NO. 1382. THE ACARINA OR MITES— BANKS. 3 



The cephalothorax. or anterior part of the boch', commonly ha.s one 

 or more pairs of .simple ocelli-like eyes. They are usually sessile, but 

 sometime.s elevated on pedicels. 



The mouth-segments form typically a truncate cone or beak. Some- 

 times it is partially or completely retracted into the bod\'. The mouth 

 parts are the mandibles and palpi. P^requently there are other parts, 

 as a hypopharynx, a lip, or detinitel^^ separated maxillae, as will be 

 mentioned under each family. Several investigators have claimed that 

 there are three or four mouth appendages, and there are structures in 

 some forms that indicate three. The mandibles generally are of two 

 joints. 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 (Oribatidaj, Gamasidfe, and Tvroglyphida?) wdiich typi- 

 calh' have chelate mandibles there is one genus with styliform piercing 

 mandibles. The palpi have never more than live joints; the last is 

 frequently provided with peculiar sensor}^ hairs. In some cases the 

 palpi have a geniculate attitude. The various forms of palpi may be 

 grouped into four classes. (1) Where they are simple, filiform, and 

 have a tactile function; (2) where they are moditied for predatory pur- 

 poses, being provided with spines, hooks, or claws; {?>) where the last 

 joint is oppo.sable to the preceding, so that the mite may bv 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 luaxilhv. 



In several families there is a distinct lingula. tongue, or hypostome, 

 which arises from the inner base of the beak, and may be divided or 

 simple. It ma\^ have a groove above, called the vomer. The hypos- 

 tome is usuallv not visible except hy dissection, Ijut in the ticks it is 

 ver}^ large and roughened with sharp teeth. 



Sometimes the ])a.sal joints of the palpi unite to form a lip, or labium. 



Above the mandibles in man}- forms is a thin corneous plate, known 



I as the epistoma. Its sides may be partlv 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 some- 

 times 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 {JJi'opodci) the anterior 

 ^ legs are also inserted into this camerostome. 



The adult mite generallj' has four pairs of legs, and the larva three 



pairs. It has been shown that the eml)ryo of certain forms {Gauiasus 



1 and Ixodes) has four pairs of legs Ijefore birth, but one pair is aborted 



I to be again developed at the nymphal stage. This is an indication that 



'l the six-legged larva is a secondar}^ development, and lessens the 



