Introduction 23 



extension of the synarthrodia! membrane produces a pulvillus which 

 resembles a cluster of setae in some cases or a coronet in others. There 

 is frequently a seta at the tip of the fixed digit but not on the movable 

 digit. Setae and sensory pores are found on the chehcerae of other 

 groups as well. 



Pedipalps: The pedipalps are the second appendages in the arach- 

 nids. In the Acarina they originate laterally from the palpal coxae 

 which are probably homologous to the fused, basal segments of a 

 primitive, biramous appendage. In recent Acarina the pedipalps are 

 never divided into more than six segments, but in the Devonian Pro- 

 tacarus crani Hirst there are seven segments. The segments of the pedi- 

 palps are named as are those of the legs, beginning with the proximal 

 element, as follows: coxa, trochanter, basifemur, telofemur, genu, tibia, 

 and tarsus. The reduction in the apparent number of segments from 

 the primitive seven to the maximum of six in living acarinids is accom- 

 plished by fusion of the basifemur and telofemur to form an undi- 

 vided femur. 



A relatively unmodified palp is found in the Onychopalpida, 

 Mesostigmata, Cryptognathidae, Sarcoptiformes, Limnocharidae, and 

 Eylaidae. In some of the above groups the palp is six-segmented but 

 in all the tarsus and tibia are similar to those of the legs. The palpal 

 tarsi of the Onychopalpida are provided with claws as are the tarsi of 

 the legs (Figure 20). 



Figure 20 Right, the palpal tarsi of Neocarits texanus Chamberlain and Mulaik, 

 1942 and left, the palpal tarsi of Holothyriis australasiae Womersley, 1935 to 

 show the palpal claws. The setae have been omitted so that the claws are 

 apparent. 



