58 SANITARY ENTOMOLOGY 



attract different species of miracidia) it burrows into the soft tissues of 

 the mollusc and reaching the respiratory chamber proceeds to develop 

 into the next stage, the sporocyst. Within the sporocyst by a process 

 of internal budding more or less numerous so-called redice develop. The 

 rediae finally leave the sporocyst and migrate into the liver of the mollusc. 

 In the redia several generations of daughter rediae may develop by 

 budding. The next stage, developed also by internal budding from the 

 redia, is the cercaria. The cercaria of some species is provided with a 

 tail by means of which it swims about in the water when it finally escapes 

 from the mollusc. The cercaria may be swallowed by or actively pene- 

 trate into some animal and become enc\'sted in this animal. Finall}' when 

 the animal harboring encysted immature flukes is swallowed by an animal 

 which can serve as a host of the adult fluke, the young flukes thus reach- 

 ing their definitive host develop to maturity and the life cycle is complete. 

 Following is given a partial list of the insects in which young flukes 

 have been recorded. The species to which the young flukes in question 

 have been assigned and the final host animals are also indicated. Further 

 investigations are likely to show that some of the flukes from insects 

 have been misidentified and do not belong to the species to which they 

 have been supposed to belong, and the data given in the list should 

 not be accepted as fully proved in any case, though there can be no doubt 

 in some of the cases cited. No distinction has been made between certain 

 and doubtful cases, except that a few that are doubtful are indicated by 

 question marks. The determination of species of young flukes found in 

 insects has generally been made solely upon their morphological similarity 

 to adults occurring in vertebrate hosts and it is quite likely that mistakes 

 have been made by investigators of these parasites just as mistakes have 

 frequently been made in the association of immature and adult parasites 

 belonging to other groups of worms. 



NEMATODA OR ROUNDWORMS 



Among parasitic worms the species of nematodes are more numerous 

 than either the species of tapeworms or flukes. Nematodes as a group 

 are not exclusively parasitic and thousands of free-living species are 

 known to exist, although comparatively few have been described. Many 

 species of nematodes are parasites of insects only and do not occur in 

 other animals. Insects therefore harbor parasitic nematodes which belong 

 to them exclusively as well as the larval stages of nematodes that occur 

 in higher animals in their adult stage. The ubiquity of free-living nema- 

 todes introduces a frequently troublesome complication into the study 

 of the life histories of monoxenous parasitic nematodes of which there 

 are many species, and the common occurrence of parasitic nematodes 



