STEPS TO PROVE INSECT CAUSATION OF DISEASE 29 



host upon the body, and the scratching of infected portions of the 

 insect's body into the blood ; or may be transmitted only by the ingestion 

 of the insect itself by its vertebrate host, or accidentally by some grazing 

 animal. In fact quite a series of disease organisms find their way into 

 their hosts because of the habit of the animals of feeding upon insects. 



6. What Is the Course of the Organism in the Insect? 



If the organism is taken up by the insect in its larval stage, it may 

 pass directly through the larva and out in its feces and may quite con- 

 ceivably pass in this manner through insect after insect larva before it 

 finally finds a vertebrate host. The organism may be taken up by the 

 larva and remain dormant in some portion of the larva's anatomy, or on 

 the other hand, it might undergo considerable development and multipli- 

 cation in the larva and remain there through all the metamorphosis of 

 the insect until the latter arrives at maturity, at which time development 

 of the organism may begin or may continue. 



Upon being taken up in the blood by the bite of the insect, the organ- 

 ism may lodge in tlie esophagus and carry out all its metamorphosis 

 there, or in some of the organs of the head and find its way into the 

 salivary glands and through the salivary secretions into a new host. 



It may, on the other hand, pass back into the gut, or into the stomach ; 

 from the stomach its path may lead in many directions. It may pass on 

 in its course of development into the rectum and out in the feces, or it 

 may enter the fatty bodies, or pass into the general cavity of the 

 insect, or it may migrate forward into the esophagus and into the labrum ; 

 and it may pass into the ^Malpighian tubules, or into the ovaries. 



The organism may enter the eggs and remain therein through their 

 development into the larA'ae, nymphs or adults, and be transmitted at 

 some stage of the development of the second generation. Some diseases 

 can pass on even to the third generation. 



7. What Is the Course of the Organism on Leaving the Insect? 



The organism may leave the insect in the saliva and immediately enter 

 the feeding puncture. It may bore through the labium of the insect at 

 tlie time of feeding and enter the puncture. It may leave the rectum of 

 the insect, or the Malpighian glands and be washed into the puncture by 

 means of the secretions of the coxal glands, or some other excretions 

 made at the time of feeding. It may be excreted in Malpighian secre- 

 tions, or rectal feces, or regurgitated in vomit, and may lie dormant on 

 the skin of the host, or on the food of the host, until it is scratched into 

 the blood, or is taken into the mouth. 



