Introduction 29 



off into the lumen of the mid-gut from whence they pass into the in- 

 testine. Mites excrete their nitrogenous wastes in the form of guanin. 

 The Notostigmata have two excretory tubules and a pair of coxal 

 glands that open on coxae i or ii. The Holothyroidea have two pairs 

 of excretory tubules and a pair of coxal glands that open on coxae i. 



The Mesostigmata have one pair of excretory tubules and from one 

 to four pairs of coxal glands. 



The Trombidiformes have no excretory tubules of the usual type 

 but the hind-gut is modified as an excretory organ in many of them. 

 Coxal glands are present in some of the Trombidiformes. 



The Sarcoptiformes have a pair of small excretory tubes. Coxal 

 glands have been found in the oribatids. 



Circulatory System: The circulatory system of most Acarina consists 

 only of the blood, which is colorless and bathes all of the organs of the 

 body. Amoeboid leucocytes are present and can best be demonstrated 

 during the quiescent stages that precede ecdysis, A simple heart is 

 present in the Holothyroidea and some of the Mesostigmata. 



Respiratory System: Of all the anatomical features of the Acarina, 

 those associated with the respiratory system are most important to the 

 systematics of the group. The suborders are established largely on the 

 basis of this system. In those mites that have tracheae the number and 

 placement of the stigmata are of first importance. In function the res- 

 piratory system is similar to that of most terrestrial arthropods. The 

 main tracheae are subdivided into tracheoles that run through most of 

 the tissues and provide for the gaseous exchanges required by the 

 metabolism of the cells. Some mites have no tracheal system. These 

 mites are usually small, and respiration must be either carried on 

 through the cuticle or by means of anaerobic reactions. 



The Notostigmata have four pairs of stigmata situated on the dorsal 

 surface of the first four segments of the opisthosoma. They open 

 through the leathery cuticle and are not supported by stigmal plates. 

 Tracheal trunks branch out from each pair of stigmata but they do 

 not anastomose. 



The Holothyroidea have two pairs of stigmata. The anterior pair is 

 situated laterally above coxa iii. It communicates with a vestibule from 

 which many tracheae extend out into the body tissues. The posterior 

 pair of stigmata open into a pair of atria from which many thin-walled 

 diverticula extend. 



