8 Trans. Acad. Sd. of St. Louis. 



TBe possession of median eyes In some of the mites, suggesting the 

 condition found in the Phalangidea, 



The close similarity of the typical chelate chelicera of the mites to 

 those of the Phalangidea. 



The possession by the two groups of very similar internal organs, 

 especially those of reproduction, and digestion. 



The similarity of hablti existing between many of the free-living 

 Acarina and the Phalangidea. 



The relation of the Acarina to the Solpugida is best 

 brought out by the study of the species of the genus 

 Rhagida, belonging to the family Eupodidae. There is a 

 remarkable resemblance of these mites to solpugids. 

 This is shown in the general shape of the body, the form 

 and shape of the legs and the palpi, but more especially 

 in the size and shape of the chelicerae. These are of 

 enormous size as in the solpugids, being equal to a fourth 

 or fifth of the total length of the body. They are greatly 

 swollen, and the movable digit works vertically as in the 

 solpugids. An investigation of their internal anatomy 

 has shown that it is among the most primitive of the 

 Trombidoidean type. 



Of the various groups of the Acarina which have been 

 suggested as being most nearly related to the primitive 

 stock, the genus Opilioacarus^ has attracted most atten- 

 tion of late. The genus is represented by four species 

 found in the following countries: Algeria, Italy, Ara- 

 bia, and South America. These mites have a segmented 

 abdomen, leg-like palpi, chelate chelicerae, two pairs of 

 eyes; while the tracheae open by means of four dorsal 

 stigmata. The possession of a segmented body is cer- 

 tainly a marked primitive character, but it must be re- 

 membered that some of the well-known groups possess 

 a more or less segmented body, for example the Tarson- 

 emidae and some of the Tetranychidae. The possession 

 of leglike palpi as well as chelate chelicerae are even 



' For a figure of one of these species and a short discussion of the 

 systematic characters see: Oudemans, A. C. A Short Survey of the 

 More Important Families of Acari. Bull. Ent. Research. 1 : 105. 1910. 



