EXTERNAL STRUCTURE: THE PALPS 571 



the true head of a tick is not recognizable, as it has become fused with 

 the rest of the body. The capitulum is the small chitinous structure 

 which bears the mouth, and which has attached to it several specialized 

 appendages. It consists of the following parts : 



The basis capituli, or mouth shield, is the dense basal portion which arti- 

 culates with the body by abroad neck-like structure; when viewed dorsally 

 it is more or less flattened and rectangular in shape ; its ventral surface 

 is usually convex. The shape of the basis capituli varies in different 

 species, and is of some taxonomic value ; it may, for instance, be triangu- 

 lar, globular, or hexagonal. The postero-lateral margins may be prominent 

 as in the case of Rhipicephalus sangnineus (Plate LXXVII, figs. 1 

 and 2) or they may be rounded off (Plate LXXIII, fig. 10). On the 

 dorsal surface of the basis capituli there is a well-marked ridge, the 

 dorsal ridge (Plate LXXIII, figs. 1, 4, 5, 8, 10, 11, 13 and 14), which 

 extends transversely across the dorsum up to the postero-lateral margin, 

 where it may end. The ridge comes in contact with the anterior edge 

 of the scutum, especially when the capitulum is raised ; the posterior 

 portion of the basis capituli is then concealed. 



On the dorsal aspect on each side of the central area there is a shallow 

 depression which contains the porose areas (Plate LXXIII, figs. 1, 4, 8, 

 10 and 14) ; these structures, which are present only in the female, consist 

 of many minute open pores ; they are of considerable taxonomic value. 

 In the male tick the dorsal surface of the basis capituli is much smoother, 

 there being no porose areas, and the ridges, except the dorsal one, are 

 poorly developed. There are, however, a few scattered pores to be seen, 

 and occasionally a few hairs, when the basis capituli is said to be 

 glabrous. 



Deeply embedded into the antero-lateral margins of the basis capituli, 

 in some species towards the ventral side, in others towards the dorsal, 

 there are two flap-like structures, the palps (Plate 

 LXXIII, all figs.) which vary considerably in shape 

 and size in the different species of ticks ; the general outline, how- 

 ever, is that of a truncated cone. In Hyalomma (Plate LXXIII, 

 figs. 4 and 5) they are long and valvate, partially obscuring the 

 mouth parts; in Haemaphysalis (figs. 11 and 12) and Rhipicephalus 

 Cfig. 9) they are short and broad, while in Ixodes (figs. 1 and 2) 

 they are long and cone-shaped. Each palp consists of four segments, 

 which differ considerably in size and form. Each has a dorsal, 

 a ventral, an external and an internal or median surface, and these vary 

 in the different genera. The first (basal or proximal) segment is nearest 



