SEVENTEENTH ANNUAL REPORT. 



391 



orly by a digits which is recurved at its anterior free extremity in a 

 dorsal hook. This digit bears either two or three processes, known as 

 the wpophyses^ which are unequal in size — the internal or lesser apophy- 

 sis, the external (away from the median line) or 

 greater apophysis, and the middle apophysis — • 

 situated between the internal and the external. 

 Each apophysis bears one or more thorn-like 

 or hook-like teeth, more or less recurved to- 

 ward the base of the capitulum. Some authors 

 attach considerable importance to the number 

 and arrangement of these teeth in classifying 

 ticks. Our own experience has been that the 

 teeth of the external apophysis are subject to 

 variation among the individuals of given spe- 

 cies, hence not reliable as systematic charac- 

 ters; while the teeth of the internal apoph3^sis 



are usually so diffi- 

 cult to recognize with 

 certainty that, what- 

 ever may be their 

 systematic value, we 

 are disposed to select 

 other characters. 



(4) The mandibu- 

 lar sheath (figs. 54, 

 55) lies dorsal of the 

 mandibles, and often 

 appears as though it 

 were a direct prolon- 

 gation of the base of 



the capitulum. The sides are curved ventrally 

 at the sides of the mandibles. The anterior 

 extremity is notched in the median line, and 

 a distinct line (fig. 52) can often be traced 

 from this point to the base of the capitulum, 

 while a slight pressure not infrequently re- 

 sults in separating the structure into two dis- 

 tinct halves corresponding to the mandibles. 

 Examination under a microscope often re- 

 veals numerous fine structures on the exposed 

 surface, giving to it the appearance of a file. 

 As stated above, the hypostome, mandibles, and mandibular sheaths, 

 taken together, represent the haustellum (sometimes called the beak). 

 These are the structures which are inserted into the skin of the animal 

 (fig. 54) upon which the ticks are parasitic. The arrangement of the 



Fig. 56.— Dorsal view of mandi- 

 ble of Argas miniatus, show- 

 ing the base h, stem s, digit d, 

 external e, internal i, and me- 

 dian m apophyses. Greatly 

 enlarged. Original. 



Fig. 57. 



-Ventral view of same. 

 Original. 



