896 
INSECTS AND THEIR RELATION 
are species of three distinct genera which may cause the 
disease — the type and severity of which varies according to 
the particular parasite involved, and the treatment to be 
effective must be accordingly modified. 
Investigation into the life history of Sarcoptes Scabei, the 
cause of sheep scab, has enabled sheep-owners to check the 
disease very materially, as it has revealed the weakest point 
for attack by the dipping-fluids and the interval between 
dipping that is best for this attack. As this disease is estimated 
to cause a loss of five million pounds sterling per annum to 
flock-masters of the British Empire, the importance of this 
insignificant parasite is apparent. 
As with the scab parasite, so also with ticks : the 
mechanical inconvenience caused by these pests is enormous. 
Have we not sometimes seen a dog whose body is literally 
covered with ticks, of all ages and sizes ? In some countries 
of the world, cattle and horses may be seen similarly covered ; 
and you can then well believe that the milk-yield of a cow so 
attacked may be reduced 40 per cent., and that calves reared 
under these conditions are stunted and take much longer to 
come to maturity. In this country it is fortunate that we 
are not quite so bad as others ; but even here our collective 
losses from mechanical annoyance by ticks must be very 
great. 
Much greater, however, are those due to the tick as a 
disease-carrier. Relapsing fever of man has caused serious 
losses among the troops in German East Africa : Redwater, 
anaplasmosis, and East Coast fever of cattle are a constant 
drain upon this country’s resources, and tick fever of dogs 
has caused many of us to lose a cherished pet. 
In order to appreciate the way in which these diseases are 
carried, we must be familiar with the life history and habits 
of the tick ; and here at once we find two definite plans, each 
followed by the members of the two main divisions into which 
the Ixodidae are divided. 
The so-called human-tick of Uganda, German East Africa, 
and the Northern Erontier, and the fowl-tick which occurs at 
Mombasa, are representatives of the one group, or Argasidae, 
the members of which live in cracks of the walls or floors in 
