A NUTRITIONAL STUDY OF INSECTS 35 



this manner yeasts often become predominant and prevent the 

 development of destructive molds. These habits of the larvae 

 may have come about through selective killing, for a mortality of 

 75 per cent was shown in cultures where the activities of the in- 

 sects were suspended by low temperatures. This high mor- 

 tality occurred only in cultures contaminated by fungi and was 

 not due to the low temperature itself, but to the uncontrolled 

 growth of the microorganisms. 



3. Another explanation m.ay be drawn from a consideration of 

 the relative food value of the substratum and the microorgan- 

 isms. The fruits or vegetables of the substratum (omitting 

 beans, com, spinach, etc., which are unlikely to form an appreci- 

 able number of breeding places have less than 2 per cent protein 

 and are relatively rich in carbohydrates. Microorganisms, on 

 the other hand, have a protein content of over 10 per cent, but 

 are poor in carbohydrates. ^^ This is shown in table 11. 



The protein content of yeast cells is readily available, as the 

 enclosing cell membrane has been shown to be closely allied to 

 pectin (Casagrandi) . Salkowski ('94) finds that the membrane 

 is composed of two layers, one of which forms ,on hydrolysis 

 d-glucose and the other glucose and mannose. The membranes 

 are readily destroyed by the digestive juices, as yeast is exten- 

 sively used as protein food for farm animals and even for man 

 (Salomon, '16). The components of the yeast nucleoprotein in- 

 clude almost every known monoamino-acid cleavage product, 



^* Kappes' ('89) analyses of Micrococcus prodigiosus scraped from the surface 

 of solid media gave on an average : Water, 85.5 per cent, and dry matter, 14.5 per 

 cent, the latter having a percentage composition of protein (N X 6.25) of 71.2 

 per cent, i.e., 10.3 per cent for the whole cell. Nucleoprotein has been separated 

 from cholera, bubonic plague, anthrax, and diphtheria bacilli and from B. pyo- 

 cyaneum megaterium and Staphylococcus pyogenes aureus (according to Ben- 

 ecke, '12) and from .yeasts by Hoppe-Seyler (71), Kossel (79), and Stutzer ('82). 



Clautrian ('95) showed the presence of glycogen in dried Boletus edulis (20 per 

 cent), Amanita muscaria (14 per cent) and yeast (31 per cent). In 1866 Hoppe- 

 Seyler found 0.25 gram lecithin and 0.44 gram cholesterin in 81 grams of dry yeast, 

 and later Naegli and Loew ('78) found 5 per cent fat (stearic and palmitic acid) 

 in the yeast cell. Yeast gum (mannan) makes up 6 to 7 per cent of yeast (by dry 

 weight), according to Salkowski ('94). Later the following gums were isolated 

 from different yeasts, viz., mucin, dextran, laevulan, mannan, arabin and galac- 

 tane (Lafar, '03). 



