ANATOMY. 59 



thicker ; it is the joint which usually gives the change 

 in position from the proximal to the distal joints. It 

 is common that Acari, particularly swift species, hold 

 the tibia and tarsus almost perpendicular, while the 

 coxa and femur are nearly horizontal; the genual, 

 which is the central joint, naturally is the one that 

 gives this change of direction. In the Oribatidae this 

 is almost universal and very distinct. In some of the 

 Tyroglyphidae however although the same thing exists 

 it is not quite so evident ; because from the much greater 

 relative size of the genual it pushes back, so to speak, 

 occupying a part of the space which would be devoted 

 to the larger femur in the Oribatidae ; and also because 

 the length of the tarsus in some Tyroglyphidae is so 

 great that the tibia is not required to be perpendicular, 

 and partly shares with the genual in producing the 

 change of direction. 



In the large families of Acarina, which have six or 

 more joints to the leg, such as the Trombididae, Gama- 

 sidae, Hydrachnidas, Halicaridae, etc., this change of 

 position by the genual is of course more or less lost ; 

 but the leg of five joints may probably be looked upon 

 as the primitive arrangement in the Acarina; the legs 

 of more numerous joints being derived from it by 

 division of the tibia, or other joints ; if this be borne 

 in mind the genual and its action as a changer of 

 direction may often be traced. 



The Tibia is not usually a very important joint as 

 regards size ; it is however of interest in most active 

 Acarina because it is the joint which bears the great 

 tactile hair near its distal end in the median line of its 

 dorsal surface; this hair, which is most developed on 

 the anterior legs, practically to a great extent serves 

 eyeless Acarina such as the Tyroglyphidae and Oriba- 

 tidaa as a substitute for the sense of sight. 



The Tarsus is most commonly the longest and 

 thinnest joint in the leg; it often diminishes rapidly 

 near its proximal end and is drawn out gradually 

 almost to a point at the distal; in the hind legs of 



