884 
INSECTS AND THEIR RELATION 
germ : so live a part, indeed, that in a great many instances 
the disease could not exist, much less be propagated, but for 
the mediation of these so-called insects. 
Let just one instance, known to all of you, suffice to make 
this point clear. A mosquito at birth is clean and harmless 
as a disease -producer, although some of us may fully appreciate 
the irritation and annoyance of its bite ; but after it has fed 
upon a man suffering from fever, or in whose blood the linger- 
ing remnants of a previous attack are to be found, that 
mosquito, provided it be of a species capable of acquiring 
infection, becomes a breeding-place for the germs of malaria. 
Among the germs taken in with the blood into the stomach 
of the mosquito, are some which have already undergone vital 
changes and have acquired sexual characters. Conjugation 
of the two sexual elements is effected, and the so-formed cell, 
having burrowed into the wall of the stomach, proceeds to 
divide and subdivide within the little cyst it has made. The 
contents eventually become so numerous and bulky that the 
cyst wall breaks, and the vast number of daughter cells are 
liberated into the body cavity, from which they wander to 
reach the salivary glands. From this destination they may 
pass down the proboscis at each subsequent bite of the mosquito, 
being aided by the discharge of saliva — an act natural to many 
animals at the thoughts of food. 
These young cells, or sporozoits, injected into man will 
again reproduce, and with the culmination of each generation 
the patient suffers from the rigor of a malarial attack. 
Here, then, is an instance in which an actual multiplication 
of the germs takes place within the insect : an instance where 
the mosquito is the true host of the disease, which it can convey 
at every succeeding bite, until the supply of sporozoits within 
the salivary glands becomes exhausted. 
Analogous (though naturally with modifications adapted 
to the natural or life history of the respective participants) 
is the relationship between the tsetse-fly and the germ of fly 
disease or sleeping sickness, between the blue-tick and Red- 
water, the dog-tick and tick fever, the human-tick and re- 
current or relapsing fever, and the brown-tick and East Coast 
fever. In each case the arthropod carrier is the true host, 
