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THE POPULAR SCIENCE MONTHLY 



is limited to one sex alone, and that the free-winged males of stylops 

 are as highly organized as most of their neighbor insects. 



The class of the sj^iders i^Aixichnida) offers collective examples of 

 degeneration and retrogression, which show how large numbers of 

 animals may acquire lower characters, contrasting with the higher 

 phases to which other members of their class have attained. The 

 mites and ticks have unquestionably originated from the same root- 

 stock as the spiders and scorj)ions. The development of the two 

 groups proves this much. But, while the latter animals have advanced 

 to a high complexity of organization, the mites and ticks have degen- 

 erated into parasitic forms or at least exemplify beings which, first 

 attaining a respectable rank in their own series, have certainly not ad- 

 vanced upon that rank. Many of the mites, however, exhibit well- 

 marked degeneration. Only on the hypothesis of sweeping retrogres- 

 sion can we account for the sing^ular and anomalous condition in which 

 a certain harmless mite, ndixx^^^ Demodex folliculorimi (Fig. 14), spends 

 its existence. This mite inhabits the sacs or follicles of the human 

 skin at the sides of the nose. It is a minute, worm-like animal, pos- 

 sessing eight degenerate rudiments of legs, and a thoroughly rudimen- 

 tary structure in other respects. Here parasitism has denuded the 



Fig. 14. Demodex (magnified). 



Fig. 15, Linguatulina. 



animal of wellnigh every attribute of its Arachnidan character, and 

 has left it in a condition analogous in many respects to sacculina itself. 

 Of the equally curious Linguatidina (Fig. 15), inhabiting the "fron- 

 tal sinuses " or forehead spaces of dogs, wolves, horses, and sheep, the 

 same remark holds good. The body here is thoroughly worm-like in 

 shape (^, c), and a digestive and nervous system are to be enumerated 

 among the possessions of the organism. ]3ut not even thje rudiments 

 of legs are to be perceived, although the mouth bears certain apologies 

 for the appendages proper to that region in the mite and spider class. 

 Yet the young linguatulina (ci) exactly resembles the early form of the 



