These are small creatures, seldom attaining the twentieth of an inch in length. They are vegetable-feeders (except, perhaps, Pelops), and are to be found in dead wood or vegetable débris, under bark, or among moss and lichen. In winter they often take refuge under stones. It is impossible at present to estimate the number of existing species, for only a few localities have been systematically worked for them, and their small size has prevented their inclusion, in any numbers, in the collections of scientific expeditions. Our knowledge of the group is likely, however, to be largely extended, for it has been found that they reach England alive and in good condition from the most remote regions if moss or other material in which they live is collected when not too dry, and hermetically sealed up in tin cases.
About twenty genera and more than 220 species are at present known. Pelops has much elongated chelicerae, with very small chelae at the end. There are ten species, found in moss and on bushes. Oribata numbers about fifty species, found in moss and on trees. Notaspis, in which the last three legs are inserted at the margin of the body, has about thirty species, found among moss and dead leaves. Nothrus is a short-legged genus with flat or concave dorsal plate, often produced into very remarkable spiny processes. There are twenty-two species found under bark and among moss and lichen. Hoploderma (Hoplophora) is remarkable for its power of shutting down its rostrum and withdrawing its legs in a manner which leaves it as unassailable as a tortoise or an armadillo.
Though the Oribatidae are all eyeless, they are distinctly sensitive to light, not wandering aimlessly till they reach a shadow, but apparently making straight for a dark spot when subjected to strong illumination. Some species have a curious habit of collecting dirt and débris on their backs, so as entirely to obscure the often very remarkable disposition of the spines and processes with which they are furnished.
Fig. [243].—Capitulum of Boophilus australis; ventral view. p1, p2, p3, p4, The four articles of the palp; m, the mandible or chelicera; d, its digit; n, the hypostome.
The next two families include the animals commonly known as Ticks, the largest and most familiar of the Mite tribe. Of recent years they have attracted much attention as the conveyers, to man and domestic animals, of certain diseases due to blood-parasites (see p. [457], n.), and our knowledge of their structure and habits has greatly increased in consequence. Hitherto they have generally been considered to constitute a single family, the Ixodidae, but a section of them so differ from the rest as to require their removal to another family, the Argasidae, so that it is necessary to employ a super-family name—Ixodoidea—to embrace the whole group.
Ticks are parasitic on mammals, birds, and reptiles, some shewing a marked partiality for a particular host, others being much more catholic in their tastes. Both sexes in the Argasidae, but the females only of the Ixodidae, are capable of great distension, but when unfed they are all somewhat flat animals with laterally extended legs and rather crab-like movement.
All Ticks possess a small, movable “false head” or capitulum bearing mouth-parts which are exceedingly characteristic of the group. The chelicerae are cutting instruments with their distal ends serrated outwardly, and there is always present a hypostome beset with recurved teeth which serve to maintain a firm hold on the tissues into which it is thrust. On either side of the chelicerae are the four-jointed palps, leg-like in the Argasidae, but more rigid and rod-like in the Ixodidae, where their inner margin is often hollowed so as to enclose the chelicerae and hypostome when the palps are apposed. There is a conspicuous pair of spiracles near the coxae of the fourth pair of legs.
Fig. [244].—Ornithodoros talaje, under surface, × 5. (After Canestrini.)