Introduction 



31 



ply other portions of the body. Apparently motor and sensory fibers 

 are incorporated in all the nerves. 



Sense Organs: As with other arthropods the setae of the Acarina 

 are primarily sensory in function. They have been discussed under the 

 section on the integument with one exception. Many of the Trom- 

 bidiformes and Sarcoptiformes possess specialized setiform, pseudo- 

 stigmatic organs or sensillae. They usually arise from a specialized 



/1/^V^^ 



Figure 28 The emergent peritremes 

 of Allothrombium sp. 



Figure 29 The brain of a Tetrany- 

 chits. (After Blauvelt 1945) 



depression in the propodosomal plate or in the region of this plate if it 

 is lacking. This depression is known as a pseudostigma and in the Ori- 

 batei is connected with air sacs that aid in respiration. The function 

 of the pseudostigma and its sensilla is unknown. That it is sensory is 

 probably correct since the sensillae are directly connected to the nerv- 

 ous system. The terrestrial mites that possess them usually have them 

 well developed, but closely related aquatic groups have them reduced 

 or lacking. Sensillae may vary in structure as widely as the body setae 

 (Figure 30). They can be readily recognized in any one species be- 

 cause they are always considerably different from the ordinary setae. 



The palps and legs of all acarinids are more or less thickly clothed 

 with setae and in many cases are provided with striated, sensory setae. 

 At times the palps or one or more pairs of legs are modified as tactile 

 organs. The tarsi are usually well provided with special sensory setae 

 and in the ticks and Rhagidiidae special organs have developed on the 

 anterior tarsi. Haller's organ is characteristic of all stages of all ticks 

 with the exception of Ceratixodes that lack it in the nymphal and per- 

 haps also in the larval stage. Haller's organ is associated with the 

 olfactory responses of ticks. It is situated on tarsus i and usually con- 



