Ewing — Significance of Parasitism in Acarina. 3 



same limits of its definition. Used in the broader sense 

 Megnin has described as parasites^ those which live at 

 the expense of others which are living. Simple as this 

 definition is, the scope of its interpretation is so great 

 that it appears almost useless, for it might well be said 

 that nearly all organisms live at the expense, directly 

 or indirectly, of other living organisms. Lenckart would 

 include as parasites^ all those creatures that inhabit a 

 living organism, and obtain nourishment from its body. 

 This definition, which is perhaps as good as any, is ob- 

 jected to by many who claim that a parasite need not 

 necessarily '' inhabit a living organism"; or in other 

 words a definite host relationship is not necessary in 

 order to establish parasitism; for example, many para- 

 sites only visit the forms subject to their attacks at feed- 

 ing times, and certainly do not inhabit them. Another 

 objection may be raised to this definition because the 

 statement of obtaining "nourishment from its body" is 

 hardly clear or sufficient. Some epizoa live upon the 

 excretions, or the secretions, or the food taken into their 

 hosts, yet I do not see how it could be said that they 

 obtained their nourishment from the body of the forms 

 they attack. So with equal impunity every definition 

 offered in the past may be criticised and rejected. 



I cannot hope, therefore, to give a definition of para- 

 sitism that will meet with general acceptance. But I 

 believe that the special forms of parasitism can be sat- 

 isfactorily defined. 



It is only in recent years that the varied nature and 

 complexity of the factors involved in parasitism have 

 been realized. Prominent parasitologists as Leuckart, 

 Kiichenmeister, Cobbold, Megnin, and Neumann, did 

 great and extensive work on the life histories of para- 

 sites. Many specialists in the various groups of parasitic 



» M6gnln, P. Les Parasites et les Maladies Parasitalres. Paris, 1880. 

 ^Leuckart, K. G. F. R. The Parasites of Man. Translated from the 

 German by W. E. Hoyle. Edinburgh, 1886. 



