ENTOMOLOGY

RELATION OF INSECTS TO THE PARASITIC WORMS 51may likewise harbor parasites other than the one that is being studied.The possibilities of confusion and of the entrance of extraneous factorsinto the problem are so many and so varied that in most cases it isonly after the most rigorously controlled experiments, combined withcareful comparative studies of the successive stages of the parasite, thatconclusions may safely be drawn. Furthermore, in working out the lifehistory of a parasitic worm it is not sufficient to prove that insects of acertain species can act as intermediate hosts under experimental conditions.Some species of parasitic worms are able to develop in more thanone species of insect, and the fact that a certain parasite can develop ina certain insect does not necessarily mean that under natural conditionsthe species of inse~t in question serves as the intermediate host of theparasite. For example, one of the common parasites of sheep and cattleis able to pass through its :rhrval stages in cockroaches. These insectsbecome readily infected if the eggs of the parasite which occur in thefeces of the final host animals are fed to them. Under natural conditions,however, cockroaches do not ingest the feces of sheep and cattle, nor arethey found in places where they are likely to be picked up by sheep andcattle. Besides cockroaches, various species of dung beetles have beenshown to be capable of acting as intermediate hosts of the parasite inquestion, and it is evident that these insects are the natural intermediatehosts. Unlike cockroaches they have plenty of opportunity both ofbecoming infected and of passing on their infection to the final hosts.A more or less intimate environmental relationship between the insecthost and the final host generally exists in the case of parasites transmittedby insects. In a number of cases the insects are coprophagous and alsolikely to be ingested by the final hosts, as in the instance just cited.Another highly interesting group of cases is that in which the insectsare ectoparasites on the final hosts, or bloodsuckers that periodicallyvisit them, and thus have particularly favorable opportunities for becominginfected with parasitic worms harbored by the animals they attackand in turn reinfecting the latter.MODE OF INFEC-TION OF INSECT HOSTSAs already stated the part which insects may take in the propagationof parasitic worms of higher animals is that of intermediate hosts, inwhich certain larval stages of the parasites are passed before they areready to enter the bodies of their final or definitive hosts in which theydevelop to maturity. The way in which the insects become infected varieswith different species of parasites. In the case of some species whichlive in the alimentary tract of the final host the eggs or larvae are dischargedfrom the body of the host in the feces. Coprophagous insects