August 18, 1905.] 



SCIENCE. 



195 



the droppings of this insect to form cen- 

 ters of development wherever they chance 

 to be deposited. More extended experi- 

 mentation on this point is urgently needed, 

 bnt one can hardly donbt that other insects, 

 and perhaps many invertebrates, function 

 in similar manner as distributors of infec- 

 tion. It should be noted that this manner 

 of distribution is not confined to bacteria 

 alone, although only scanty evidence is at 

 hand concerning the mechanical transport 

 of other forms. Thus Grassi found that 

 flies sucked up with water eggs of various 

 parasites, both tapeworms and round 

 worms (Tcenia solium, Oxyuris, Trichuris), 

 and that these eggs were recovered unal- 

 tered from the dejections of the flies, while 

 he also caught some flies with the aliment- 

 ary canal full of these eggs. This is posi- 

 tive evidence that the fly is able to ingest 

 solid bodies of some size through the suck- 

 ing proboscis. At the same time he saw 

 flies feed on the eggs of Trichuris, on his 

 laboratory table, and later found the eggs 

 in droppings deposited in the kitchen in the 

 story beneath, at a distance of ten meters 

 from the place where the insects had been 

 feeding. Such internal transportation evi- 

 dently insures far greater freedom from 

 damage and adverse conditions, as well as 

 much wider dissemination than were the 

 spores or eggs merely adherent to external 

 organs. Thus living cholera bacilli have 

 been voided by a fly some days after the 

 original contamination. In the course of 

 this period of time the fly could have 

 wandered to some distance from the place 

 of infection. 



]\Iany investigations have shown, how- 

 ever, that small larvte or adult worms like 

 trichinae are digested by the various ani- 

 mals to which they were fed, and have en- 

 tirely disappeared in the course of a few 

 hours. Such experiments have been made 

 with frogs, salamanders, land and water 



beetles, maggots and earth worms. Stiles 

 tried some years ago a most interesting 

 experiment which throws much light upon 

 this subject. He placed fly maggots with 

 some Ascaris lumhricoides and the latter 

 were devoured together with the eggs they 

 contained. Not only the fly larvae, but 

 also the pupge and the adult insects which 

 developed from them were found to contain 

 eggs of the Ascaris. As the experiment 

 was carried out in very warm weather the 

 Ascaris eggs developed rapidly and were 

 present in the insects in various stages. 

 Evidently then the adult fly would serve as 

 a disseminator of the parasite, and if the 

 eggs attained the proper stage of develop- 

 ment the fly might infect man directly by 

 depositing them on articles of food. It is 

 known that certain seeds will develop only 

 after having passed through the intestine 

 of birds and it may well be that a similar 

 biological environment is necessary in the 

 case of some disease germs. Some such 

 condition would serve to explain the curi- 

 ous inability to infect experimentally by 

 direct transfer where the disease is yet 

 readily and abundantly transferred in 

 nature. But the transferring insect would 

 not be a mere mechanical carrier, it would 

 constitute a necessary link in the life his- 

 tory. 



There are many such cases already 

 known, but in most of them at least, the 

 disease-producing organism passes through 

 some phase in its life history in the dis- 

 seminating animal which thus becomes an 

 intermediate host, a necessary and not a 

 casual element in the life cycle. Such 

 forms are in no sense mechanical carriers 

 and it is evident that the limits between 

 these two groups depend partly at least on 

 the extent of our knowledge, since a more 

 careful investigation may show that some 

 instances of transfer which are regarded 

 to-day as purely mechanical involve in 



