Introduction 



21 



it can be seen as a toothed structure between the pedipalps (Figure 



14). The so-called hypostome of the ticks is not homologous to the 



hypostome of the other groups but is in reality 



formed largely from the pedipalps although its 



basal portion is formed in part by the true 



hypostome. 



Chelicerae: The chelicerae of the Acarina usu- 

 ally terminate in a chela that is composed of a 

 dorsaL fixed digit and a ventral, movable digit. 

 In all of the suborders such chelate chelicerae 

 are to be found. In the Notostigmata, Holo- 

 thyroidea, and the Tetrapodili only chelate 

 chelicerae are known. The chelicerae of the 

 Tetrapodili are modified, however, in that the 

 movable digit has become stylet-like and is 

 the sole effective piercing element (Figure 15). 

 The chela is formed from the tibia and tarsus. 

 Fundamental segmentation of the chelicerae 

 is obscure. In most arachnids the chelicerae con- 

 sist of three segments. In some of the younger 

 stages of certain mesostigmatids the chelicerae 

 have six segments. In most other acarinids, how- 

 ever, the chelicerae are divided into two or three 

 segments. 



The primitive type of chelate chelicera is superficially similar in 

 structure to the chelae of many decapod crustaceans. Each digit is pro- 

 vided with teeth which may or may not oppose each other. This type 

 of chela is used for grasping prey or crushing other types of food 

 (Figure 16). In the males of many of the mesostigmatids the movable 

 digit is modified as an accessory copulatory organ (Figure 17). It is 

 used to transfer^ the spermatophores from the 

 genital opening of the male to that of the female. 

 Even though the movable digit is greatly modified 

 the fixed digit is unchanged. 



The primitive chelate type is modified in many 

 ways. Both elements may become minute and 

 needle-like and together form a piercing structure 

 such as is found in Dermanyssus (Figure 18). The 

 fixed digit may disappear and the movable digit 



Figure 14 The hypo- 

 stome of Ixodes rasiis 

 Neumann, 1899. 



Figure 15 The che- 

 licera of Eriophyes 

 pini (Nalepa), 1887. 



