68 Trans. Acad. Sci. of St. Louis. 



portant mite parasites arose. It is found generally associated with, 

 scale insects. — Fig. 5. Hemisar copies malus (Shimer). This is the 

 most efficient of all the natural enemies of the Oyster Shell Scale. 

 It is hardly a real parasite as yet, but has taken to feeding upon live 

 eggs of the scale insects and will occasionally attack the mature 

 insects themselves. The diet upon decaying organic matter has been 

 entirely forsaken. Degeneration is noticed in the shortening of the 

 legs and in the reduction of the mouth-parts. Specialization is shown 

 in the development of suckers situated each on a tarsal pedicel. The 

 homologues of these suckers can be seen in Fig. 4, where they are 

 present as a flat padlike tarsal appendage. — Fig. 6. Notoedres notoe- 

 dres (M6gn.). The itch mite of the rat. Here parasitism is firmly 

 established. Degeneration has continued in the shortening of the 

 legs, and the shortening or loss of their tarsal suckers, the reduction 

 of the mouth-parts and a change of the original bodily form. — Fig. 7. 

 Cnemidocoptes mutans (Robin), dorsal view. The itch mite causing 

 "Scaly Leg" among chickens; a true parasite. Here the individual 

 has become so degenerate that only stumps of legs are left which 

 bear no tarsal appendages. The mouth-parts are very simple while 

 the body is only a round fleshy mass entirely devoid of hairs except 

 for a single posterior pair. 



Plate III. — Converging Adaptation. — The two small parasitic mites 

 figured at the bottom of this plate have a very great superficial re- 

 semblance to each other, and for a long time were supposed to be 

 closely related forms. The one represented by Fig. 10, is now known 

 to be closely related to the predaceous Cheyletidae, forms that are 

 very highly developed; tbe other, represented by Fig. 11, is on the 

 contrary most nearly related to the simplest of the free-living Acarina, 

 the Tyroglyphidae. — Fig. 8. Cheyletus seminivorus Packard. A mem- 

 ber of the family Cheyletidae. The Cheyletidae possess highly de- 

 yeloped and specialized mouth-parts, an elaborate tracheal system, 

 have the sense of touch highly developed, and usually possess eyes. — 

 Fig. 9. Tyroglyphus lintneri Osborn, one of the "Cheese Mites." The 

 members of the family Tyroglyphidae, the "Cheese Mites," are prob- 

 ably the most degenerate and the simplest of all the free-living mites. 

 The mouth-parts while rather prominent are very simple; no tracheae 

 are present, the sense of touch is poorly developed, and they are all 

 blind. — Fig. 10. Harpy rhynchns hrevis Ewing. The members of the 

 genus Harpyrhynchus differ from their predaceous free ancestral 

 types in the great shortening of the legs, the degeneration of the 

 mouth-parts, the loss of the eyes, etc. Internally they still have left 

 a tracheal system, but it is very vestigial.— Fig. 11. Sar copies scaUei 

 De Geer, the itch mite of man. Superficially it resembles the species 

 of Harpyrhynchus just mentioned. Since its free type was very 

 simple the degenerative process has been less. 



Plate IV.— Diverging Adaptation.— The different types of parasitic 

 Acarina here figured, though very diverse in their appearance and 

 structure, apparently have descended from a common scavenger an- 



