242 
MITES AND TICKS. 
any trace of segmentation; it is confluent with the cephalothorax, the fusion between 
the two being so complete, that, as in the harvest-spiders of the group Palpatores, 
the anterior sternal plates of the abdomen are thrust far forward between the 
coxrn of the cephalothoracic limbs. As in all Arachnida, the mouth is adapted for 
sucking, but the jaws are often partially united, and form, with a plate termed 
the epistome, and the labium, a beak. The epistome is often of large size, and is 
attached to the front border of the carapace; the mandibles are either pincer-like 
or simply pointed at the tip, forming piercing organs. The palpi, which resemble 
a pair of small legs, have their basal segments, or maxillae, united together and to 
the labium, to form a conspicuous plate or hypostome, constituting the floor of the 
mouth. These organs forming the mouth-parts are often separated from the rest 
of the cephalothorax by a membranous joint, and constitute a kind of movable 
head, the capitulum. In many cases there are no traces of special respiratory 
organs, breathing being effected by means of the skin; but, when present, such 
organs take the form of tracheal tubes, the apertures of which vary in position. 
They may, for instance, lie in the head between the mandibles and palpi, or far 
back in the body at the base of the legs of the last pair; but in some species 
they occupy intermediate positions, and open in front either of the first, second, 
or third legs. Another character of some value in separating the ticks from the 
harvest-spiders, is that in the former the young undergo a metamorphosis in the 
course of growth, being hatched from the egg as six-footed larvae, which later 
acquire the fourth pair of legs. The order may be divided into the typical mites 
and ticks (Acarina), and an aberrant worm-like group (Vermiformia). 
The Acarina include a number of families severally distinguished by the 
position of the respiratory stigmata, and the form of the mandibles and palpi. In 
the velvety mites ( Trombidiidoe ) the integument is soft and covered with variously 
coloured hairs; the legs are adapted for walking or running, the mandibles are 
pointed at the tip, and the stigmata open in the 
anterior portion of the body. There is usually 
a pair of eyes on the carapace, although these 
may be wanting. These mites, which may be 
either parasitic or leading a free life, feed by 
sucking the juices of animals or plants. A 
fairly common British species is Trombidium 
holosericeum, the second name referring to the 
clothing of crimson silky hairs covering its body. 
In the six-footed larval stage these mites live 
parasitically upon harvest-spiders, to which they 
cling, and resemble a cluster of bright red beads. 
Before attaining maturity they fall from their 
host to the ground, where, after undergoing their 
final moult, they lead a free wandering life, living 
upon minute insects such as aphides. In tropical countries mites of this genus reach 
a large size, measuring half an inch in length. They are beautiful and striking- 
objects, resembling tufts of bright blood-red plush. 
Nearly allied are the spinning-mites ( Tetranychidce ), which live exclusively 
velvety mite, Trombidium holosericeum, 
from below (enlarged eiglit times; nat. 
size on the leaf). 
