386 
INSECTS AND THEIR RELATION 
group of the Protozoa, or animal, rather than vegetable, 
cells. 
To this group belong the viruses of horse-sickness, heart- 
water, gastro-enteritis of sheep, and several others of very 
considerable importance to mankind and his property. 
Our present discussion will, then, resolve itself into one 
concerning the Arthropoda and the Protozoa — both families 
of the Animal Kingdom, and therefore worthy of attention 
by zoologists and other workers in natural history. If I 
can induce some stock-owners to see the affinity between 
natural history and the very tangible interests of their pockets, 
then I feel certain this Society will materially benefit by their 
membership, and thereby from the encouragement which they 
will give to the study of such families as ticks and the various 
biting-flies — a study which will be to the mutual advantage of 
all members. 
The phylum Arthropoda is divided into five classes : 
Crustacea, or lobsters and crabs ; Insecta, or six-legged 
arthropods, including flies ; Arachnida , including spiders, 
ticks, and mites ; Myriapoda, or centipedes and millipedes ; 
and Protracheata, a group of animals of interest chiefly because 
they constitute a missing link between the earth-worm and 
leech family and the Arthropoda. 
Of these five classes, two only — the Insecta and the Arach- 
nida — include forms which in virtue of their habits of sucking 
mammalian blood may be the true hosts of their diseases. 
The class Insecta is subdivided into a very large number 
of orders, including the locust and mantis ; the white ant and 
thrips ; beetles ; ants and bees ; butterflies and moths, &c. 
But those of interest to us to-night are the Diptera or flies, 
Syphonoptera or fleas, Hemiptera or bugs, and Anoplura or lice. 
All known biting Insecta belong to one or other of these four 
orders ; and though fleas, bugs, and lice are known to be carriers 
of such human diseases as plague, Kala Azar, and typhus fever 
(not typhoid), they have not yet been proved to be responsible 
for carrying any disease in stock, although the effects of the 
local irritation produced by lice and fleas are well known to 
both horse- and dog-owners. 
The number of species of lice and fleas is considerable in 
