Arach´nida (ἁρἁχνη, spider) is the Class of spiders, scorpions, and mites.
Aranéida.—This Order contains the more highly developed forms of the Class, among which are the common spiders of houses and gardens; and some of their structures are very curious and interesting.
The head of spiders is united or fused with the thorax, forming one piece, which is called the ceph´alothorax (κεφαλἠ, head, θὡραξ, chest).
The claw-jaws, or man´dibles, are terminated by a curved and pointed claw, with which the spiders hold their prey. It is traversed by a slender canal, containing a slender tube or duct leading from a poison-gland, and opening near its point; and when the insect prey is transfixed by the mandible, the poison is pressed out and enters the wound.
Near the root or base of the mandibles on each side is a jointed feeler, or pal´pus; but spiders have no anten´næ. The eyes are simple, forming separate round shining dots, and are called ocel´li (ocel´lus, a little eye); they are usually placed on the top of the head, and are often arranged in a geometrical form, as a triangle, &c.
The legs are four pairs; they are hairy, and terminated by two or three claws, which are fringed with minute teeth, or pec´tinate. These claws serve to comb the fibres of the web, just as we comb our hair with a common comb.
The spinnerets, with which spiders form their web, are very curious organs. They are situated at the under and hind part of the body, and consist of two or three cones, or papillæ, on each side. On the summits of these papillæ are very numerous bristle-like tubes, through which the secretion of certain glands passes; this secretion, when hardened by exposure to the air, forms the fibres of the web.
On carefully examining a spider’s web, the radial fibres, or those which pass from the centre to the circumference, will be found to be smooth, these fibres serve to fix the web; while the cross fibres are covered with numerous viscid globules, which serve to attach flies or other prey to them. This difference of the fibres is best observed with a hand-lens.
Acarína, or the Order of Mites.—Here belongs cheese-mite, Ac´arus domes´ticus ([Pl. IX.] fig. 36). Its body is somewhat milky white, oval, and furnished with feathery hairs. When viewed from beneath, there is seen a transverse line, indicating the separation of the thorax from the abdomen; and another line in front of this, with four minute tubercles, from each of which arises a hair. The head is pointed and beak-like, forming a ros´trum (rostrum, a beak), consisting of two mandibles pressed together; these can only be seen to be separate when dissected apart with the mounted needles. Each mandible is chélate (χηλἠ, forceps), or has the form of a lobster’s claw; and beneath the two mandibles is a flat membranous under lip or labium, consolidated on each side with a palp. The legs are four pairs, as in all the Arachnida; they are pinkish, 6-jointed, and terminated by a leaflike sucker and a minute claw.
The males (fig. 38) are smaller than the females, the fore legs being much stouter, and furnished with a blunt tooth (fig. 38 a). The eggs can often be distinguished within the body of the female (fig. 36); they are oval and granular.