Cancer 
255 
Saul (1910) and Dahl (1910) go much further, since they attribute 
the production of the malignant growth to the presence of mites 
which Saul had found in cancers. These Dahl described as belonging 
to a new species, which he designated Tarsonemus hominis. These 
findings have since been confirmed by several workers. Neverthe¬ 
less, the presence of the mite is so rare that it cannot be regarded as 
an important factor in the causation of the disease. The theory 
that cancer is caused by an external parasite is given little credence 
by investigators in this field. 
In conclusion, it should be noted that the medical and entomolog¬ 
ical literature of the past few years abounds in suggestions, and in 
unsupported direct statements that various other diseases are insect- 
borne. Knab (1912) has well said “Since the discovery that certain 
blood-sucking insects are the secondary hosts of pathogenic para¬ 
sites, nearly every insect that sucks blood, whether habitually or 
occasionally, has been suspected or considered a possible transmitter 
of disease. No thought seems to have been given to the conditions 
and the characteristics of the individual species of blood-sucking 
insects, which make disease transmission possible.” 
He points out that “in order to be a potential transmitter of human 
blood-parasites, an insect must be closely associated with man and 
normally have opportunity to suck his blood repeatedly. It is not 
sufficient that occasional specimens bite man, as, for example, is the 
case with forest mosquitoes. Although a person may be bitten by a 
large number of such mosquitoes, the chances that any of these 
mosquitoes survive to develop the parasites in question, (assuming 
such development to be possible), and then find opportunity to bite 
and infect another person, are altogether too remote. Applying 
this criterion, not only the majority of mosquitoes but many other 
blood-sucking insects, such as Tabanidae and Simuliidse, may be 
confidently eliminated. Moreover, these insects are mostly in 
evidence only during a brief season, so that we have an additional 
difficulty of a very long interval during which there could be no prop¬ 
agation of the disease in question.” He makes an exception of 
tick-borne diseases, where the parasites are directly transmitted from 
the tick host to its offspring and where, for this reason, the insect 
remains a potential transmitter for a very long period. He also 
cites the trypanosome diseases as possible exceptions, since the causa¬ 
tive organisms apparently thrive in a number of different vertebrate 
hosts and may be transmitted from cattle, or wild animals, to man. 
