45 8 ARACHNIDA ACARINA chap. 



p. 467), which are nearly all covered by an extremely hard and 

 coriaceous armature. 



Eyes are sometimes absent, sometimes present in varying 

 numbers. They seem here to be of remarkably little systematic 

 importance, as otherwise closely allied species may be either 

 eyed or eyeless. 



Normally Mites possess the usual Arachnid appendages, 

 chelicerae, pedipalpi, and four pairs of ambulatory legs. The 

 anterior appendages are, however, subject to a very great degree 

 of modification, while in one Family, the Eriophyidae (Phytop- 

 tidae), the legs are apparently reduced to two pairs. 



The chelicerae are sometimes chelate, in which case they are 

 two-jointed, the distal joint or movable finger being always 

 articulated helotv the immovable finger. Sometimes they ter- 

 minate in a single claw or blade, the movable joint being obsolete. 

 In the Ticks they exist as two long styles or piercing weapons, 

 serrate on the outer edge. 



The pedipalpi vary very much in structure, according to the 

 habits of the particular form to which they belong. In the 

 Sarcoptidae (see p. 466) they are hardly recognisable owing to 

 the extent to wliich they have coalesced with the maxillary 

 plate. In many of the free-living forms they are leg-like feeling 

 organs, but in others they are raptorial, being not precisely 

 chelate, but terminating in a " finger-and-thumb '' arrangement 

 which is of use in holding prey. The extreme development of 

 the raptorial palp is found in Cheyletics (see p. 473), in which 

 the whole appendage is remarkably thick and strong, and the 

 " finger " is a powerful chitinous claw, while the " thumb " is 

 replaced by movable pectinated spines of chitin. The Water-mites 

 have a palpus adapted for anchoring themselves to water-weeds, 

 the last joint being articulated terminally with the penultimate 

 joint, and bending down upon it. Finally, in the " Snouted- 

 mites " (Bdellidae, see p. 471) the palpi are tactile or antenni- 

 form, often strongly recalling the antennae of weevils. 



The maxillary plates which arise from the basal joints of the 

 pedipalps are always more or less fused, in the Mites, to form a 

 single median transverse plate, constituting the lower lip or 

 " labium " of some authors. In some of the Oribatidae the 

 fusion of the maxillae is only complete at the base, and the free 

 points are still of some use as masticating organs. In those free 



