CHAPTER V 



Relation of Insects to the Parasitic Worms of Vertebrates * 

 B. H. Ransom 



The only important part insects are known to play in the propagation 

 of parasitic worms that affect human beings and other vertebrates is 

 that of true intermediate hosts necessary to the existence of the parasites 

 in some of their stages of development. Observations have been recorded 

 in the literature showing that flies and other insects may swallow the 

 eggs of various parasites of man such as hookworms, whipworms and 

 other nematodes in whose life history no intermediate hosts are required, 

 also the eggs of tapeworms in whose normal life history it is known that 

 insects are not concerned, for example, Taenia saginata, whose inter- 

 mediate host is the ox. It has been supposed that insects may thus act 

 as mechanical carriers for such parasites, but as a matter of fact definite 

 •evidence of the importance of insects as mechanical carriers of the eggs 

 or larvae of parasitic worms has not yet been brought forth. On the 

 contrary there are reasons to suppose that in some cases at least the 

 swallowing of the eggs or larvae of parasites by insects that can act 

 only as mechanical carriers and not as intermediate hosts, reduces rather 

 than increases the chances of the young parasites continuing their 

 development and reaching a host in which they can become mature. 

 Among the parasitic worms affecting man and other vertebrates it is 

 those forms requiring intermediate hosts, so-called heteroxenous parasites, 

 that are of special interest so far as insect transmission is concerned. 

 The monoxenous parasites, or those requiring no intermediate host, may 

 practically be left out of consideration, with the admission that the 

 mechanical carriage of monoxenous parasitic worms by insects may in 

 the future be proved to have an importance not yet demonstrated. 



A complete demonstration of the part played by an insect in the life 

 history of a given species of parasite is often a difficult matter. The 

 animal which serves as the final host may be subject to infection not 

 only with the species of parasite under investigation but also with other 

 species liable to be confused with it in some of its stages. The insect 



*This lecture was read to the class on December 16, 1918, and distributed January, 

 1919. It has been revised up to date. The names of insects have been revised by 

 the editor. 



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