Eiving — Significance of Parasitism in Acarina. 53 



or feathers only to be caught by other hairs or feathers 

 on account of these long spines and thus are saved from 

 a complete loss of a host. This fact, I think, would sug- 

 gest itself immediately to any one who had ever endeav- 

 ored to pick off these live parasites from their hosts, or 

 who has watched domestic animals trying to rid them- 

 selves of them. 



The degeneration of the special senses, an almost uni- 

 versal accompaniment of parasitism, is fully illustrated 

 in the Acarina. Of the special senses found in the free- 

 living mites that of touch is undoubtedly the highest de- 

 veloped; sight is present in some cases, but many of the 

 free forms are without eyes. It appears to be estab- 

 lished that many free forms have the faculty of smell 

 and also that of taste. The possession of the sense of 

 hearing has never been satisfactorily proved, I believe, 

 for any of the Arachnida, not even in the spiders, where 

 stridulating organs are found; so, of course, it is sup- 

 posed to be wanting in the mites. 



All of the special senses which are found in the free 

 forms show successive stages of degeneration in accord- 

 ance with the progressive stages of parasitism. In prac- 

 tically all of the free forms we find special tactile struct- 

 ures present, in the form of long specially adapted palpi 

 or front legs, the possession of tactile bristles, or even 

 special organs developed for this purpose. In the Sar- 

 coptidae, some of the Cheyletidae, in the Listrophoridae, 

 and in other groups all these tactile organs have atro- 

 phied. Although many of the free forms, closely re- 

 sembling the types from which parasitic forms evidently 

 arose, have well-developed eyes, these structures appear- 

 to be universally absent in parasitic forms. Here the 

 degenerative process has been complete. No evidence^ 

 appears to be obtained that the sense of smell has been 

 lost or has become weakened in the parasitic forms. But 

 since the development of this sense in the free forms is 

 slight, it would be difficult to get reliable data upon its. 

 decline, if present, in the parasitic forms. 



