PARASITES 73 



daily toll from the tissues of the tree ; between the burglar and 

 the blackmailer. The general result is the same, although the 

 methods employed are different. 



3. Although the relative sizes of carnivores and parasites 

 are so markedly different in the vast majority of cases, it is 

 really rather difficult to draw the Hne in all cases between the 

 two classes of animals, since there are a number of species 

 which combine to some extent the characteristics of both 

 types. It is possible and interesting to trace a complete 

 series between the two extremes. A hookworm lodged in the 

 intestine of a mammal is a typical parasite, destroying the 

 tissues of its host, and, in the adult stage, entirely confined to 

 it. A louse is also clearly a typical parasite, although it can 

 walk about from one host to another. Fleas, however, are less 

 constant in their attendance upon their host, since they are 

 able to live for some days at a time without feeding, and may 

 be found walking about at large, in the open or in nests of their 

 host. When we come to blood-sucking flies it is quite diffi- 

 cult to know whether to class them as parasitic or carnivorous. 

 Their habits and size may be the same as those of fleas, and 

 the time which is spent with the host may be no less than that 

 of the flea, but they are less closely attached to any one host. 

 This time varies considerably : a complete series could be 

 traced between species which spend a great deal of their time 

 in the company of their host-animal {e.g. the horse-fly (Hippo- 

 hosca equina) which lives close to its host), and others which 

 lead highly independent lives {e.g. the Tabanids, which are 

 generally active fliers and sit about waiting for some animal 

 to pass). 



4. We can find a similar graded series amongst animals 

 which live by robbing other animals of their food. The adults 

 of certain Filaria worms live embedded in the muscles of their 

 hosts. The larvae of these worms live in the blood or the 

 lymph- vessels of the host. Then there are tapeworms, which 

 inhabit the intestine and absorb food which has previously 

 been made soluble by the digestive juices of the host. Other 

 animals {e.g. nematode worms), often living in the same place 

 as the tapeworms, exist by eating solid food particles, which 



