( cxlii ) 



the facts embodied in this paper are compiled from the notes 

 my friends have kindly given me. 



Parasitism is not an original form of existence; on the 

 contrary, it is an acquired habit, acquired slowly through 

 ages. Parasites are derived from non-parasitic forms, and 

 the alteration of habit is accompanied by corresponding 

 morphological changes. Parasitism and non-parasitism are 

 two conditions somewhat analogous to pathological and normal 

 states, the pathological being a modification of the normal or 

 healthy tissue. The study of parasites and parasitism is 

 fascinating and delightful to the speculative mind, as a 

 comparison between the various parasitic insects and their 

 non-parasitic relatives enables the observer to trace changes 

 and modifications which are more apparent among parasites 

 than among normal insects. 



Ectoparasitism is a mode of life adopted by the members 

 of several orders of insects either in one stage of the life of 

 the individual or throughout its entire existence. In some 

 cases it is only the young stages which adopt an ectoparasitic 

 existence, for example, many mites; in others it is the 

 imago only which is an ectoparasite, for example, fleas ; while 

 in others again, the parasitic habit obtains from birth to 

 death, as in the case of Anoplura and Mallophaga. Some 

 of these Epizoa never leave the host on which they dwell 

 and feed, while others are temporary visitors only when 

 they are in need of food. A third association *appears to 

 occur in at least one case, the curious Hemimerus, a parasite 

 on an African rat, which appears to use its host more as a 

 means of locomotion than for any other purpose, it being 

 supposed that the Hemimerus does not secure any food from 

 the skin of its host. 



The great variation which obtains in the degree of para- 

 sitism, in the number of hosts frequented, and in the orders 

 from which parasites are derived, has naturally produced 

 numerous and varied species of parasitic tracheates. Not- 

 withstanding this fact, even the casual observer must notice 

 the repeated recurrence in widely different orders of similar 

 morphological details. While there is no general uniformity, 

 many Epizoa exhibit points of remarkable resemblance. 



