DESCRIPTIONS OF GENERA AND SPECIES. 203 



and its palpi; in the curious foreign species H. 

 flagellifer, however, the remainer of the cephalothorax 

 appears from the discoverer's drawings to be drawn 

 out anteriorly so as to form a kind of hood, which 

 hides the rostrum. 



Mandibles (PL III, figs. 2, 13, 19). These, as before 

 stated, form the great characteristic of the genus ; in- 

 stead of the broad and robust chelate mandibles of all 

 other Tyroglyphidse, which are holding and tearing 

 organs, those of Histiostoma are long, thin blades on 

 edge ; usually diminishing somewhat in breadth toward 

 their distal ends, and occasionally ending there in a 

 piercing point or small pointed hook turned downward. 

 The lower edge of these blades is cut into a number of 

 teeth like a saw ; these may extend the whole length 

 of the mandible, or may occupy its distal end only ; 

 they may be of even size throughout, as in H. pyri- 

 forme (PI. Ill, fig. 19), or may be of unequal sizes, as 

 in H. rostro-sermtum (PL III, fig. 2). In the Italian 

 species, H. fimetarium of Canestrini and Berlese,* the 

 mandible is stated by those eminent acarologists to be 

 entirely without teeth on its lower edge, which differs 

 from all other known mandibles in the genus. I 

 have not succeeded in finding any species with the 

 mandibles in this condition. In H. pyriforme the 

 teeth are difficult to see ; in H. pulchrum much more 

 difficult ; but I have not found any species where the 

 teeth cannot be demonstrated if the mandible be 

 dissected out and examined by an amplification of about 

 500 diameters, with good apochromatic objectives and 

 sub-stage condenser, and the light skilfully managed ; 

 but I do not for a moment doubt that H. fimetarium, 

 which I have not seen, has the knife-like mandible, 

 as it has been examined by such very competent 

 observers. 



It is curious that in the Oribatidae, another family of 

 Acari, which in some respects is rather allied to the 

 Tyroglyphidas, one of which is the strong chelate 



* "Nuovi Acari," in ' Atti Soc. Veneto-Trent,' vol. vii, p. 150 (1881). 



