526 Journal of Agricultural Research voi. x. no. 10 



A study of the sections from the above aphids shows that the precipi- 

 tate did not adhere well to the tissues and, owing to its color, it is dis- 

 cernible in sections only with difficulty. Nevertheless, small particles 

 of it were observed occasionally on the mteguments and in the trachese, 

 showing that a soap solution not containing a stain passes into the respi- 

 ratory system of aphids when used as a spray. 



Owing to the color and adhering ability of precipitated carmine acid, 

 the sections from the aphids sprayed with the soap-solution extract 

 colored with this stain are much more satisfactory to study. A careful 

 study of them shows the following: Much of the red precipitate adheres 

 to the outside of the integuments, but none of it has passed through 

 them; much of it also lies in the spiracles and at various places in prac- 

 tically all the large tracheae in the abdomen, thorax, and head, and in 

 the bases of all the legs, but none was observed in the lumens passing 

 through the proboscides. In the abdomens and bases of the legs the fat 

 cells surrounding the tracheae are generally stained red, indicating that 

 the liquid had passed through the tracheal walls. At one place in a 

 thorax a trachea, bearing some of the precipitate, runs along beside a 

 large muscle which is also slightly stained. More or less precipitate 

 was also observed in the tracheae lying against the optic lobes, the brain, 

 and thoracic ganglion; and occasionally it was noted that the colored 

 liquid had passed through the tracheal walls and had stained the nerve 

 cells near by. Only a few small particles of the precipitate were ob- 

 served in the interior of the optic lobes and brain. 



The preceding results indicate that the nerve tissue is the one 

 vitally affected, because the spray solution does not seem sufficiently 

 distributed in the other tissues to cause fatality, whereas only a few parti- 

 cles of any toxic substance in the brain and ganglia usually cause death. 



SUMMARY 



Throughout this investigation the experimental results obtained almost 

 invariably support the results of the quantitative determinations. The 

 data obtained pertaining to this portion of the work are as follows : 



(i) Medium-sized quassia chips (samples 3 and 4, Table III) soaked for 

 two hours in water yield during the first extraction about 60 per cent of 

 their total soluble matter, but only about 15 per cent during the second 

 extraction; if soaked for 24 hours, they yield during the first extraction 

 about 60 per cent, but during the second extraction about 10 per cent. 

 Experiments with first and second extracts always showed that the first 

 extract was the more eft'ective, but the ratio of effectiveness of the first to 

 the second extract was only about 9 to 7 ; this is true for both water ex- 

 tracts and soap-solution extracts. In practical spraying neither one of 

 the water extracts nor the second soap-solution extract was efficient, but 

 the first soap-solution extract was sometimes efficient. 



