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 sty li form 

 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 

 (Uropoda) the anterior legs are also inserted into this camerostome. 



The adult mite as a rule has four pairs of legs and the larva three pairs. It has 

 been shown that the embryo of certain forms (Parasitidse and Ixodes) has four pairs 

 of legs before birth, but one pair is aborted, to be again developed at the nymphal 

 stage. This is an indication that the six-legged larva is a secondary development 

 and lessens the apparent difference between Acarina and other Arachnida. The 

 legs (fig. 4) are composed of from five to seven segments; in some forms the apical 

 joints are subdi\ided but do not form genuine segments. The length and character 

 of the joints vary in the different families, but usually there are distinguishable the 

 following parts: Coxa, trochanter, femur, patella, tibia, and tarsus. In some cases 

 the femora are divided into two parts, or there is a suture near the base indicating a 

 trochantin. The legs are pro\ided with hairs and spines, sometimes modified for 

 some particular function. In several groups there are organs on the anterior legs 

 which appear to have a sensory function. The last joint or tarsus is commonly ter- 



FiG. 3.— Palpi of various forms: a, Bdella; b, Cheyktus; c, Gamasus; 

 d,SciTus; e, Histiostoma; f, Eupodes; g , Sarcoptes; U, Arrenurus; 

 i, Trombidium. (Author's illustration.) 



