54 BRITISH TYROGLYPHID.E. 



anchylosed to, and continuous with, the dorsal cuticle, 

 the position is scarcely capable of alteration. 



The mandibles in the genus Histiostoma are totally 

 different from those of the remainder of the family ; 

 they are not chelate ; they are saw-like, or in one 

 instance knife-like. A tolerably full description of the 

 variation of these mandibles in the known species will 

 be found in the description of the genus and in the 

 accounts of the various species, therefore I will not 

 duplicate it here. It is probable that the Histiostoma 

 mandible may be considered as derived from the 

 ordinary chelate mandible by the suppression of the 

 distal joint, which forms the moveable arm of the chela ; 

 but if so a considerable amount of correlated modifica- 

 tion must be admitted to have taken place in the organ, 

 and the mode of action has become very different from 

 that of the original form. Instead of the slow and 

 irregular tearing movement of the chelate mandibles of 

 the ordinary Tyroglyphidse, those of Histiostoma have 

 a rapid, alternate, regular, forward and backward 

 motion, forming a sawing action. The creature wades 

 forward in the film of liquid in which Acari of this 

 genus are usually half immersed, and the mandibles 

 cut the vegetable cells as it advances, releasing the 

 cell contents which are passed into the mouth by 

 means of the palpi. 



Exceptional genera witli saw-like or knife-like man- 

 dibles of one joint are found in other families of 

 Acarina, which usually have those organs chelate, as 

 in the Oribatidae, the Gamasidae, etc. 



The Maxillary Lip and Maxillae. In all Acarina the 

 maxillae are united together so as to form a broad lip 

 below the mouth, known as the maxillary lip or hypo- 

 stome. In many families, such as the Gamasidae, the 

 Trombididae, the true Sarcoptidae, etc., the blending 

 is so complete that all trace of the maxillae as separate, 

 free, paired organs is lost. In the Oribatidge, however, 

 it is only the basal portions of the maxillae which are 

 fused ; the distal parts form broad, almost free blades, 



