the insect’s body undergoes a more or less prolonged series 
of changes before it is in a fit state again to infect a healthy animal’s 
blood. The fact that insects have been found to have parasites of their 
own which are extremely similar to certain forms of mammalian blood 
parasites renders the matter more complicated, as does also the remark¬ 
able hereditary transmission of infective power exhibited by certain 
Ticks. The consideration of the Arachnids is outside the field covered 
by this book, but the Ticks are of great importance as pests of cattle and 
dogs, which they infect with spirillar diseases (“ Tick-fever,” etc.) 
and with Piroplasmosis (Biliary fever), while they are also responsible 
for an often fatal disease of fowls and for a human relapsing fever, a 
remarkable feature being that in some species the infection is not trans¬ 
mitted by the Tick which bites a diseased animal, but by that Ticks’ 
young ones. As regards the Rhynchota, there is a strong presumption 
that Bed-bugs are responsible for the spread of human disease, and it 
appears that they are capable of harbouring the organism which causes 
Kala-azar and possibly of transmitting it by their bite (Rogers and 
Patton). Comparatively little attention has yet been paid to the Pedi- 
culidce which infest animals in India, but the human head-louse has been 
shown to transmit a spirillar fever among school-children (Mackie). Of 
those Diptera which chiefly attack cattle (Hi'ppobosca, Stomoxys, and 
Tabanidce) all three families are suspected of being the agents whereby 
Surra, a serious cattle-disease, is spread, and investigations are now 
