504 On iJw Joints of the Mandibles in the Acari. 



LXYII. — On the Morphological Significance of the Joints of 

 the Mandibles in the Acari. By Dr. A. C. Oudemans*. 



In tlie legs of the Acari the following joints are generally 

 distinctly recogniz;ible : — coxa, trochanter, femur, genu 

 (patella), tibia, and tarsus. Soinetimes two or more of these 

 joints are fused together ; thus in the case of certain species 

 we meet with a trochanterofemur or a femorogenu. It may 

 also happen that a joint is divided into two or more small 

 joints. Thus, for instance, the femur may be divided into a 

 basifemur and a telofemur, or in a tarsus a basitarsus, a 

 mesotarsus, and a telotarsus may clearly be distinguished. 



If we now compare tiie maxillte of the Parasiti lie (Gamas- 

 idas) with the legs in the same family, we arrive at the 

 conclusion that the coxse of the two maxillae have become 

 fused into a hypostoma, and that the rest of the joints have 

 remained freely movable and form the palpi, in which we can 

 recognize distinctly trochanter, femur, genu, tibia, and tarsus. 

 The tarsus is, however, not attached to the tibia exactly at 

 its distal extremity, but obliquely on its under and inner side. 

 The tarsus itself bears beneath on the inner side and proxi- 

 nially a much deformed hair, which is capable of being 

 moved by muscles (a three-pronged fork without a handle). 



On examining a mandible we find that it is generally 

 composed of tliree joints, namely, (I) a cylindrical joint, 

 (2) a likewise cylindrical joint, which is movably united to 

 the first joint by means of two lateral condyles and narrows 

 abruptly at its distal extremity. At the proximal end of 

 this narrowed portion there lies on the ventral side a slit or 

 pit, in which the third joint is movably wedged, almost in the 

 game way as that in which one of the rami of our lower jaw 

 is attached to the temporal bone, that is by meaiis of a poste- 

 rior and upper condyle. The third joint and the attenuated 

 distal portion of the second together form the chela, with a 

 digitus tixus above and a digitus mobilis below. 



In many families or orders the mandible consists, however, 

 of only two joints — namely, of those described above, the 

 first and second are fused together, and then constitute a 

 somewhat difFrently shaped, more powerful, and more or less 

 pyritorm structure, the head of which is situated proximally, 

 wliile the stalk represents distally the digitus tixus. 



* Translated bv E. E. Austeu from the ' Zoologisclier Anzeiger,' 

 Bd. xxix. No. 20 (Jan. 8, 1906), pp. 63S-639. 



