Fig. 411.—Spinnerets of Spider greatly enlarged.
The eight legs and feet of the spider (one only is represented [Fig. 410], No. 4) are curiously constructed. Each foot, when magnified, is seen to be armed with strong horny claws, with serrations on their under-surface. By this arrangement the spider is enabled to regulate the issue of its web from the spinnerets. In addition, a remarkable comb-like claw is provided for the purpose of separating certain threads which enter into the composition of the delicate web, so that everything is arranged and planned in the most geometrical order, while the mouth or jaws with their two movable poison-fangs convert the Arachnidæ into formidable and dangerous foes. The maternal industry and instincts of spiders, the ballooning habits of others, the cave dwellers, with their limited vision, combined with an increased delicacy of touch and hearing, their disguise of feigned death when a strong enemy approaches, are all of the most interesting character.
One of the more remarkable, the Argyroneta aquatica (diving spider), weaves itself a curious little bell-shaped globule, which it takes with it to the bottom of the water, whither it retires to devour its prey. Notwithstanding its aquatic habits, this, like the rest of its species, is fitted only for aerial respiration; it therefore carries down, entangled amongst the hairs of its body, a small bubble of air. This contrivance presents us with the earliest form of diving-bell.
Mites and Ticks constitute a group which for diversity of structure, number of species and individuals, and minuteness of size, has no equal. The typical genus of the family—Ixodidæ—being wholly parasitic in their habits, are so modified in organisation, so marked by degeneration, that some authors have proposed to remove them into a class by themselves. One leading character distinguishes the whole: the abdomen rarely presents a trace of segmentation, but is confluent with the cephalothorax, the fusion between the two being so complete that, as in the harvest spiders belonging to Palpatores, the anterior sternal plates of the abdomen are thrust far forward between the coxæ of the cephalothoracic limbs. As in Arachnidæ, however, 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 mandibles are either pincer-like, or simply pointed at the tip, forming piercing organs; the palpi have their basal segments, or maxillæ, united, which form a conspicuous plate, or hypostomes, constituting the floor of the mouth. These organs are often seen to be separated from the rest of the cephalothorax by a membranous joint, and constitute a kind of head, the capitulum. In most cases no trace of special respiratory organs can be found. Another characteristic of value in separating ticks from harvest-spiders is that in the former the young undergo a metamorphosis in the course of growth, being hatched as six-footed larvæ, and acquiring later in life a fourth pair of legs.
Fig. 412.
A. Atax spinipes, water mite seen from below; B. Water Scorpion infested by Atax.
The Acariæ include a number of families, all distinguished by the position of the respiratory stigmata and the form of the mandibles and palpi. In the velvety mites (Trombidiidæ), the integument is soft and covered with variously-coloured fine hairs, and the legs are adapted for walking, running or swimming. The latter live in fresh-water ponds, creeping over the leaves of aquatic plants. The fresh-water mites (Atax spinipes, [Fig. 412]) swim about freely by means of vigorous strokes of their legs, which act as oars. In the adult the body is more or less spherical, and usually of a bright red or greenish colour. The males of one species have a curious blunt tail-like prolongation from the hinder end of the abdomen. The eggs are laid in the spring on the stems of water plants, and the six-footed larvæ when hatched attach themselves to water-bugs (Nepa) or water-beetles (Dytiscus) by means of a large sucker developed on the front of the head.
Fig. 413.—Ixodes ricinus or Sheep-tick (under surface). The small circle encloses one life-size.