Air-breathing (by ordinary or modified tracheæ); the lower forms breathe
with the skin. Body at most consists of two chief regions, since head
and thorax are always fused together, making up a cephalothorax (Fig.
124); but this may again be united with the always unsegmented abdomen
into a single piece (Fig. 127). The last is the case with the mites, in
which, therefore, the characters of segmented animals can only be
recognized in the limbs. In the true spiders (Fig. 124) the body
consists of cephalothorax and abdomen. Arachnids have always four pairs
of legs, which, in the true spiders, are attached to the cephalothorax,
in the mites to the front part of the unsegmented body.
[Illustration:
FIG. 124.—A Spider (_Salticus scenicus_).
]
The chief Orders belonging here are: (1) true =Spiders= (_Araneida_);
(2) =Scorpions= (_Scorpionida_); (3) =Harvestmen= (_Opilionida_); (4)
=Mites= (_Acaridea_). Only the last contains species of importance
agriculturally.
ORDER: Acaridea (MITES).
Small arachnids, in which the cephalothorax and abdomen are fused
together into one piece (Fig. 127). The just-hatched young have three
pairs of legs, the adults, of course, four.
Family: =Acaridæ= (_True Mites_).
Soft skin. No tracheæ, no eyes. Legs short, often with a sucker at the
end. Here belong the =Cheese Mite= (_Acarus siro_), the =Meal Mite=
(_Tyroglyphus farinæ_), and several other species living in dead organic
substances; also—
The =Itch=, or =Mange Mites=.
These live as parasites on or in the epidermis, and cause the _itch_ or
_mange_ (scabies) in man, as well as in several domestic animals. By
means of the structure and habits three genera are distinguished:—