RELATION OF INSECTS TO THE PARASITIC WORMS 51 



may likewise harbor parasites other than the one that is being studied. 

 The possibilities of confusion and of the entrance of extraneous factors 

 into the problem are so many and so varied that in most cases it is 

 only after the most rigorously controlled experiments, combined with 

 careful comparative studies of the successive stages of the parasite, that 

 conclusions may safely be drawn. Furthermore, in working out the life 

 history of a parasitic worm it is not sufficient to prove that insects of a 

 certain species can act as intermediate hosts under experimental condi- 

 tions. Some species of parasitic worms are able to develop in more than 

 one species of insect, and the fact that a certain parasite can develop in 

 a certain insect does not necessarily mean that under natural conditions 

 the species of insect in question serves as the intermediate host of the 

 parasite. For example, one of the common parasites of sheep and cattle 

 is able to pass through its larval stages in cockroaches. These insects 

 become readily infected if the eggs of the parasite which occur in the 

 feces of the final host animals are fed to them. Under natural conditions, 

 however, cockroaches do not ingest the feces of sheep and cattle, nor are 

 they found in places where they are likely to be picked up by sheep and 

 cattle-. Besides cockroaches, various species of dung beetles have been 

 shown to be capable of acting as intermediate hosts of the parasite in 

 question, and it is evident that these insects are the natural intermediate 

 hosts. Unlike cockroaches they have plenty of opportunity both of 

 becoming infected and of passing on their infection to the final hosts. 



A more or less intimate environmental relationship between the insect 

 host and the final host generally exists in the case of parasites transmitted 

 by insects. In a number of cases the insects are coprophagous and also 

 likely to be ingested by the final hosts, as in the instance just cited. 

 Another highly interesting group of cases is that in which the insects 

 are ectoparasites on the final hosts, or bloodsuckers that periodically 

 visit them, and thus have particularly favorable opportunities for becom- 

 ing infected with parasitic worms harbored by the animals they attack 

 and in turn reinfecting the latter. 



MODE OF INFECTION OF INSECT HOSTS 



As already stated the part which insects may take in the propagation 

 of parasitic worms of higher animals is that of intermediate hosts, in 

 which certain larval stages of the parasites are passed before they are 

 ready to enter the bodies of their final or definitive hosts in which they 

 develop to maturity. The way in which the insects become infected varies 

 with different species of parasites. In the case of some species which 

 live in the alimentary tract of the final host the eggs or larvae are dis- 

 charged from the body of the host in the feces. Coprophagous insects 



