Gr. H. F. Nuttall 
25 
its digits by moving and retracting them, it cannot do so with its hypostome 
which is a rigid structure whose chitin is continuous with that forming the 
basis capituli. The hypostome is the most readily regenerated element of 
the mouthparts because it consists almost entirely of chitin, its formative 
tissues lying beneath the stout chitinous base whilst it is developing in the 
Fig. 5. Amblyomma hebraeum $. Capitulum in dorsal and ventral aspect (Original, ca x 16). 
Fig. 6. Hyalomma aegyptium $. Capitulum in dorsal and ventral aspect 
(Nuttall, 1911 ca. x 16). 
immature stages prior to a moult. In this position, the formative tissues are 
not liable to be hurt, whereas in the case of the palps and chelicerae as with 
the legs, the formative tissues are more directly accessible to injury. 
The power of regenerating mutilated mouthparts possessed by immature 
ticks is of paramount importance in connection with their maintenance in 
