220 



did not occur when many larvae were present. These surface growths, 

 which proved to be largely yeast cells, appeared however after pupa- 

 tion, or if only a few larvae are present. It was also found that 

 adult flies, pupae, larvae and eggs invariably carried yeast cells. 

 Experiments showed that a loose symbiosis exists between the two 

 organisms, but that yeast is not present in the eggs or pupae of Droso- 

 phila. 



From the fact that fungus growths disappear in the presence of 

 larvae it was inferred that they fed upon the micro-organisms present, 

 this being established as a fact by experiment, which also proved 

 that the fungus is merely the food of the insect, since the larvae grew 

 more rapidly on abundant dead yeast than on less abundant living 

 yeast. There is also evidence that other micro-organisms may furnish 

 food for these flies. 



Similar food relations may be common- to a number of organisms, 

 as for example the house-fly, which oviposits only in the presence of 

 the odour of fermentation and always has a certain form of 

 bacteria on its body in great abundance, the larvae being unable to 

 survive in garbage which gives an acid reaction. Those insects that 

 live in unusual media, such as strong salt water and petroleimi, may be 

 associated with micro-organisms that have unusual powers of oxidation. 



Since Drosophila can be reared on a synthetic medium consisting of 

 inorganic salts, sugars and ammonium tartrate, it was supposed to 

 possess a synthetic power as great as that of bacteria. It is, however, 

 the yeast cells that are capable of synthesising the proteins from such 

 a culture medium, and the insect depends on these cells for its proteins, 

 having no greater synthetic power than is common to higher animals. 

 Adult flies do not require proteins, but survive for a much longer 

 period on sugar agar than on yeast agar, while proteins are necessary 

 to the development of rapidly growing larvae. 



The habit of the larva of constantly agitating the surface and 

 carrying yeast cells throughout the medium, greatly increases fermen- 

 tation, and the resulting increase in alcohol may serve as a protection 

 to the larvae against destructive moulds and putrefactive bacteria. 

 The resulting anaerobic conditions may also increase the food value 

 of the yeast cells by increasing the percentage of albumen contained 

 in them. 



The number of larvae present determines the extent to which the 

 yeast will be spread though a culture medium, the depth to which 

 the larvae can go below the surface being also an important factor. 

 This depth is determined by the consistency of the medium, since the 

 larvae will go to the depth at which a bubble of air will remain attached 

 to the two projecting posterior spiracles. The moment this bubble 

 is lost the larva goes upward until another is found. Thus a medium 

 of a jelly-like homogeneous consistency would appear to be the best 

 for the artificial rearing of these larvae, 



Brues (C. T.). Adult Hymenopterous Parasites attached to the Body 

 of their Host. — Proc. National Acad. Sci., Washington, D.C., iii, 

 no. 2, February 1917, pp. 136-140, 1 fig. 



A small parasitic Hymenopteron of the family Scelionidae para- 

 sitises the eggs of the Deccan grasshopper {Colemania sphenarioides, 



