712 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
taches to his name, “ that the necessary and sufficient condition for the 
experimental transmission of the plague was the biting of a man by 
the fleas of an infected rat.” . 
What precedes cgncerns the present importance of insects in med- 
icine, and I have endeavored to show how in recent years the ideas 
relative to the pathological power of these animals has been singu- 
larly improved. But there is in parasitology another subject of 
which the practical importance is just as great as that of the preced- : 
ing one. This is the rdle played by ticks in the dissemination of cer- 
tain diseases. Like the first, this subject has been so changed by 
recent work that if, in order to compare them, one should place side 
by side two of its classical expositions, the one made to-day and the 
other ten years ago, these expositions would not be in anywise similar- 
I shall endeavor to prove this in the lines which follow. 
It is superfiuous to recall here the old condition of the question, 
known to everyone; my shorter and simpler task should be confined. 
to an indication of the actual state of the subject. 
First, I desire to say a word or two to bring to mind certain neces- 
sary points of biology. The ticks, as everybody knows, are arthropods 
familiar to all, and especially to hunters, who call them by the name 
of “racins,” or ‘“ wood fleas.” They constitute in the order Acarina 
the family Ixodide, of which. the principal genera are Jzxodes, 
Rhipicephalus, Dermacentor, Haemaphysalis, and Argas. The most 
of them, so far as domestic animals are concerned, are essentially only 
temporary parasites, for they attack them only during one phase of 
their evolution, that of the adult female. But, in exchange, these 
females can attach themselves almost indifferently to any of our 
larger mammals, and it is an error to believe, as is frequently done, 
that each species of animal has its peculiar species of tick. Indeed, 
there is not a dog tick, a cattle tick, a sheep tick, or a human tick, and 
if this be so, it is because, far from choosing their victims, the 
Ixodide cast themselves on the first vertebrate that comes their way. 
These parasites fix themselves on the body of their hosts, in the 
integuments of which their beak is firmly implanted. They remain in 
this situation for a fortnight, during which they constantly suck 
blood, and they are then found under the double influence of the 
nourishment imbibed and the progeny which develops, swollen 
progressively to such a degree that an individual measuring 3 or 4 
millimeters long at the outset ends by becoming from 12 to 15 milli- 
meters long. 
At the end of this period the female is ordinarily satiated. She 
detaches herself spontaneously, owing to inflammation and gangrene, 
which softens the borders of the place of implantation. She tumbles 
to the ground, reaches a hiding place, such as the base of a tuft of 
