572 MEDICAL ENTOMOLOGY 



to the basis capituli ; it is usually small and nearly always hidden when 

 the palps are examined from the dorsal surface ; ventrally this segment is 

 seen as a small knob. It is best studied in specimens in which the capi- 

 tulum has been removed and cleared in caustic potash solution. The 

 second segment is usually the longest and most prominent, and with the 

 third forms more than half the total length of the palp ; it varies much in 

 shape. In Hyalomma (Plate LXXIII, fig. 5) it is long and deeply scooped 

 out, and is markedly convex on its external surface. The third segment 

 is generally smaller than the second, and is often deeply excavated on its 

 median surface ; on the inner side of its distal end it has a hollow into 

 which the fourth segment (Plate LXXIII, figs. 2, 3, 5, 6, 7, 9, 11, 12, 13 

 and 15) is inserted. This latter is always small and bears a certain 

 number of sensory hairs. The second and third segments, when deeply 

 excavated, ensheath and protect the mandibles and hypostome. On their 

 internal margins these segments are often armed with peculiar feathery 

 hairs, the number and arrangement of which are of some importance 

 in the identification of the species. The other surfaces usually have 

 pores and simple hairs. 



The paired mandibular sheaths* (Plate LXXIII, figs. 8, 10 and 14), with 

 their contained mandibles or chelicerae, are the next structures of import- 

 ance to note. Each sheath is continuous w r ith the ante- 

 Mandibular sheaths .. ri , , . .^ ,. , ,. . , 



and mandibles nor P ortlon * tne basis capituli, and lies in close contact 



with its fellow of the opposite side. At its distal 

 or free extremity the sheath forms a thin membrane which is invaginated 

 and attached to the shaft of the mandible, so that the two move 

 together. The outer surface of the sheath, except in its proximal 

 third, is covered with minute denticles arranged in rows close to each 

 Other, giving the surface the appearance of a file. Each mandible 

 (Plate LXXXV, figs. 8 and 9) consists of a cylindrical bar of chitin, 

 the shaft, which in a partially cleared tick may be seen to project 

 into the body cavity for a considerable distance below the base of the 

 eapitulum. Its dilated and proximal portion forms the bulb, which is 

 specially grooved to allow space for the attachment of the retractors and 

 extensors of the mandible. The shaft is cylindrical in shape with a ridge 

 on its median surface, and is thickened at its distal end to allow of the 

 articulation of the digits. Each digit (Plate LXXXV, figs. 8 and 9) is 

 an irregularly shaped piece of chitin articulating with the end of the 

 shaft of the mandible, and consists of two moveable articles. The large 

 internal one, which is continued forwards into a long hooked tooth 

 * The sucking apparatus is described in the section dealing with the internal anatomy. 



