34 Acarology 



ings and the posterior pair of legs, are similar to the adults. In some 

 prostigmatid mites the larva is quite different from the adult and in 

 reality metamorphoses into the nymph. The nymphs are usually simi- 

 lar to the adults. In the oribatids, uropodids, and the acarids, however, 

 certain of the nymphs differ markedly from the adults. 



Classification: Most acarologists, entomologists, and zoologists con- 

 sider the Acarina to be an order of the class Arachnida. As long as the 

 Arachnida are subdivided primarily into orders this course reflects the 

 relationships of the Acarina as well as any other. However, when cer- 

 tain of the orders are grouped together into subclasses as is done by 

 Petrunkevitch 1949 and others, it might seem desirable to consider 

 the Acarina as a separate subclass. The Acarina are readily separable 

 from other arachnids in that they possess a distinct gnathosoma. Fur- 

 thermore they are never divided so that a distinct cephalothorax and 

 abdomen are clearly recognizable. The phylogeny of the Acarina is 

 obscure and most students of the group consider them to be polyphy- 

 letic in origin. In the present work the Acarina will be considered as 

 an order. 



A diagnostic classification of the Acarina follows: 



Phylum Arthropoda: Metameric animals with an exoskeleton and jointed 

 appendages. 



Subphyliim Chelicerata: Arthropods without antennae or mandibles. 

 Mouth parts consist of pedipalps and chelicerae. 



Class Arachnida: Chelicerates that lack gill books. 



Order Acarina: Arachnids with the mouth parts more or less dis- 

 tinctly set off from the rest of the body on a false head, capitulum, 

 or gnathosoma. Posterior segmentation is greatly reduced or absent. 

 Primary sclerites are largely replaced by secondary plates of divers 

 origins. Larval stages normally have three pairs of legs; nymphal 

 and adult stages usually have four pairs of legs. Usually minute 

 except for ticks and a few mites. 

 Suborder Onychopalpida (Chapter II): Acarina with typical ambu- 

 lacral claws on the pedipalps and more than one pair of idiosomal 

 stigmata. 

 Suborder Mesostigmata (Chapter III) : Acarina with a single pair of 

 stigmata lateral to the legs that is usually associated with an elon- 

 gated peritreme, or if absent degenerate parasites of the respiratory 

 tract of vertebrates. Haller's organ absent. Hypostome not devel- 

 oped for piercing. 



