CHAPTER III 



The Suborder Mesostigmata G. Canestrini^ 1819 



THE Mesostigmata and Ixodides are usually combined to form a 

 single suborder, the Parasitiformes. The evidence for this arrange- 

 ment is good on morphological grounds, especially when the position 

 of the stigmata and the structure of the gnathosoma are considered. 

 However, two other suborders — the Trombidiformes and Sarcopti- 

 formes — are apparently more closely related to each other than are 

 the Mesostigmata and Ixodides. The relationship among these sub- 

 orders, however, is not expressed in the classification. It seems desir- 

 able for simplicity to omit the term Parasitiformes from the classifica- 

 tion. This omission is not intended to deny relationship between the 

 suborders but is made to increase the utility of the classification. 



The Mesostigmata are readily recognized by their gnathosoma, lat- 

 eral stigmata, tritosternum, dorsal plates, ventral plates, genital open- 

 ings, and legs. They are diverse in the details of their structure but are 

 nevertheless a fairly compact group. Vitzthum 1931 estimated that 

 there were 1,290 species at that time, but since 1931 numerous new 

 species have been described. Even at present almost every small col- 

 lection of mites contains a few new species of Mesostigmata. Since so 

 many forms await discovery and description, the classification is of 

 course in a state of flux. 



In a recent series of papers Tragardh has investigated the compara- 

 tive anatomy of their sclerotized structures and has succeeded in ob- 

 taining some degree of order out of the previous chaos. This has been 

 achieved largely by studying the ventral plates of the females and the 

 structures associated with the genital openings. Although his groups 

 are established on the basis of the structure of females, his more recent 

 investigations show that the ventral plates of the males may also be 

 useful in demonstrating relationships. Despite Tragardh's excellent 



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