ecomes hard and black and in man are known as blackheads. If one of
these blackheads is forced out and the fatty substance dissolved with
ether the mites may be found in all stages of development. The young
have six legs, the adult eight. The body is elongated and transversely
wrinkled. In man they are usually found about the nose and chin and neck
where they do no particular harm except to mar the appearance of the
host and to indicate that his skin has not had the care it should have.
Very recently certain investigators have found that the leprae bacilli
are often closely associated with these face mites and believe that they
may possibly aid in the dissemination of leprosy. It is also thought
that they may sometimes be the cause of cancer, but as yet these
theories have not been proven by any conclusive experiment.

In dogs and cats these same or very similar parasites cause great
suffering. In bad cases the hair falls out and the skin becomes scabby.
Horses, cattle and sheep are also attacked. The disease caused by these
mites on domestic animals is not usually considered curable except in
its very early stages when salves or ointments may help some.

_Itch-mites._ "As slow as the seven-years' itch" is an expression, the
meaning of which many could appreciate from personal experience, for it
certainly seemed to take no end of time to get rid of the itch once it
was contracted. Just why seven years should have been set for the limit
of the disease is not clear, for if the little roundish mites that cause
the disease live for seven years on a host they are not going to move
out voluntarily even if their seven-year lease has expired.

[Illustration: FIG. 18--_Ornithodorus moubata_, the Tick that Transmits
Relapsing Fever. From Boyce's "Mosquito or Man."]

[Illustration: FIG. 19--The follicle mite (_Demodex folliculorum_).
(After Murray.)]

[Illustration: FIG. 20--Itch-mite (_Sarcoptes scabiei_). (After
Murray.)]

[Illustration: FIG. 21--Harvest-mites or "jiggers." (_Leptus irritaus_
and _L. americanus_.) (After Riley.)]

The minute whitish mites (Fig. 20) that cause this disgusting disease
are barely visible to the naked eye. They are usually very sluggish but
become more active when warmed. They live in burrows just beneath the
outer layer of skin, sometimes extending deeper and causing most intense
itching. As the female burrows, she lays her eggs from which come the
young mites that are to spread the infectio