The Arthropoda are generally divided into five groups (Crustacea,[322] Protracheata, Arachnoidea, Myriapoda,[323] and Insecta or Hexapoda), of which only the Arachnoidea and the Hexapoda interest us here.
A. ARACHNOIDEA (Spiders, Mites, etc.).
The head and thorax are always united together; the abdomen is either segmented or without exterior segmentation, in which case it is united with the cephalothorax.[324] The number of pairs of appendages amount to six, of which the two front pairs, the cheliceræ and the pedipalpi, are attached to the head region and the four remaining pairs to the thoracic region.[325] The abdomen in the adult condition has no appendages. The Arachnoids are air-breathers, and for this purpose are either provided with tracheæ or with so-called lung-sacs, or they breathe through the surface of the body. Some aquatic forms breathe by gills.
There are eight or ten orders of Arachnoids,[326] of which, however, only two, the Acarina and the Linguatulida, have to be considered here.[327]
Order. Acarina (Mites).
Small Arachnoids, the three parts of the body of which are, as a rule, coalesced; it is only rarely that a faint line indicates the division between a cephalothorax and abdomen. The two appendages on the head are designed for biting or puncturing and sucking, and vary according to their use. The cheliceræ[328] are fang-like jaws or puncturing bristles forming a kind of rostrum, the pedipalpi are claw-like or shear-shaped, or form a suctorial proboscis.[329] The four pairs of legs are usually well developed, more rarely they are rudimentary or have partly vanished; many parasitic forms are provided with pedunculated suckers [ambulacra—F. V. T.]. Respiratory organs (tracheal tufts) may be present or absent. The nervous system is reduced to a minimum, eyes are usually lacking. The intestine, situated in the central part, generally has three blind appendages; the anus is situated on the venter above the posterior end. Sexes separated; nearly all the species deposit eggs, from which six-legged larvæ hatch. The Acarina live either free in the water or in moist soil, or they are parasitic on plants and animals.[330]
Family. Trombidiidæ (Running Mites).
Soft-skinned Acarina with tracheæ and with two eyes, usually pedunculated; they are often brightly coloured; cheliceræ lancet- or claw-shaped; pedipalpi claw-like; legs composed of six segments, with suctorial discs between the terminal ungues.[331] Larvæ six-legged. To the latter belong the larvæ of several species of Trombidium such as:—
Genus. Trombidium, Latreille (and Leptus).