94 PROC. ENT. SOC. WASH., VOL. 20, NO. 5, MAY, I'lls 



bites of the stable fly (Stomoxys calcitrans), horse flies (Tabanus 

 striatus and other species), the horn fly (Haematobia irritans) 

 and mosquitoes (Psorophora sayi and Aedes sylvestris) several of 

 which insects have just been proven carriers by Harry Morris. 

 Many bacteria pass uninjured and unaltered through the intestines 

 of insects. 



Type 3. The relationship between vertebrate host and organ- 

 ism still simple:, but the organism able to multiply and even in- 

 crease its virulence by passage through the intestine of an insect, 

 but not necessarily having an intermediate host. Cao has shown 

 that many pathogenic bacteria are taken up by flesh and feces 

 breeding insects and are able to multiply in the insect and acquire 

 greater virulence, as for example, the bacilli of fowl cholera, and of 

 anthrax in the cockroach. 



Type 4- An interesting type of relationship is that of normally 

 saprophytic, nonpathogenic organisms, which after a passage 

 through certain insects acquire an intense pathogenicity, and if by 

 chance they should become inoculated in a vertebrate by contact 

 of the insect with an injury, would cause serious disease symptoms. 

 Possibly this is one of the manners in which new bacterial diseases 

 originate. Cao has conducted many experiments with cock- 

 roaches, may beetle larvae and fly larvae, in which he has obtained 

 such results as these. Among the organisms thus becoming 

 virulent were Bacillus fluorescens liquefaciens and non-liquefaciens , 

 Bacillus subtilis, Sarcina aurantiaca and Sarcina lutea. 



Type 5. The relationship three-sided, that is with two distinct 

 hosts in each of which the parasite must pass part of its life cycle, 

 as for example the organisms of malaria, sleeping sickness, Texas 

 fever, etc. 



Type 6. The relationship multiple, in which there is a normal 

 wild animal reservoir of the organism, an insect vector, and one or 

 more other animal hosts, as Rocky Mountain spotted fever, many 

 trypanosome diseases, and possibly the majority of protozoan 

 diseases. 



Type 7. An accidental relationship in which an apparently 

 normal insect organism is inoculated and becomes pathogenic to a 

 vertebrate. Experimental inoculations with Herpetomonas and 

 Leptomonas from insects have produced diseases in animals. 

 Some authors believe that possibly if not probably the Leishmania 

 and Trypanosome diseases originated in this manner. 



The complications of these types of relationships are many in 

 fact almost every case is different, and very few are perfectly 

 simple as described above. 



In the three and multiple sided relationships, wherein an insect 

 often serves as the intermediate host, are found some of the most 

 interesting biological phases of the subject. 



