16 



Acarology 



of setae are recognizable: the setae proper, and the sensory setae 

 which are obviously different from the majority of body setae. 



The setae proper ("Eigentliche Haare" of Vitzthum 1940, "Polls 

 proprement dits" of Grandjean 1935) are of many types (Figure 

 10). They may be simple, pilose, plumose, capitate, spatulate, cordate, 

 palmate, pilidiform, pinnate, chambered, dentate, furcate, or may be 

 so irregular that only a careful description will suffice for their identi- 



Figure 9 The arrangement 

 of the integumental glands 

 of Megaluracarus globator 

 (Miiller), 1776. (After Vitz- 

 thum 1940) 



fication. It is difficult to grasp the significance of the diversity of form 

 exhibited by the setae. Their form and number in any species are 

 usually constant. For this reason they are very useful as taxonomic 

 characters. 



The primitive type of seta was most likely attenuate and pilose. The 

 primitive arrangement of the setae corresponded with the primary 

 segmentation of the animal, each segment being provided with a row 

 of from two to six dorsal setae at its posterior margin. This primitive 

 arrangement is maintained with little modification in certain of the 

 Trombidiformes and Sarcoptiformes. In all of the larger groups, how- 

 ever, reduction or multiplication of the setae has occurred many times 

 so that the primitive arrangement is obscured in many species. 



The type of sensory seta most frequently seen is a striated sensory 



