Ewing — Significance of Parasitism in Acarina. 



The Zoological Position of the Acarina, 



The Acarina consist of a more or less natural group 

 of the class Arachnida. The saclike form of the body, the 

 absence of a distinct head, the possession of four pairs of 

 legs, the usually very incomplete metamorphosis, as well 

 as the structure and relationships of the internal organs, 

 are only a few points which demonstrate their undoubted 

 position in this class. The absence or incomplete de- 

 velopment of many of the special senses, the loss of 

 many specialized structures found in a high grade of de- 

 velopment in some of the other groups of Arachnida, 

 as well as the frequent assumption of a parasitic life, all 

 point toward a line of degenerate descent from the an- 

 cestral stock of the class to which they belong. It must 

 be remembered, however, that in some respects they show 

 a development of specialization that is perhaps not ex- 

 celled by any of the various groups of the arachnids. 



The nearest affinity of the order appears to be with the 

 Phalangidea, yet in som.e respects they are more nearly 

 related to the Solpugida. This relationship of the Aca- 

 rina to the Phalangidea has been most clearly shown by 

 the discovery and study of some of the smaller tropical 

 phalangids. Some of these phalangids are exceedingly 

 mitelike in general appearance, and a careful study of 

 them brings out other points of relationship. Among 

 these characters might be mentioned their smallness, 

 the shortness of the legs and their reduced number 

 of segments, the more nearly saclike shape of the body, 

 the length of the palpi, and the form of the body. But 

 many of the characters of the Acarina may be used to 

 demonstrate their affinity with the more common of the 

 Phalangidea. These may be listed as below:— 



A general similarity in the shape of the body, especially in the 

 breadth and frequently depressed condition of the abdomen. 



The possession of a segmented abdomen by several forms of the 

 Acarina, showing probably a homologous state to that of the Phalan- 

 gidea. 



