he third time they feed and
develop until fully mature and the female is ready to lay her eggs. The
Texas fever tick, and some others, as we shall see, do not drop to the
ground to molt but once having gained a host remain on it until ready to
deposit their eggs.

The young ticks have only six legs (Fig. 15) but after the first molt
they all have eight.

TICKS AND DISEASE

_Texas Fever._ Ever since stockmen began driving southern cattle into
states further north it has been noted that the roads over which they
were driven became a source of great danger to northern cattle. Often
80% to 90% of the native cattle died after a herd of southern cattle
passed through their region and the losses became so great that both
state and national laws were passed prohibiting the driving or shipping
of southern cattle into northern states.

[Illustration: FIG. 14--Castor Bean Tick (_Ixodes ricinus_) not fully
gorged.]

[Illustration: FIG. 15--Texas fever tick, just hatched; has only six
legs.]

[Illustration: FIG. 16--Texas fever tick (_Margaropus annulatus_) young
adult not fully gorged.]

[Illustration: FIG. 17--_Amblyomma variegatum_ several ticks belonging
to this genus transmit _Piroplasma_ which cause various diseases of
domestic animals.]

But for years the cause of this fever, which came to be known as the
Texas fever, was not known. The southern cattle themselves seemed
healthy enough and it was difficult to understand how they could give
the disease to the others. It was early noticed, too, that it was not
necessary for the northern cattle to come in direct contact with the
others in order to contract the disease. Indeed the disease was not
contracted in this way at all. All that was necessary for them was to
pass along the same roads or feed in the same pastures or ranges. Still
more puzzling was the fact that these places did not seem to become a
source of danger until some weeks after the southern cattle had passed
over them and then they might remain dangerous for months.

In 1886 Dr. Theobald Smith of the Bureau of Animal Industry, United
States Department of Agriculture, found that the fever was caused by the
presence in the infected cattle of a minute Sporozoan parasite
(_Piroplasma bigeminum_). Further investigations and experiments proved
conclusively that this parasite was transmitted from the infected to the
well animal only by the common cattle tick now known as the Texas fever
tick (Fig. 16).

T