CHAPTER VI. 
ARACHNIDA. 
While in the more restricted sense the animals of this division are 
not insects, they are commonly looked upon in that hght, and since 
many of the parasitic forms have habits similar to those of the true 
insects, and can in many instances be treated by very similar or iden- 
tical measures, often at the same time, there is the best of reason for 
including a discussion of them in a work having the scope of this paper. 
While the group as a whole includes the spiders, scorpions, harvest 
men, etc., the parasitic forms are included in the order Acarina, and it 
will be unnecessary to enlarge on what has been said in the introduc- 
tory chapter relative to the characteristics of the group as a whole, but 
confine what is said to the order Acarina. 
Order ACARINA. 
These are commonly known as mites, ticks, scao insects, mange 
insects, etc., and are in general distinguished by having no prominent 
separation between the different regions of the body, the head, thorax, 
and abdomen, forming one closely connected structure. They have 
eight legs, except in the early stages, when there are but six; the eyes 
are often small or obsolete, the spiracles reduced to one pair, some- 
times apparently wanting; the mouth parts are fitted for piercing, 
biting, or in some cases for combined biting and suction, there being 
usually a pair of slender, sharp mandibles capable of penetrating the 
skin of the host animals. Much variation of habit exists, and ranges 
from free forms to strictly parasitic forms. 
HARVEST MITES; CHIGGERS. 
In the ramily Trombidiid, which includes normally plant-feeding 
species, we find a few species which have adopted a phase of para- 
sitism which, though apparently abnormal, results in extreme annoy- 
ance to the animals affected. 
Apparently the most abundant species in this country is the Leptus 
irritans of Riley, which is illustrated herewith. This oceurs in a large 
portion of the United States and occasions during the summer months 
an enormous amount of suffering. It ranges north in the Mississippi 
Valley into central Lowa, at least, appearing there by the latter part of 
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