54 Trans. Acad. Sci. of St. Louis. 



The degeneration of the locomotor appendages is also 

 as complete as that of the special senses. We get all 

 degrees of degeneration from the type of legs found in 

 the free forms; consisting of from five to seven distinct 

 segments, clothed with bristles and ending in special 

 adaptive tarsal appendages, as claws, caruncles, and pul- 

 villi; to even entirely limbless sarcoptids.^^ Bridging the 

 space between these two extremes, two common types 

 of legs might be mentioned, that of the Eriophyidae and 

 that of the Demodecidae. In the former family the legs 

 are reduced to four in number; and although they con- 

 sist of five segments each, they are weak and unable to 

 sustain the weight of the body (See PL VII, Fig. 29). 

 In the latter family the legs are of the normal number, 

 yet are greatly reduced, consisting of only three short, 

 stumpy segments and scarcely extending beyond the mar- 

 gins of the body (PL IV, Fig. 14). 



All of these changes thus far mentioned, the assump- 

 tion of a flattened form of body, the possession of back- 

 wardly directed spines and bristles, of hooks and claws 

 for adherence, and the degenerative process affecting 

 the special senses and the locomotor organs, can only 

 be fully accounted for by the action of natural selection 

 working in response to similar environmental conditions. 

 It has resulted in bringing forms descended from widely 

 different groups to the assumption of a variety of sim- 

 ilar characters, so that now they appear to be closely re- 

 lated forms. It illustrates excellently adaptive converg- 

 ence (Note the similarity of Figs. 10 and 11 on PL III, 

 and compare with illustrations of living forms. Figs. 8 

 and 9, probably very similar to the ancestral types of 

 each). 



Divergence and Special Adaptations. 



Wliile the assumption of similar parasitic habits has 

 in many cases resulted in similar adaptations so that 

 primitively widely divergent forms now are structurally 



" TrSgardh, In 1902, discovered some sarcoptids under the elytra of a 

 species of Pimelia In which the extremities were entirely suppressed. 

 See Zool. Anzeig. 25: 617-8 (3 figs.). 



