Introduction 



19 



The propodosomal plate has been known under a variety of names 

 in the different groups of mites. In the ticks and in larvae of the Trom- 

 bidiidae, Trombiculidae, and related groups it is known as the scutum. 

 In the adults of many of the Prostigmata it is called the crista metopica. 

 In the oribatids that are capable of withdrawing the gnathosoma and 

 legs into a cavity in the hystero- 

 soma the propodosomal plate 

 closes the opening like a trap door. 

 In this group, the Ptyctima, the 

 propodosomal plate is known as 

 the aspis. The anterior portion of 

 the dorsal plates or shield of other 

 mites, or in some cases the entire 

 dorsal plate, is in reality the 

 propodosomal plate. 



The secondary plates or shields are formed in a number of ways. 

 In the Pterygosomidae the setal bases become enlarged and fuse to 

 form plates. The cuticle in the region of the genital and anal openings 

 frequently thickens and thus anal and genital plates are formed. At 

 times the cornea of the eyes will be supported at the periphery by a 

 sclerotized ring that becomes enlarged to form an ocular plate. The 

 integument may harden without apparent reason. Areas where glands 

 open or muscles attach frequently develop into plates. In some groups 

 sternal plates are formed by a fusion of the coxae, parts of which may 

 sink below the surface to form internal apodemata. The arrangement 

 of the secondary plates is quite different in different groups of mites 

 and thus discussion of them must be considered separately for each 

 group. 



Figure 12 Resinacariis resinatus Vitz- 

 thum, 1927. A lateral view that shows 

 the tergites. (After Vitzthum 1940) 



Gnathosoma: *The gnathosoma bears the mouth parts, the chelicerae, 

 and the pedipalps. The mouth is hidden by the pedipalps and che- 

 licerae. The gnathosoma is reduced in size and appears to consist only 

 of its appendages and their projections. It is constructed in different 

 ways in the different groups of mites; since fusion, expansion, and de- 

 generation of its component parts have progressed in many ways, its 

 relation to the rest of the body is not the same in all groups. At times 

 the gnathosoma may be enclosed in a camerostome or cavity in the 

 idiosoma, or it may form a prominent snout or beak. 



Dorsally the gnathosoma is usually covered, at least in part, by a 

 tectum (Snodgrass 1948) or epistome as other authors call it. The 



