8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



When a comparison is made between the leg of Labidostomma and 

 that of a free-Hving member of the family Gamasidae (pi. 2, fig. 4) 

 the homology of the segments is easily determined. In a gamasid mite 

 there are also the eight full rings to a leg but here the last two are 

 immovably joined. It has been customary to consider these two 

 together as a divided tarsus, but this practice appears to be without 

 justification. A comparison, segment for segment, between the leg 

 of Labidostomma and that of a gamasid indicates that the short 

 penultimate segment of the latter should be the tibia. The other 

 segments of the gamasid leg are almost identical with those of Labido- 

 stomma except that the precoxa is more coneshaped than platelike. 



An excellent intergrading of connecting species exists between the 

 Gamasidae and the Ixodoidea, hence a comparison of the leg of the 

 latter with that of the former should be illuminating. This is given 

 in plates 2 and 3. Here again the homology is clearly indicated. The 

 subcoxa has become platelike, the femur is somewhat larger, and the 

 penultimate segment is better developed, giving rise to most of the 

 muscles moving the claws. In the tick leg both an extensor and a 

 flexor muscle of the claws is present (pi. 3, fig. 5), the latter arising 

 chiefly if not entirely from the patella. 



It appears that in the Arachnida a segment additional to any found 

 in insects must be recognized. It is here regarded as the patella. The 

 patella, whatever its fate among the immediate ancestors of insects 

 may have been, appears to be totally lost in that class. Not even a 

 rudiment of it has been successfully accounted for. In the Arachnida. 

 a group much more nearly related to the Crustacea, it remains, repre- 

 senting probably the meropodite of a primitive crustacean leg accord- 

 ing to Hansen's interpretation. 



Among the orders of Arachnida, the mite leg, l)e it of the eight- 

 segmented or the five-segmented type, is characterized particularly by 

 the passing of the large flexor muscles entirely through one segment 

 to the base of the next. This condition is especially apparent in the 

 leg of a cheese mite, family Tyroglyphidae, as shown by Michael 

 (1901). In part it may have been brought about by the shortening 

 of the segments of the legs but probably also by a shifting of the 

 position of the origin of the flexor muscles entirely to the dorsal wall 

 of the segments. 



LEC-SEGMENTATION IN VARIOUS ARACHNID GROUPS WITH SPECIAL 

 REFERENCE TO THE PSEUDOSCORPIONIDA 



Three well-known Arachnid groups have a leg that is typically 

 seven-segmented, not counting the foot. They are the scorpions, the 



