INTRODUCTION 15 



with the female Culicidce. Many Culicid^ — as on small 

 tropical islands — may seldom get a chance of sucking 

 blood ; others such as Stegomyia fasciata, which is so 

 constantly associated with man, in tropical latitudes, as to 

 be known as a house-mosquito, may make a habit of it. 

 Here we see a more or less predatory insect becoming 

 commensal like a house-fly, and thus illustrating both aspects 

 of parasitism. 



The bed-bugs belong to an Order which includes a large 

 number of predatory species : the ticks belong to a Class 

 which is pre-eminently predaceous : hardly any modification 

 of the structure and habits of these two groups of animals is 

 necessary to convert them into parasites, without any bias 

 from association. Here we have an instance of a predatory 

 parasite that does not always form a fixed and constant 

 association with any particular kind of animal. 



From loose parasites of this kind we pass to parasites 

 like lice {Siphunculatd), which are definitely associated with 

 animals of a certain kind — mammals. Here the whole 

 organisation is profoundly modified in the way of adapta- 

 tion to the particular mode of life. Wings are absent ; legs 

 are converted into grapnels for clinging to hair ; the mouth- 

 parts form a long suctorial tube, which can be firmly anchored 

 in the host's skin ; the eggs are firmly attached to the host's 

 hairs ; and metamorphosis is suppressed, so that the young 

 louse when it leaves the egg is a finished parasite like its 

 parents. 



The few instances selected above are quite sufficient to 

 show that there is no clear line of separation either between 

 commensalism and parasitism, or between piracy and para- 

 sitism ; but that we can pass by easy gradations and many 

 circuits from innocent casual attachments and chance 

 predatory onslaughts to those definite associations of two 

 diiferent species, for the benefit of one of them and to the 

 detriment of the other, which constitute true parasitism. 



(b) Arthropoda as " Carriers " of Disease. 



(i) The term "carrier" has been used almost as loosely as 

 the term parasite, the looseness indeed being inherent in the 



