This family includes the Mites, Ticks, and Water-mites, some of which are parasitic, whilst others are free, and some are even aquatic in their habits. The mouth is formed for suction, or for biting. There is no definite line of demarcation between the unsegmented abdomen and the cephalothorax.
Fig. 164. - Arachnida. a Pycnogonum littorale ; b Tetranychns telarius, one of the " Sociable " mites; c Hydrachna globulus, one of the " Water-mites."
Fig. 165. - A, Demodex folliculorum, greatly magnified. B, Emydium testudo, one of the Tardigrada, greatly magnified. C, Sarcoptes scabiei, the Itch-mite, greatly magnified.
In the true Acari (tig. 164, b), of which the Cheese-mite may be taken as an example, there are four pairs of legs, adapted for walking, and the mouth is provided with distinct mandibles. Besides the Cheese-mite (A. domesticus), another well-known species is the Acarus destructor, which feeds upon various zoological specimens, and is very annoying to the naturalist. In the Sarcoptes scabiei - the cause of the skin-disease known as the "itch" - the two anterior pairs of legs are provided with suckers, and the two posterior are terminated by bristles; the mouth, also, is furnished with bristles (fig. 165, C). In the Ticks (Ixodes) the mouth is provided with a beak, or " rostrum," which enables them to pierce the skin and retain their hold firmly. In the Hydrachnidae (fig. 164, c), or Water-mites, the head is furnished with two or four ocelli, and there are four pairs of hairy natatory legs. They are parasitic, during at least a portion of their existence, upon Water - beetles and other aquatic insects. They pass through a metamorphosis, the larva being hexapod, or having only three pairs of legs. The Garden-mites (Trombididae) and Spider-mites (Ganasidae) live upon plants; the Wood-mites (Oribatidae) and Harvest-ticks (Zeptidae) are to be found amongst moss and herbage, or creeping upon trees or stones; whilst the true Ticks (Ixodidae) attach themselves parasitically by means of their suctorial mouth to the bodies of various Mammals, such as sheep, oxen, dogs, etc. Several Mites (Thalassarachna, Pontarachna, etc.) have been found to inhabit salt water, and several species of Trombididae live habitually between tide-marks.
Another member of the Acarina is the curious little Demodex folliculorum (fig. 165, A), which is found in the sebaceous follicles of man, especially in the neighbourhood of the nose.
It is probable that very few, if any, individuals are exempt from this harmless parasite.