. The Acarina or mites. A review of the group for the use of economic entomologists. Mites. THE ACAEINA OR MITES. 9 The last is often opposable to a projection of the preceding, so as to become chelate. However, in many forms the mandibles are slender, needle-like, and suited for piercing. In each of the three large families (Oribatidse, Parasitidse, and Tyroglyph- idse) which typically have chelate mandibles there is one genus with styliform piercing mandibles. The palpi have never more than five joints; the last is fre- quently provided with peculiar sensory hairs. In some cases the palpi ha
. The Acarina or mites. A review of the group for the use of economic entomologists. Mites. THE ACAEINA OR MITES. 9 The last is often opposable to a projection of the preceding, so as to become chelate. However, in many forms the mandibles are slender, needle-like, and suited for piercing. In each of the three large families (Oribatidse, Parasitidse, and Tyroglyph- idse) which typically have chelate mandibles there is one genus with styliform piercing mandibles. The palpi have never more than five joints; the last is fre- quently provided with peculiar sensory hairs. In some cases the palpi have a genicu- late attitude. The various forms of palpi (fig. 3) may be grouped into four classes: (1) \\Tiere they are simple, filiform, and have a tactile function; (2) where they are modified for predatory purposes, being provided with spines, hooks, or claws; (3) where the last point is opposable to the preceding, so that the mite may by its palpi cling to some object; (4) where they have become obsolete and are more or less united to the rostrum. The basal joints of the palpi are at times differentiated to form maxillae. In several families there is a distinct lingula, tongue, or liypostome, which arises from the inner base of the beak and may be divided or simple. It may have a groove above, called the vomer. The hypostome is usually not visible except by dissec- tion, but in the ticks it is very large and roughened with sharp teeth. Sometimes the basal joints of the palpi unite to form a Up or labium. Above the mandibles in many forms is a thin corneous plate, known as the epistoma. Its sides may be partly united to the beak or lip below and thus form a tube, called the oral tube, for it is through it that the mandibles are protruded. The pharynx or sucking portion of the alimentary tract is sometimes prolonged forward into a sort of cone between the mouth parts. The opening of the body into which the mouth parts are inserted is known as the camerostome. In one group (Uro
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