Ewing — Significance of Parasitism in Acarina. 65 



Those which are confined or limited to a single host species. 



Those which may be found upon only closely related host species, 

 usually species of the same genus or of very closely related genera. 



Those which are found upon host species which are very widely 

 separated from each other, belonging to distantly related families or 

 even to different classes. 



Our present distribution of the parasitic Acarina ac- 

 cording to host species is dependent upon many factors, 

 some of the more important being : 



The nature of the food of the parasite, whether it be blood, living 

 tissues, cast epidermal cells, etc. 



The means by which the parasite attaches itself to its host. 



The ability of the parasite to withstand fasting when detached 

 from its normal host. 



The degree of locomotor ability possessed by the parasite when 

 detached from its normal host. 



The ability of the parasite to sustain or partially sustain itself 

 when detached from its host upon the diet of a free-living species; in 

 other words a facultative or partially facultative parasite. 



The life history of the parasite especially in regard to the number 

 and methods of its transformations. 



The shedding or molting habits of the host or hosts. 



The wandering or migration habits of the host or hosts. 



The surroundings and the materials of the home or nest for the 

 young of the host or hosts. 



The feeding habits of the host especially in regard to preying or 

 scavenger habits. 



The limits and nature of the geographical distribution of the host 

 or hosts. 



General climatic conditions of the country, especially in regard 

 to temperature. 



Some of the conditions actually found to favor the 

 greatest distribution among the most widely separated 

 hosts are: 



A blood diet (This enables the parasite to engorge itself with an 

 enormous amount of food in a short time and then become detached 

 and live for long periods until a new host is found). 



The possession of ambulatory legs. 



The possession of a clinging apparatus designed to adhere to the 

 flesh itself (Special apparatuses for clinging to either hairs, or 

 feathers, etc., disqualifying their possessors from adhering to hosts 

 which do not have these). 



Oviparous reproduction with a large number of eggs laid when 

 away from any host. 



The ability to withstand great changes in temperature and moisture. 



