Introduction 13 



ments, each one of which bears a pair of legs. The opisthosoma is 

 usually composed of six segments. Variation in the number of opis- 

 thosomal segments is found and this accounts for variation in the total 

 number of segments suggested above. The Notostigmata may have as 

 many as ten or eleven opisthosomal segments. Pachygnathus may have 

 eight or nine and Parhypocthonius has seven. 



The gnathosoma is usually set off from the rest of the body at least 

 dorsally; but in some groups (Uropodina, Spelaeorhynchidae, Crypto- 

 stigmatina, and others), the segments of the propodosoma extend over 

 the gnathosoma and enclose it in a cavity or camerostome. In one fam- 

 ily of the Prostigmata (Smaridiidae) the gnathosoma is protrusible on 

 a long, narrow, trunklike stalk that may be as long as the mite itself. 

 The gnathosoma is narrow and in no way can be said to correspond 

 to the head of other arthropods. It is frequently called the capitulum, 

 but the term gnathosoma is to be preferred since it bears only the 

 mouth and mouth parts. 



The propodosoma with the first two pairs of legs is frequently sepa- 

 rated from the metapodosoma by a deep furrow. The Trombiculidae 

 exhibit this separation very well. It is for this reason that the gnatho- 

 soma and propodosoma together are referred to as the proterosoma 

 while the metapodosoma and opisthosoma which are usually fused in- 

 sensibly with one another are designated as the hysterosoma. 



The prosoma corresponds to the cephalothorax of other arachnids 

 but in no case are the anterior segments so fused that they form a 

 distinct body region that can be designated as a cephalothorax in con- 

 tradistinction to a posterior abdomen. However, Speleorchestes is 

 exceptional in that the gnathosoma, propodosoma, metapodosoma, and 

 opisthosoma are apparently each distinct from the others. 



The podosoma is that portion of the body that bears the walking 

 legs. The idiosoma includes the body of the mite posterior to the 

 gnathosoma. In many mites and all the ticks the only clearly recog- 

 nizable body divisions are gnathosoma and idiosoma. 



Integument: As is the case with other arthropods the integument of 

 the Acarina consists essentially of a single layer of epithelial cells, the 

 hypodermis, and sclerotized layers which they secrete. The cuticle 

 (Figure 6) can be subdivided into four distinct layers which are char- 

 acterized by Vitzthum 1940 as follows: 



1. Tectostracum — the outermost covering which is always very thin and 

 never pigmented 



