A Study of the Processes of Digestion in Certain Insects

Abstract

Aspects of the processes of digestion in the insects Blattella germanica, Per-iplaneta americana, and Tenebrio molitor are reported. In Blattella starved for 2 days, a meal of colored starch reaches the midgut within 10 min. and the rectum within 5 hrs. The pH of the gut contents on a starch diet is approx. 4.5 in the crop, 60 in the midgut, and 8.0 in the hindgut. A protein diet raises the pH of the crop to about 6.0, but does not change that of the other regions. There is a gradient of decreasing redox potential from the crop to the hindgut where the Eh approximates -0.1 v. at pH 8. Concurrent quantitative enzyme estimations and cyto-logical investigations on Blattella have proved that the presence of cytoplasmic globules, hitherto generally referred to as cyto-logical evidence of secretory activity, is not associated with an increase in enzyme concn. in the gut contents. The greatest enzyme concns. are found when the cytoplasm is cytologically uniform. The secretory globules are more probably signs of cell breakdown than an indication of secretory activity. Digestive enzymes are still present in Blattella midgut contents after 3 days'' starvation, but the enzymes studied increase in concn. when the insect is fed, irrespective of the diet. A digestive enzyme of Blattella decreases in amount when the insect is fed a diet of that particular enzyme substrate for some time. The enzyme concn. is fairly slow to regain its former level. Evidence is presented that stimulation of epithelial regeneration of the midgut of Tenebrio is effected ty a factor carried in the blood. There is some evidence, mainly morphological, against the nervous control of midgut secretion. A study of the localization of various substances shows that different materials may be absorbed in different regions of the gut. Fore-, mid-, and-hind-guts, and the midgut caeca may all be involved in absorption. The histopathology of a number of insecticides suggests that, except for arsenic compounds, changes produced in the midgut are not sufficient to account for death of the insect.