576 



NA TURE 



[October 14, 1897 



test-tubes. Moulds were in all cases abundant. In those cases 

 where the grapes had been left in the apparatus some days after 

 the ants were dead, moulds developed rapidly on the grapes, 

 and probably destroyed all the yeasts ; for in the test-tube 

 cultures moulds were abundant, and no yeasts were observed. 



Many experiments were carried out by Dr. Amedeo Berlese 

 to prove that yeasts are abundantly distributed through various 

 kinds of flies. 



Small pieces of meat, carefully sterilised by washing in 

 sublimate solution and then in water, were exposed on a terrace, 

 some inside a close wire net which prevented contact with 

 insects, and others so as to favour the visits of flying insects 

 only. Several individuals of the Sarcophaga carnaria were 

 noticed visiting the exposed meat. After two hours' exposure 

 to the flies, and thirteen hours' exposure to the air, the pieces 

 of meat were dropped into test-tubes containing sterilised must, 

 and the sediments obtained after fermentation examined for 

 yeasts. It was found that whenever the meat had been exposed 

 to flies, yeasts were fajr more abundant than when the flies had 

 been excluded. Saccharotnyces apicidattis was the yeast that 

 appeared more abundantly disseminated by the meat-flies ; far 

 less abundant was Saccharotnyces ellipsoideus. It was calculated 

 that the quantity of yeasts carried to a piece of meat by flies in a 

 given time is about twenty-six times the quantity that would be 

 carried by air alone. 



In other experiments, flies were made to visit grapes that had 

 been previously carefully sterilised. The flies were attracted to 

 the grapes by concealing near them, but so that the flies could 

 not touch them, pieces of meat. The flies that were chiefly 

 attracted and alighted on the grapes were Sarcophaga cai-naria 



Fig. 2. 



and the blue meat-fly, Calliphora erythrocephala. The grapes 

 were found abundantly infected with Saccharomyces apiculatus, 

 and in a far minor degree with S. ellipsoideiis and S. pastorianus : 

 moulds and Dematium were abundant. In control experiments, 

 where flies had not been allowed to touch the grapes, these con- 

 tained no yeasts. Strong infections with S. apiculatus were also 

 obtained by imprisoning blue meat-flies and Sarcophaga inside a 

 wire netting in which sterilised grapes had been hung. 



On experimenting in a similar manner with the cellar-midge or 

 vinegar-fly, the Drosophila cellaris, it was found that it conveyed 

 in great abundance Saccharomyces ellipsoideus and pastorianus, 

 and also in smaller quantity Saccharomyces apiculatus, besides, 

 of course. Bacterium aceti, Dtmatitun, and many moulds. 

 Sterilised grapes, when visited by the cellar-midge, cause rapid 

 fermentation of the grape-must in which they are sown. 



How do winged insects convey yeasts ? 



Several experiments with various kinds of flies showed that 

 ferments are often more abundant in the bodies of the flies 

 than in their legs and feet. It is also easy to observe the 

 presence of cells, similar to those of yeast, in the excrements of 

 flies. Dr. Berlese was thus brought to study experimentally the 

 passage of yeast cells through the digestive tract of meat-flies 

 and cellar- midges. 



The best experiments to prove the passage and the preserva- 

 tion of yeast cells inside the bodies of insects were made with 

 meat-flies. The living flies, as shown in Fig. 2, were laid on 

 their backs on a glass plate and pinioned there by glueing the 



extremities of their wings to the glass ; the legs of the fly thus 

 secured were removed, in order to prevent infection of the 

 external part of the body by the legs. The external part of the 

 body of each fly was carefully sterilised by repeated brush 

 washings with corrosive sublimate solution. Thus fixed 

 on its back, externally sterilised, and secured inside a glass 

 Petri-box, each fly was regularly fed for several days, either 

 with sterilised grape-must, or with pure cultures of yeasts in 

 must. The excretions of the fly were easily collected by means 

 of a sterilised platinum loop, and examined by inoculation in 

 sterilised must for yeast cells. This method gave full assurance 

 that any living cell found in the excretions had passed through 

 the intestine. The pinioned flies did not lose their desire for 

 food, but eagerly sucked up with their proboscis the grape-must 

 presented to them on the platinum loop, living on for several 

 days, when kept at a temperature of from 18° to 20° C. It was 

 observed that when the flies had Vjeen kept fasting they eagerly 

 and completely sucked up all the yeast-laden must offered to 

 them ; but when the flies had been well fed they became more 

 dainty, and sucked only the liquid portion of the sweet drop, 

 leaving a residual semi-solid lump consisting largely of yeast 

 cells ; this would prove that in the act of suction with the 

 proboscis, flies can probably at will use a filtering process to 

 separate the solid from the liquid parts. In numerous experi- 



FlG. 



ments with blue meat-flies, flesh-flies, and cellar-midges, it was 

 proved that when the flies are fed with sterilised must the excreta 

 contain no yeasts, especially if gathered after repeated evacua- 

 tions ; but if, on the contrary, the flies are fed with pure cultures 

 of Saccharomyces apiculattis, or of other yeasts, the excreta soon 

 contain in great numbers the yeast that was in the food. The 

 yeast cells contained in the excreta are living, for, when sown in 

 suitable liquids, they multiply with great rapidity. Figs. 3 and 

 4 give an example of the multiplication, during eighteen hours, 

 of the cells of a yeast {Saccharomyces pastorianus) emitted by a 

 blue meat-fly, which had been fed with grape-must containing 

 that yeast. The vitality of the yeast cells is also unimpaired 

 when given to the flies with meat juice containing no sugar. 

 The temperatures prevailing during ingestion of yeast cells in 

 flies influence the rapidity of their multiplication in the excreta. 

 Thus, when Saccharomyces apiculatus is given to the flies, the 

 cells in the excreta are in active germination when the prevailing 

 temperature is from 20° to 25° C. ; on the other hand, germ- 

 inating cells are scarce if the fly has been kept at 8° to 10° C. 

 This suggests that inside the digestive tract of the insect ex- 

 ternal conditions may influence the multiplication of the yeast 

 cells. To prove this, some blue meat-flies were fed only once 



NO. 1459, VOL. 56] 



