18 Trans. Acad. Sci. of St. Louis. 



One zoophagous parasitic group, showing affinity to the 

 plant feeding free forms, is known: 

 The parasitic Pediculoldidae. 



Very many facts could be brought together, showing 

 that each of the four phylogenetically separate sub- 

 groups of parasitic forms included in the first group of 

 parasites indicated above has been evolved from pre- 

 daceous ancestors; but, owing to the limits of space, only 

 two of the subgroups will be considered here, the para- 

 sitic Cheyletidae and the parasitic Peritremata, exclud- 

 ing those few forms found in the Uropodidae, which 

 evidently had a separate origin. 



The parasitic Cheyletidae both in habits and in their 

 superficial externsfl characters appear to be related to 

 the Sarcoptidae and the Analgesidae, and have been 

 placed with such forms by some authors; but the follow- 

 ing structural characters will, I think, show that their 

 affinity with the Cheyletidae should not be disputed: 



They possess tracheae, hence these must have been either evolved 

 from tracheate ancestors, or the tracheal system must have been evolved 

 independently in forms of parasitic habits. The latter alternative is 

 something almost entirely beyond our belief, since it is In contradiction 

 to the common influence of parasitism; and if so, would represent a 

 process so exceptional in the history of arthropod evolution as to be 

 only duplicated in an extremely few Instances, covering vast periods of 

 geological time. 



The absence of anal suckers and tarsal suckers, and the possession 

 of almost normal tarsal claws allies them with the Cheyletidae rather 

 than the Analgesidae. 



In the case of the gefius Harpyrhynchus the enormous size of the 

 palpi, so similar to those of the free Cheyletidae certainly establishes 

 the aflBnIty of these forms with the Cheyletidae. 



Passing to a study of the habits of the Cheyletidae, the 

 steps in the origin of parasitism in this family become 

 more clear. 



The free-living Cheyletidae are very small creatures, 

 many of them being not more than half a millimeter in 

 length. Their palpi are enormously developed (PI. Ill, 

 Fig. 8), and instead of working vertically, as is true of 



