INTRODUCTION 13 



the following order: — Insecta, Arachnida, Myriapoda, and 

 Crustacea, leaving the Prototracheata out of account alto- 

 gether for reasons already explained. 



IV. — Arthropoda in their General Relation 

 TO Human Pathology. 



Ever since the days when Aaron stretched forth his rod 

 and smote the land of Egypt with lice and flies, the Arthro- 

 poda — whatever else may be said of them — have had an evil 

 reputation as the source of some of the sharpest and readiest 

 scourges of mankind. 



At first treated merely with disdain or disgust, as 

 pertinacious aggressors or as simple parasites, the species 

 that commonly annoy man are now, as a result of a series of 

 discoveries brilliantly initiated by Manson in 1879, looked at 

 in a much more serious way ; indeed, the present tendency is 

 to go to the other extreme, and to regard them all as vested 

 with lethal powers over man and beast. 



This introductory chapter cannot therefore be concluded 

 until the question of Arthropod parasites and carriers of 

 disease has been briefly looked at from the purely biological 

 standpoint. 



(a) Parasitism ; and Parasitic Arthropods. 



The term parasite (Tro/oao-jTo? = one who lives at another's 

 expense) has been used almost as loosely as the adverbs 

 " awfully " and " absolutely." Some apply it indiscriminately 

 to any animal that commonly associates with other species. 

 Others, with more approach to exactness, apply it to any 

 animal that can and commonly does live by extracting nutri- 

 ment from the tissues of other living animals. Others, still 

 more precise, restrict the term to an animal that is specially 

 modified, or adapted, for obtaining its nutriment solely from 

 the body of some other living animal, generally to the detri- 

 ment of the latter. 



To form a just conception of the term parasite the 

 two factors of association and offence must be considered 

 separately. 



As regards association : Two animals of different kinds 



