GASTRIC C.^CA OF THE HETEROPTERA. 14! 



Anisops platycnemis Fieb. 

 Nepidae. 



Ranatra qitadridentata Stal. 

 Belostomidae. 



Benacus griseus Say. 



Belostoma fluminea Say. 

 Yeliidee. 



Hebrus americana Uhl. 



Gerris marginatus Say. 



G err is regimis Say. 



FUNCTIONAL RELATION OF C^CAL BACTERIA TO THE HOST 



INSECT. 



In considering the possible relation of the caecal bacteria to the 

 life processes of the insect, a digestive function is at once sug- 

 gested on account of the great number and apparent limitation 

 of these organisms to the digestive tract of the host. One point, 

 however, that was rather puzzling and decidedly difficult to 

 understand on this basis was the peculiar localization of the 

 infection in relation to the digestive portion of the gut. 



Digestive caeca are very common in other groups of insects, 

 occurring notably in the Orthoptera, Coleoptera, and Diptera, as 

 well as in several other orders, but with a few isolated exceptions 

 these organs are invariably located toward the anterior end of the 

 midgut, often serving as reservoirs for the food in which certain 

 definite digestive processes take place. In the Heteroptera, 

 however, the caeca, and consequently the intestinal bacteria, are 

 located at the very posterior end of the midgut, and these 

 organs appear to have lost their direct digestive function, since 

 neither they nor the fourth stomach itself ever seem to contain 

 any food, the last stages of digestion apparently taking place in the 

 third stomach, which is usually found to contain a mass of food 

 material in process of digestion. Just how to correlate the singu- 

 lar localization of the intestinal bacteria in these insects with any 

 digestive process was by no means clear, and this remained more 

 or less of a mystery until explained by culture experiments. 



It is a well-known fact that most insects, whether feeding on 

 solid or liquid food, support a great variety of saprophytic and 



